**Strategies to Improve Music Perception in Cochlear Implantees**

Joshua Kuang-Chao Chen1,2, Catherine McMahon3 and Lieber Po-Hung Li1,2,4,\* *1Department of Otolaryngology, Cheng Hsin General Hospital 2Faculty of Medicine, School of Medicine, National Yang-Ming University 3Center for Language Sciences, Macquarie University 4Integrated Brain Research Laboratory, Taipei Veterans General Hospital 1,2,4Taiwan 3Australia* 

#### **1. Introduction**

58 Cochlear Implant Research Updates

Millar J., Tong Y., Clark G. (1984). Speech processing for cochlear implant prostheses, *Journal* 

Misiti M., Misiti Y., Oppenheim G., Poggi J. M. (2000) *Wavelets Toolbox Users Guide*, The

Moore J. A., Teagle H. F. B. (2002). An introduction to cochlear implant technology,

Nogueira W., Giese A., Edler B., Buchner A. (2006). Wavelet Packet Filterbank for Speech

Parkins C., Anderson S. (Eds.) (1983). Cochlear prostheses, *An international symposium New* 

Pereyra M. C., Mohlenkampy M. J. (2004). Wavelets, their friends, and what they can do for

Mexico, Albuquerque NM 87131-0001 USA, 23.08.2005, Available from

Quatieri (2001). *Discrete-Time Speech Signal Processing*, Principles and Practice. PrenticeHall Saeed V. V. (2000). *Advanced Signal Processing and Noise Reduction*, 2nd Edition, John Wiley

Rioul O., Vetterli M. (1991). Wavelets and signal processing, *IEEE SP Magazine*, pages 14–38,

Sheikhzadeh H., Abutalebi H. R. (2001). An improved wavelet-based speech enhancement

Strang G., Nguyen T. (1997). *Wavelets and Filter Banks*, Wellesley-Cambridge Press, second

Wilson B. S., Finley C. C., Lawson D. T., Wolford R. D., Eddington D. K., Rabinowitz W. M. (1991). Better speech recognition with cochlear implants, *Nature*, 352, pp. 236-238 Wilson B., (R. Tyler, ed.) (1993). *Signal processing in Cochlear Implants: Audiological* 

Yuan X. (2003). *Auditory Model-based Bionic Wavelet Transform*, Speech and Signal Processing

activation, and programming Language, Speech, and Hearing Services in Schools,

you, June 20, Department of Mathematics and Statistics, MSC03 2150, Un. of New

*of Speech and Hearing Research*, vol. 27, pp. 280-296

Processing Strategies in Cochlear Implants, *ICASSP*

Polikar R. (1999). The wavelet tutorial, 06.06.2000, Available from

http://users.rowan.edu/ polikar/WAVELETS/WTtutorial.html

*Foundations*, pp. 35-86, Singular Publishing Group, Inc.

MathWorks, Wavelet Toolbox

*York*, New York Academy of Sciences

http://www.math.unm.edu/crisp

system, *Eurospeech*, pp. 1855-1858

edition, ISBN 0-9614088-7-1

Lab., Milwaukee, Wisconsin

pp.153-161

and Sons

October 1991

Cochlear implants have been an effective device for the management of patients with total or profound hearing loss over the past few decades. Significant improvements in speech and language can be observed in implantees following rehabilitation. In spite of remarkable linguistic perception, however, it is difficult for these patients to enjoy music although we did see some "superstars" for music performance in our patients. This article aimed to clarify current opinions on the strategies to improve music perception ability in this population of subjects. In part I, we included one of our previous work (Chen et al., 2010) talking about the effect of music training on pitch perception in prelingually deafened children with a cochlear implant. In part II, other factors related to the improvement of music perception in cochlear implantees were discussed, including residual hearing, bimodal hearing, and coding strategies. Evidences from results of our researches and from literature review will both be presented.

#### **2. Part I: Music training improves pitch perception in prelingually deafened children with cochlear implants**

#### **2.1 Introduction**

Cochlear implants have been an effective device for the management of deaf children over the past few decades. Significant improvements in speech and language can be observed in implanted children following rehabilitation. In spite of remarkable linguistic perception, however, it is difficult for these children to enjoy music (Galvin et al., 2007; McDermott, 2004). Essential attributes of music include rhythm, timbre, and pitch. Previous studies have shown that perception of rhythm is easier than timbre and pitch for cochlear implant users (Gfeller & Lansing, 1991). Recognition of timbre depends, at least partly, on the

<sup>\*</sup> Corresponding Author

Strategies to Improve Music Perception in Cochlear Implantees 61

was obtained from parents with a protocol approved by Institutional Ethics Committee of

No Gender Device HA O P A A>5 D D>5 1 F 6 20 Clarion 0 48 y 45.3 37.1 45.7 40.0 48.6 46.7 2 M 5 42 Clarion 3 17 y 56.1 60.0 41.9 46.7 64.8 80.0 3 M 6 36 Nucleus 12 33 y 44.6 42.9 55.2 76.7 26.7 40.0 4 M 10 78 Nucleus 36 11 y 60.7 48.6 52.4 53.3 70.5 76.7 5 F 10 64 Nucleus 30 22 y 88.2 94.3 91.4 93.3 80.0 93.3 6 F 6 53 Nucleus 0 10 y 36.4 40.0 31.4 30.0 41.9 36.7 7 M 8 57 Clarion 0 34 y 50.5 65.7 55.2 53.3 41.0 46.7 8 M 6 36 Nucleus 0 33 n 48.2 5.7 52.4 66.7 52.4 50.0 9 M 6 54 Clarion 24 26 y 55.7 57.1 71.4 73.3 41.0 33.3 10 M 5 17 Nucleus 0 46 y 46.9 40.0 43.8 53.3 51.4 43.3 11 F 6 58 Nucleus 3 13 y 46.2 42.9 41.9 46.7 55.2 33.3 12 F 7 29 Nucleus 6 55 y 52.1 94.3 28.6 73.3 67.6 10.0 13 M 8 22 Nucleus 0 69 y 52.5 45.7 43.8 50.0 61.0 63.3 14 F 5 37 Nucleus 0 19 n 17.4 25.7 20.0 26.7 8.6 20.0 15 F 6 48 Nucleus 0 23 y 69.2 68.6 67.6 83.3 67.6 66.7 16 F 5 32 Nucleus 0 24 y 38.7 97.1 27.6 33.3 34.3 30.0 17 M 8 65 Clarion 0 25 y 56.1 94.3 68.6 76.7 33.3 26.7 18 M 5 30 Med El 0 31 y 55.4 62.9 55.2 70.0 50.5 50.0 19 M 6 37 Nucleus 2 31 y 56.1 88.6 61.0 63.3 44.8 33.3 20 M 8 68 Clarion 14 36 n 37.4 42.9 37.1 36.7 38.1 30.0 21 M 6 45 Clarion 0 33 y 46.2 20.0 41.9 36.7 56.2 66.7 22 M 6 36 Clarion 14 16 y 68.2 82.9 72.4 80.0 54.3 73.3 23 M 14 163 Clarion 0 17 y 89.2 97.1 95.2 93.3 79.0 90.0 24 F 5 53 Clarion 6 15 n 9.5 17.1 9.5 23.3 5.7 0.0 25 M 5 34 Clarion 0 30 n 50.2 5.7 30.5 30.0 81.0 83.3 26 M 5 32 Clarion 20 35 y 92.5 100.0 91.4 93.3 90.5 93.3 27 M 8 86 Clarion 2 22 y 36.4 0.0 41.0 40.0 41.9 40.0 No, participant number; Age, y/o; Age\*, age at implantation; Device, type of cochlear implant; DuM, duration of musical training (months); DuC, duration of cochlear implant use (months); HA, use of

Correct rate (%)

DuM (mo)

DuC (mo)

hearing aid in the other ear; Correct rate, percentage of correct response for pitch-interval differentiation; O, overall correct rate; P, correct rate for prime pitch interval; A, correct rate for ascending interval; A>5, correct rate for ascending interval over 5 semitones; D, correct rate for

Experiments were conducted in an acoustically-shielded room using a tuned piano (YAMAHATM, Japan). Subjects sat upright with eyes open, facing away from the piano at a distance of about 1 meter, and were instructed to attend to the auditory stimuli during experiments. A modification of a two-alternative forced choice task was used. Each teststimulus consisted of two sequential piano tones, ranging from C (256 Hz) to B (495 Hz). To

descending interval; D>5, correct rate for descending interval over 5 semitones.

Table 1. General data for all participants.

**2.2.2 Experiment paradigm** 

Cheng-Hsin General Hospital.

(yr)

Age\* (mo)

Age

discrimination of pitch in terms of fundamental frequency (Gfeller et al., 2002). The ability to differentiate pitch thus plays an important role in perception of music for implanted children. Fundamental traits of pitch acoustically transmitted to the auditory pathway of cochlear implantees via the apparatus are much less precise than those of normal-hearing subjects (Sucher & McDermott, 2007). Built-in restrictions for pitch perception in contemporary systems of cochlear implants arise from the electrical model of temporospatial stimulation, which in turn leads to a finite spectral resolution (McDermott, 2004). Efforts have been made to improve pitch resolution of cochlear implants for tonal languages and music perception (Busby & Plant, 2005; Firszt et al., 2007; Hamzavi & Arnoldner, 2006). However, the conclusions have been indecisive.

Neural correlates crucial for music processing have been demonstrated in cochlear implantees in an electroencephalographic study (Koelsch et al., 2004). Furthermore, magnetoencephalographic evidence of auditory plasticity has been noted in sudden deafness (Li, 2003, 2006). This plasticity facilitates tone perception in cochlear implantees, which can be mirrored by the progressive optimization of neuromagnetic responses evoked by auditory stimuli after implantation (Pantev et al., 2006). Considering limitations of cochlear implant processing strategies for pitch differentiation, education might have a major effect on improvement of music processing by inducing plastic changes in the central auditory pathway of cochlear implantees (Pantev et al., 1998). In fact, musical training has been found to be associated with improved pitch appraisal abilities in normal-hearing subjects, and comparatively poor music performance in cochlear implantees might be ascribed in part to an inadequate exposure to music (Sucher & McDermott, 2007). However, few studies exist on music performance in implanted children, and the effect of training on music perception in prelingually deafened children with cochlear implants has not been addressed. In the present study, twenty-seven prelingually deafened children with monaural cochlear implant were recruited to investigate whether or not musical education improved pitch perception. Thirteen subjects received structured training on music before and/or after implantation. Music perception was evaluated by using a test-set of pitch differentiation. To mirror real-world auditory environments, pure tones were presented using a tuned piano. Effect of age, gender, pitch-interval size, age of implantation, and type of cochlear implant were also addressed.

#### **2.2 Patients and methods**

#### **2.2.1 Subjects**

Twenty-seven subjects with congenital/prelingual deafness of profound degree (eighteen males and nine females; 5~14y/o, mean=6.7) were studied (Table 1). No other neurological deficits were identified. Thirteen subjects used Nucleus24 (Cochlear™, Australia)(left=6, right=7), thirteen subjects used Clarion (Advanced Bionics™, USA)(left=7, right=6), and one subject used Med-El (MED-EL™, Austria) cochlear implant system (right). Elapsed time for the evaluation of pitch perception after cochlear implantation ranged from 10 to 69 months (mean=29). Thirteen subjects attended the same style of structured music classes at YAMAHA Music School (2~36 months, mean=13.2). The programs included training of listening, singing, score-reading, and instruments-playing. They attended classes with normal-hearing children. Subject 4 and 5 have had musical education before the implantation. The study conformed to the Declaration of Helsinki. Written informed consent

discrimination of pitch in terms of fundamental frequency (Gfeller et al., 2002). The ability to differentiate pitch thus plays an important role in perception of music for implanted children. Fundamental traits of pitch acoustically transmitted to the auditory pathway of cochlear implantees via the apparatus are much less precise than those of normal-hearing subjects (Sucher & McDermott, 2007). Built-in restrictions for pitch perception in contemporary systems of cochlear implants arise from the electrical model of temporospatial stimulation, which in turn leads to a finite spectral resolution (McDermott, 2004). Efforts have been made to improve pitch resolution of cochlear implants for tonal languages and music perception (Busby & Plant, 2005; Firszt et al., 2007; Hamzavi & Arnoldner, 2006).

Neural correlates crucial for music processing have been demonstrated in cochlear implantees in an electroencephalographic study (Koelsch et al., 2004). Furthermore, magnetoencephalographic evidence of auditory plasticity has been noted in sudden deafness (Li, 2003, 2006). This plasticity facilitates tone perception in cochlear implantees, which can be mirrored by the progressive optimization of neuromagnetic responses evoked by auditory stimuli after implantation (Pantev et al., 2006). Considering limitations of cochlear implant processing strategies for pitch differentiation, education might have a major effect on improvement of music processing by inducing plastic changes in the central auditory pathway of cochlear implantees (Pantev et al., 1998). In fact, musical training has been found to be associated with improved pitch appraisal abilities in normal-hearing subjects, and comparatively poor music performance in cochlear implantees might be ascribed in part to an inadequate exposure to music (Sucher & McDermott, 2007). However, few studies exist on music performance in implanted children, and the effect of training on music perception in prelingually deafened children with cochlear implants has not been addressed. In the present study, twenty-seven prelingually deafened children with monaural cochlear implant were recruited to investigate whether or not musical education improved pitch perception. Thirteen subjects received structured training on music before and/or after implantation. Music perception was evaluated by using a test-set of pitch differentiation. To mirror real-world auditory environments, pure tones were presented using a tuned piano. Effect of age, gender, pitch-interval size, age of implantation, and type

Twenty-seven subjects with congenital/prelingual deafness of profound degree (eighteen males and nine females; 5~14y/o, mean=6.7) were studied (Table 1). No other neurological deficits were identified. Thirteen subjects used Nucleus24 (Cochlear™, Australia)(left=6, right=7), thirteen subjects used Clarion (Advanced Bionics™, USA)(left=7, right=6), and one subject used Med-El (MED-EL™, Austria) cochlear implant system (right). Elapsed time for the evaluation of pitch perception after cochlear implantation ranged from 10 to 69 months (mean=29). Thirteen subjects attended the same style of structured music classes at YAMAHA Music School (2~36 months, mean=13.2). The programs included training of listening, singing, score-reading, and instruments-playing. They attended classes with normal-hearing children. Subject 4 and 5 have had musical education before the implantation. The study conformed to the Declaration of Helsinki. Written informed consent

However, the conclusions have been indecisive.

of cochlear implant were also addressed.

**2.2 Patients and methods** 

**2.2.1 Subjects** 


was obtained from parents with a protocol approved by Institutional Ethics Committee of Cheng-Hsin General Hospital.

No, participant number; Age, y/o; Age\*, age at implantation; Device, type of cochlear implant; DuM, duration of musical training (months); DuC, duration of cochlear implant use (months); HA, use of hearing aid in the other ear; Correct rate, percentage of correct response for pitch-interval differentiation; O, overall correct rate; P, correct rate for prime pitch interval; A, correct rate for ascending interval; A>5, correct rate for ascending interval over 5 semitones; D, correct rate for descending interval; D>5, correct rate for descending interval over 5 semitones.

Table 1. General data for all participants.

#### **2.2.2 Experiment paradigm**

Experiments were conducted in an acoustically-shielded room using a tuned piano (YAMAHATM, Japan). Subjects sat upright with eyes open, facing away from the piano at a distance of about 1 meter, and were instructed to attend to the auditory stimuli during experiments. A modification of a two-alternative forced choice task was used. Each teststimulus consisted of two sequential piano tones, ranging from C (256 Hz) to B (495 Hz). To

Strategies to Improve Music Perception in Cochlear Implantees 63

girls/subjects ≤6 y/o, respectively. The mean correct rate of overall task performance was better for boys (56%)/subjects >6 y/o (58%) than for girls (45%)/subjects ≤6 y/o (49%), respectively, although the difference was insignificant (p=0.237 for gender, p=0.243 for age; Table 2). There were no differences in the performance of pitch perception between various

Total Gender Age

O 15 12 13 5 3 6 7 2 8 10 P 13 14 9 9 4 5 5 4 8 10 A 13 14 11 7 2 7 5 4 8 10 A>5 16 11 13 5 3 6 7 2 9 9 D 15 12 11 7 4 5 5 4 10 8 D>5 12 15 10 8 2 7 4 5 8 10 Age, y/o; Correct rate at cutoff value of 50% for pitch perception; O, overall correct rate; P, correct rate for prime pitch interval; A, correct rate for ascending interval; A>5, correct rate for ascending interval over 5 semitones; D, correct rate for descending interval; D>5, correct rate for descending interval over 5

Table 2. Differences in correct rate for pitch perception (cutoff value=50%) by gender and age.

52.1 54.7 50.9 51.4

p=0.887

O P A D A>5 D>5

Fig. 1. Differences of correct rate for pitch perception by pitch-interval size. There were no differences in the performance of pitch perception between various conditions of pitchinterval size (F(5,156) = 0.342, p=0.887). O, overall correct rate; P, correct rate for prime pitch interval; A, correct rate for ascending interval; A>5, correct rate for ascending interval over 5 semitones; D, correct rate for descending interval; D>5, correct rate for descending interval

≧50% <50% ≧50% <50% ≧50% <50% ≧50% <50% ≧50% <50%

Boy Girl > 6 yrs ≦ 6 yrs

57.2

50.2

conditions of pitch-interval size (F(5,156)=0.342, p=0.887; Figure 1).

Pitch Interval

semitones.

0

over 5 semitones.

30

Correct Rate (%)

60

avoid the possible effect of intensity variation on the test, the loudness was monitored on site by a sound-pressure meter and was maintained within 70±6 dB SPL for loudness matching of different pitch tones. The first note was any of the following: C, D (294 Hz), E (330 Hz), F (349 Hz), G (392 Hz), A (440 Hz) or B. Once the first note was determined, the second note was presented randomly from C to B. The interval of two notes was thus between prime degree (two same notes, e.g."C-C") and major-seventh degree (eleven semitones, e.g."C-B"), either ascending or descending in direction. A total of 49 (7x7) tonepairs were delivered to a subject in one experiment. The task was divided into two stages depending on the response. Each time after presentation of the stimuli, the subject would be asked whether the two notes were the same (i.e.,prime degree) or not. When the two notes were the same, the answer was recorded as correct or incorrect. When the two notes were different and the answer was incorrect, the answer was recorded as incorrect. When the two notes were different and the answer was correct, the subject would then be asked if the second tone was higher or lower than the first tone, and this subsequent answer was recorded as correct or incorrect. There was no feedback to subjects on their answers. Each tone-pair was presented five times. To avoid the effect of random guessing of the results, the answer needed to be correctly answered at least three times (≥60% correct) for a single tonepair recognition response to be recorded as correct. The correct rate for each subject was obtained by averaging the number of correct responses across the number of total tone pairs (49). The programming of speech processors for each subject varied, based on the speech intelligibility programs optimal for respective users.

#### **2.2.3 Data analysis**

Statistical analysis was performed using the software of SAS8.1 (SAS Institute Inc., USA). Performance of pitch perception in terms of correct rate was grouped into six sets for statistical analysis: overall, prime degree, ascending interval, ascending interval larger than perfect-fourth degree (five semitones, e.g."C-F"), descending interval, and descending interval larger than perfect-fourth degree. Differences in the performance of pitch perception by pitch-interval size were analyzed using analysis of variance. Differences of correct rate for pitch perception (cutoff value=50%) in terms of age were evaluated by dividing subjects into two groups: subjects >6 and subjects ≤6 y/o. Gender and age differences in overall task performance of pitch perception were evaluated using t-test. Correlations between pitch perception and period of musical training, age of implantation, or type of cochlear implant were evaluated using simple correlation analysis for three conditions respectively: all subjects, subjects divided into two groups by age (>6 and ≤6 y/o), and subjects divided into two groups by duration of cochlear implant use (>18 and ≤18 months). Threshold for statistical significance was set at P < 0.05.

#### **2.3 Results**

#### **2.3.1 Differences of correct rate for pitch perception by pitch-interval size, gender, and age**

Overall, the correct rate for pitch perception varied between 9.5% and 92.5% (Table 1). Fifteen subjects (13 male and 2 female, mean age=7.3 y/o) accomplished the test with a correct rate ≥50% (i.e., chance level). When subjects were divided by gender/age, boys/subjects >6 y/o tended to accomplish the test with a correct rate ≥50% than

avoid the possible effect of intensity variation on the test, the loudness was monitored on site by a sound-pressure meter and was maintained within 70±6 dB SPL for loudness matching of different pitch tones. The first note was any of the following: C, D (294 Hz), E (330 Hz), F (349 Hz), G (392 Hz), A (440 Hz) or B. Once the first note was determined, the second note was presented randomly from C to B. The interval of two notes was thus between prime degree (two same notes, e.g."C-C") and major-seventh degree (eleven semitones, e.g."C-B"), either ascending or descending in direction. A total of 49 (7x7) tonepairs were delivered to a subject in one experiment. The task was divided into two stages depending on the response. Each time after presentation of the stimuli, the subject would be asked whether the two notes were the same (i.e.,prime degree) or not. When the two notes were the same, the answer was recorded as correct or incorrect. When the two notes were different and the answer was incorrect, the answer was recorded as incorrect. When the two notes were different and the answer was correct, the subject would then be asked if the second tone was higher or lower than the first tone, and this subsequent answer was recorded as correct or incorrect. There was no feedback to subjects on their answers. Each tone-pair was presented five times. To avoid the effect of random guessing of the results, the answer needed to be correctly answered at least three times (≥60% correct) for a single tonepair recognition response to be recorded as correct. The correct rate for each subject was obtained by averaging the number of correct responses across the number of total tone pairs (49). The programming of speech processors for each subject varied, based on the speech

Statistical analysis was performed using the software of SAS8.1 (SAS Institute Inc., USA). Performance of pitch perception in terms of correct rate was grouped into six sets for statistical analysis: overall, prime degree, ascending interval, ascending interval larger than perfect-fourth degree (five semitones, e.g."C-F"), descending interval, and descending interval larger than perfect-fourth degree. Differences in the performance of pitch perception by pitch-interval size were analyzed using analysis of variance. Differences of correct rate for pitch perception (cutoff value=50%) in terms of age were evaluated by dividing subjects into two groups: subjects >6 and subjects ≤6 y/o. Gender and age differences in overall task performance of pitch perception were evaluated using t-test. Correlations between pitch perception and period of musical training, age of implantation, or type of cochlear implant were evaluated using simple correlation analysis for three conditions respectively: all subjects, subjects divided into two groups by age (>6 and ≤6 y/o), and subjects divided into two groups by duration of cochlear implant use (>18 and ≤18

**2.3.1 Differences of correct rate for pitch perception by pitch-interval size, gender,** 

Overall, the correct rate for pitch perception varied between 9.5% and 92.5% (Table 1). Fifteen subjects (13 male and 2 female, mean age=7.3 y/o) accomplished the test with a correct rate ≥50% (i.e., chance level). When subjects were divided by gender/age, boys/subjects >6 y/o tended to accomplish the test with a correct rate ≥50% than

intelligibility programs optimal for respective users.

months). Threshold for statistical significance was set at P < 0.05.

**2.2.3 Data analysis** 

**2.3 Results** 

**and age** 

girls/subjects ≤6 y/o, respectively. The mean correct rate of overall task performance was better for boys (56%)/subjects >6 y/o (58%) than for girls (45%)/subjects ≤6 y/o (49%), respectively, although the difference was insignificant (p=0.237 for gender, p=0.243 for age; Table 2). There were no differences in the performance of pitch perception between various conditions of pitch-interval size (F(5,156)=0.342, p=0.887; Figure 1).


Age, y/o; Correct rate at cutoff value of 50% for pitch perception; O, overall correct rate; P, correct rate for prime pitch interval; A, correct rate for ascending interval; A>5, correct rate for ascending interval over 5 semitones; D, correct rate for descending interval; D>5, correct rate for descending interval over 5 semitones.

Table 2. Differences in correct rate for pitch perception (cutoff value=50%) by gender and age.

Fig. 1. Differences of correct rate for pitch perception by pitch-interval size. There were no differences in the performance of pitch perception between various conditions of pitchinterval size (F(5,156) = 0.342, p=0.887). O, overall correct rate; P, correct rate for prime pitch interval; A, correct rate for ascending interval; A>5, correct rate for ascending interval over 5 semitones; D, correct rate for descending interval; D>5, correct rate for descending interval over 5 semitones.

Strategies to Improve Music Perception in Cochlear Implantees 65

Variable *r2 p r2 p r2 p r2 p r2 p r2 p*  DuM(mo) 0.564 0.010† 0.353 0.127 0.625 0.003† 0.549 0.012† 0.295 0.207 0.305 0.191 Device -0.005 0.983 -0.201 0.396 0.071 0.767 -0.240 0.308 0.064 0.787 0.114 0.632 Age\* 0.020 0.932 -0.051 0.832 0.238 0.312 0.064 0.787 -0.163 0.492 -0.043 0.859

Correlation between variables and correct rate of pitch perception (Duration of cochlear implant use >

Variable *r2 p r2 p r2 p r2 p r2 p r2 p*  DuM(mo) 0.133 0.776 -0.057 0.903 0.072 0.878 0.078 0.868 0.216 0.642 0.265 0.566 Device 0.169 0.717 0.402 0.371 0.246 0.594 0.369 0.415 -0.109 0.816 0.194 0.677 Age\* 0.595 0.159 0.539 0.212 0.657 0.109 0.603 0.152 0.500 0.253 0.421 0.346

Correlation between variables and correct rate of pitch perception (Duration of cochlear implant use ≤

Table 4c. and 4d. Correlation between variables and correct rate of pitch perception adjusted

months to assess the effect of implant use duration on the significance of correlation. For children with duration of implant use >18 months, the duration of musical training significantly correlated with correct rate of overall (r2=0.564, p=0.010) and ascending pitchinterval (r2=0.625, p=0.003) perception; there is no correlation between pitch perception and age of implantation or type of cochlear implant. For children with duration of implant use ≤18 months, there is no correlation between pitch perception and duration of musical

In the present study, the size of the pitch interval did not considerably affect the performance of pitch perception in subjects of prelingually deafened children with a cochlear implant (Figure 1, Table 1). For the pitch perception of descending interval >5 semitones, however, the correct rate was lower than for that of descending interval ≤5 semitones. This finding was paradoxical since it's reasonable to infer that a larger pitch interval is easier to perceive correctly than a smaller one. It might imply a general intricacy in pitch perception of descending interval for cochlear implant users of all age, since scores of "falling" melodic contour perception was much lower than those of "rising" one (even lower than chance level) for adult cochlear implantees in one previous study (Galvin et al.,

Various factors have been reported to affect the pitch perception in implanted children. The insignificant effect of pitch-interval size on the differentiation tasks in the present study could be ascribed partly to the channel-setting of sound frequency and/or tone perception changes caused by cochlear implants (Nardo et al., 2007; Reiss et al., 2007).

for duration of cochlear implant use (> 18 or ≤ 18 months).

training, age of implantation, or type of cochlear implant.

**2.4.1 Insignificant effect of pitch-interval size on pitch perception** 

18 months, n=20).

18 months, n=7).

**2.4 Discussion** 

2007; McDermott, 2004).

For description, see Table 3.

O P A A>5 D D>5

O P A A>5 D D>5

#### **2.3.2 Correlation between pitch perception and period of musical training, age of implantation, or type of cochlear implant (Table 3, 4a~d)**

For all subjects combined, the duration of musical training positively correlated with the correct rate of overall (r2=0.389, p=0.045) and ascending pitch-interval (r2=0.402, p=0.038) perception. There is no correlation between pitch perception and the age of implantation or type of cochlear implant.

To assess the effect of age on the significance of correlation, additional analysis was conducted with children separated by age >6 and ≤6 y/o (i.e., preschool). For children >6 y/o, there is no correlation between pitch perception and duration of musical training, age of implantation, or type of cochlear implant. For children ≤6 y/o, the duration of musical training strongly correlated with correct rate of ascending pitch-interval (r2=0.618, p=0.006) and ascending pitch-interval over 5 semitones (r2=0.584, p=0.011) perception; there is no correlation between pitch perception and age of implantation or type of cochlear implant.


Threshold for statistical significance using simple correlation analysis was set at P < 0.05 (denoted as †). r2, correlation coefficient; DuM, duration of musical training (months); Device, type of cochlear implant; Age\*, age at implantation; O, overall correct rate; P, correct rate for prime pitch interval; A, correct rate for ascending interval; A>5, correct rate for ascending interval over 5 semitones; D, correct rate for descending interval; D>5, correct rate for descending interval over 5 semitones.

Table 3. Correlation between variables and correct rate of pitch perception.

Since some patients >6 y/o have had a longer period of music training, additional analysis was conducted with children separated by duration of cochlear implant use >18 and ≤18




Correlation between variables and correct rate of pitch perception (≤ 6 years old, n=18).

For description, see Table 3.

Table 4a. and 4b. Correlation between variables and correct rate of pitch perception adjusted for age (> 6 or ≤ 6 years old).

Strategies to Improve Music Perception in Cochlear Implantees 65


Correlation between variables and correct rate of pitch perception (Duration of cochlear implant use > 18 months, n=20).


Correlation between variables and correct rate of pitch perception (Duration of cochlear implant use ≤ 18 months, n=7).

For description, see Table 3.

64 Cochlear Implant Research Updates

For all subjects combined, the duration of musical training positively correlated with the correct rate of overall (r2=0.389, p=0.045) and ascending pitch-interval (r2=0.402, p=0.038) perception. There is no correlation between pitch perception and the age of implantation or

To assess the effect of age on the significance of correlation, additional analysis was conducted with children separated by age >6 and ≤6 y/o (i.e., preschool). For children >6 y/o, there is no correlation between pitch perception and duration of musical training, age of implantation, or type of cochlear implant. For children ≤6 y/o, the duration of musical training strongly correlated with correct rate of ascending pitch-interval (r2=0.618, p=0.006) and ascending pitch-interval over 5 semitones (r2=0.584, p=0.011) perception; there is no correlation between pitch perception and age of implantation or type of cochlear implant.

Variable *r2 p r2 p r2 p r2 p r2 p r2 p* DuM(mo) 0.389 0.045† 0.238 0.232 0.402 0.038† 0.366 0.061 0.271 0.172 0.303 0.124 Device 0.046 0.818 -0.085 0.675 0.111 0.581 -0.099 0.624 0.026 0.897 0.149 0.459 Age\* 0.293 0.138 0.146 0.466 0.381 0.050 0.229 0.251 0.154 0.445 0.226 0.257 Threshold for statistical significance using simple correlation analysis was set at P < 0.05 (denoted as †). r2, correlation coefficient; DuM, duration of musical training (months); Device, type of cochlear implant; Age\*, age at implantation; O, overall correct rate; P, correct rate for prime pitch interval; A, correct rate for ascending interval; A>5, correct rate for ascending interval over 5 semitones; D, correct rate for

Since some patients >6 y/o have had a longer period of music training, additional analysis was conducted with children separated by duration of cochlear implant use >18 and ≤18

Variable *r2 p r2 p r2 p r2 p r2 p r2 p* DuM(mo) 0.293 0.445 0.012 0.975 0.145 0.710 0.074 0.850 0.459 0.214 0.442 0.234 Device -0.261 0.497 -0.169 0.664 0.120 0.758 -0.183 0.637 -0.660 0.053 -0.253 0.511 Age\* 0.493 0.178 0.115 0.768 0.635 0.066 0.358 0.344 0.252 0.513 0.492 0.178

Variable *r2 p r2 p r2 p r2 p r2 p r2 p* DuM(mo) 0.435 0.071 0.382 0.118 0.618 0.006† 0.584 0.011† 0.098 0.698 0.151 0.550 Device 0.132 0.602 -0.101 0.691 0.070 0.783 -0.110 0.663 0.231 0.357 0.338 0.170 Age\* -0.189 0.453 -0.122 0.631 -0.126 0.619 -0.117 0.645 -0.176 0.486 -0.254 0.310

Table 4a. and 4b. Correlation between variables and correct rate of pitch perception adjusted

Correlation between variables and correct rate of pitch perception (> 6 years old, n=9).

Correlation between variables and correct rate of pitch perception (≤ 6 years old, n=18).

O P A A>5 D D>5

O P A A>5 D D>5

descending interval; D>5, correct rate for descending interval over 5 semitones. Table 3. Correlation between variables and correct rate of pitch perception.

O P A A>5 D D>5

**2.3.2 Correlation between pitch perception and period of musical training, age of** 

**implantation, or type of cochlear implant (Table 3, 4a~d)** 

type of cochlear implant.

For description, see Table 3.

for age (> 6 or ≤ 6 years old).

Table 4c. and 4d. Correlation between variables and correct rate of pitch perception adjusted for duration of cochlear implant use (> 18 or ≤ 18 months).

months to assess the effect of implant use duration on the significance of correlation. For children with duration of implant use >18 months, the duration of musical training significantly correlated with correct rate of overall (r2=0.564, p=0.010) and ascending pitchinterval (r2=0.625, p=0.003) perception; there is no correlation between pitch perception and age of implantation or type of cochlear implant. For children with duration of implant use ≤18 months, there is no correlation between pitch perception and duration of musical training, age of implantation, or type of cochlear implant.

#### **2.4 Discussion**

#### **2.4.1 Insignificant effect of pitch-interval size on pitch perception**

In the present study, the size of the pitch interval did not considerably affect the performance of pitch perception in subjects of prelingually deafened children with a cochlear implant (Figure 1, Table 1). For the pitch perception of descending interval >5 semitones, however, the correct rate was lower than for that of descending interval ≤5 semitones. This finding was paradoxical since it's reasonable to infer that a larger pitch interval is easier to perceive correctly than a smaller one. It might imply a general intricacy in pitch perception of descending interval for cochlear implant users of all age, since scores of "falling" melodic contour perception was much lower than those of "rising" one (even lower than chance level) for adult cochlear implantees in one previous study (Galvin et al., 2007; McDermott, 2004).

Various factors have been reported to affect the pitch perception in implanted children. The insignificant effect of pitch-interval size on the differentiation tasks in the present study could be ascribed partly to the channel-setting of sound frequency and/or tone perception changes caused by cochlear implants (Nardo et al., 2007; Reiss et al., 2007).

Strategies to Improve Music Perception in Cochlear Implantees 67

Our finding was in line with a previous study, in which structured training was suggested to have positive correlation with recognition and appraisal of the timbre of musical instruments by postlingually deafened cochlear implant recipients (Gfeller et al., 2002). After twelve weeks of training, those implant recipients assigned to the training group showed significant improvement in timbre recognition and appraisal compared to the control group. The effect of training in music perception of prelingual cochlear implantees, however, was not addressed in the aforementioned study. As far as we know, our present research is the first study ever reporting such finding of enhanced music perception by musical training in

Mechanisms underlying the enhanced performance of pitch perception after musical training in those prelingually deafened children with cochlear implants remained unclear. One possibility is the modification of disorganized tonotopy through auditory plasticity in the central auditory pathway of our subjects. The reinstatement of afferent input via cochlear implantation could consequently launch a cascade of plastic changes in the auditory system. Such reorganization, probably coupled with essential changes in neurotransmission or neuromodulation, might assist in reducing further deterioration in the nervous system resulting from cessation of electrical input due to cochlear damage (Durham et al., 2000; Illing & Reisch, 2006). This might reverse the disrupted tonotopic maps toward a relatively "normal" organization (Guiraud et al., 2007), which in turn may lead to a better development of frequency tuning in the auditory cortices. In normal-hearing children, improved music perception via music education has been revealed by increased auditory evoked fields, possibly due to a greater number and/or synchronous activity of neurons (Pantev et al., 1998). With the intervention of musical training, it seemed that the modified organization of tonotopy in subjects of prelingually deafened children could also be further optimized for a more delicate resolution of frequency spectrum, as is indexed by a better

prelingually deafened children with cochlear implants.

performance of pitch perception in the present study.

**2.4.3 Effect of age and duration of cochlear implant use on pitch perception** 

In the present study, the performance of pitch perception is better in children with cochlear implants >6 y/o than those ≤6 y/o (Table 2). This might be due to the younger children not understanding the test itself. Actually, some of our older children appear to have longer training periods (Table 1). Our finding was in line with previous studies in which older children with cochlear implants tended to score higher on tonal-language performance (Huang et al*.*, 2005; Lee & van Hasselt, 2005). At least partly, this could also be attributed to the aforementioned influence of auditory plasticity. In an operational context, the generally longer duration of auditory rehabilitation and thus more cognitive experiences of acoustic stimulation lead to the enhanced skills for musical perception of our older children with longer duration of cochlear implant use (Table 4c~d). Nevertheless, the effect of musical training is much more significant for children ≤6 y/o than those >6 y/o (Table 4a~b). The seemingly gender effect observed in Table 2 might actually be due to the age effect, since the mean age of boys (6.9 y/o) was larger than that of girls (6.2 y/o), though the difference was not significant (p=0.404, t-test). Our finding thus verified that later pitch sensations in implanted children possibly reflected higher-level and/or experience-dependent plastic changes in the auditory pathway (Reiss et al., 2007), and that musical training in the

Obvious disparity could occur between frequencies assigned to electrodes and those actually perceived by cochlear recipients possibly related to the channel-setting of frequency during mapping (Nardo et al., 2007). After appropriate mapping, pitch perception via cochlear implants might still have great spectral variations for years, which can echo the extent of damage of peripheral innervations patterns in the early stage and plasticity-dependent modifications in the later stage of implant use (Reiss et al., 2007). In fact, effect of musical training was much more significant for pitch perception of ascending interval >5 semitones in children with duration of cochlear implant use >18 months. Our results showed that a duration ≤18 months of cochlear implant use might not be long enough for the plasticity-dependent adaptation of aforementioned disparity to happen (Table 4c~d).

Another possibility for better results with smaller intervals might be the use of loudnessinstead of pitch-cues for tone discrimination. It has been shown that a musical note at the center of a frequency band for one electrode may be louder than that at edge of the frequency band (Singh et al., 2009). Besides, a musical note at the edge of the band may activate two electrodes instead of one (Donaldson et al., 2005). The way these different musical intervals align with the frequency ranges allocated to each electrode (i.e., MAPs) potentially provide additional cues for tones discrimination. However, it has been revealed that electrode activation differences did not influence recognition performance with low- (104–262Hz) and middle-frequency (207–523Hz) melodies (Singh et al., 2009). Since the frequency range in our study lies between 256 and 495Hz, electrode activation differences did not seem to be a confounding factor in our study.

One more plausible explanation is the abnormal frequency-coding resolution resulting from the disorganization of tonotopic maps in the auditory cortices of those prelingually deafened children. Topographically arranged representations of frequency-tuning maps (i.e.,tonotopy) have been known to exist in the auditory system (Huffman & Cramer, 2007). The orderly maps of tonotopy start at the cochlea and continue through to the auditory cortex. Mechanisms underlying the development of tonotopic maps remained unknown. In previous studies, however, deprivation of auditory input due to cochlear ablation and/or misexpression of essential proteins in the auditory pathway in neonatal birds and mammals have been shown to affect the normal development of tonotopic maps (Harrison et al., 1998; Huffman & Cramer, 2007; Yu et al., 2007; Zhang et al., 2005). This might in turn lead to a diminished capacity of the auditory system to decode the acoustic information in terms of frequency resolution (Harrison et al., 1998; Huffman & Cramer, 2007; Yu et al., 2007), which could underpin our finding of the insignificant effect for pitch-interval size on the differentiation tasks.

#### **2.4.2 Musical training improving pitch perception**

One major and novel finding in this study is that the duration of musical training correlates with music perception in subjects of prelingually deafened children with a cochlear implant. That is, higher scores for the performance of pitch perception positively correlated with a longer duration of musical training in implanted children. Furthermore, the performance for the perception of ascending interval was significantly enhanced after the musical training (Table 3).

Obvious disparity could occur between frequencies assigned to electrodes and those actually perceived by cochlear recipients possibly related to the channel-setting of frequency during mapping (Nardo et al., 2007). After appropriate mapping, pitch perception via cochlear implants might still have great spectral variations for years, which can echo the extent of damage of peripheral innervations patterns in the early stage and plasticity-dependent modifications in the later stage of implant use (Reiss et al., 2007). In fact, effect of musical training was much more significant for pitch perception of ascending interval >5 semitones in children with duration of cochlear implant use >18 months. Our results showed that a duration ≤18 months of cochlear implant use might not be long enough for the plasticity-dependent adaptation of aforementioned disparity to

Another possibility for better results with smaller intervals might be the use of loudnessinstead of pitch-cues for tone discrimination. It has been shown that a musical note at the center of a frequency band for one electrode may be louder than that at edge of the frequency band (Singh et al., 2009). Besides, a musical note at the edge of the band may activate two electrodes instead of one (Donaldson et al., 2005). The way these different musical intervals align with the frequency ranges allocated to each electrode (i.e., MAPs) potentially provide additional cues for tones discrimination. However, it has been revealed that electrode activation differences did not influence recognition performance with low- (104–262Hz) and middle-frequency (207–523Hz) melodies (Singh et al., 2009). Since the frequency range in our study lies between 256 and 495Hz, electrode activation differences

One more plausible explanation is the abnormal frequency-coding resolution resulting from the disorganization of tonotopic maps in the auditory cortices of those prelingually deafened children. Topographically arranged representations of frequency-tuning maps (i.e.,tonotopy) have been known to exist in the auditory system (Huffman & Cramer, 2007). The orderly maps of tonotopy start at the cochlea and continue through to the auditory cortex. Mechanisms underlying the development of tonotopic maps remained unknown. In previous studies, however, deprivation of auditory input due to cochlear ablation and/or misexpression of essential proteins in the auditory pathway in neonatal birds and mammals have been shown to affect the normal development of tonotopic maps (Harrison et al., 1998; Huffman & Cramer, 2007; Yu et al., 2007; Zhang et al., 2005). This might in turn lead to a diminished capacity of the auditory system to decode the acoustic information in terms of frequency resolution (Harrison et al., 1998; Huffman & Cramer, 2007; Yu et al., 2007), which could underpin our finding of the insignificant effect for pitch-interval size on the

One major and novel finding in this study is that the duration of musical training correlates with music perception in subjects of prelingually deafened children with a cochlear implant. That is, higher scores for the performance of pitch perception positively correlated with a longer duration of musical training in implanted children. Furthermore, the performance for the perception of ascending interval was significantly enhanced after the musical training

happen (Table 4c~d).

differentiation tasks.

(Table 3).

did not seem to be a confounding factor in our study.

**2.4.2 Musical training improving pitch perception** 

Our finding was in line with a previous study, in which structured training was suggested to have positive correlation with recognition and appraisal of the timbre of musical instruments by postlingually deafened cochlear implant recipients (Gfeller et al., 2002). After twelve weeks of training, those implant recipients assigned to the training group showed significant improvement in timbre recognition and appraisal compared to the control group. The effect of training in music perception of prelingual cochlear implantees, however, was not addressed in the aforementioned study. As far as we know, our present research is the first study ever reporting such finding of enhanced music perception by musical training in prelingually deafened children with cochlear implants.

Mechanisms underlying the enhanced performance of pitch perception after musical training in those prelingually deafened children with cochlear implants remained unclear. One possibility is the modification of disorganized tonotopy through auditory plasticity in the central auditory pathway of our subjects. The reinstatement of afferent input via cochlear implantation could consequently launch a cascade of plastic changes in the auditory system. Such reorganization, probably coupled with essential changes in neurotransmission or neuromodulation, might assist in reducing further deterioration in the nervous system resulting from cessation of electrical input due to cochlear damage (Durham et al., 2000; Illing & Reisch, 2006). This might reverse the disrupted tonotopic maps toward a relatively "normal" organization (Guiraud et al., 2007), which in turn may lead to a better development of frequency tuning in the auditory cortices. In normal-hearing children, improved music perception via music education has been revealed by increased auditory evoked fields, possibly due to a greater number and/or synchronous activity of neurons (Pantev et al., 1998). With the intervention of musical training, it seemed that the modified organization of tonotopy in subjects of prelingually deafened children could also be further optimized for a more delicate resolution of frequency spectrum, as is indexed by a better performance of pitch perception in the present study.

#### **2.4.3 Effect of age and duration of cochlear implant use on pitch perception**

In the present study, the performance of pitch perception is better in children with cochlear implants >6 y/o than those ≤6 y/o (Table 2). This might be due to the younger children not understanding the test itself. Actually, some of our older children appear to have longer training periods (Table 1). Our finding was in line with previous studies in which older children with cochlear implants tended to score higher on tonal-language performance (Huang et al*.*, 2005; Lee & van Hasselt, 2005). At least partly, this could also be attributed to the aforementioned influence of auditory plasticity. In an operational context, the generally longer duration of auditory rehabilitation and thus more cognitive experiences of acoustic stimulation lead to the enhanced skills for musical perception of our older children with longer duration of cochlear implant use (Table 4c~d). Nevertheless, the effect of musical training is much more significant for children ≤6 y/o than those >6 y/o (Table 4a~b). The seemingly gender effect observed in Table 2 might actually be due to the age effect, since the mean age of boys (6.9 y/o) was larger than that of girls (6.2 y/o), though the difference was not significant (p=0.404, t-test). Our finding thus verified that later pitch sensations in implanted children possibly reflected higher-level and/or experience-dependent plastic changes in the auditory pathway (Reiss et al., 2007), and that musical training in the

Strategies to Improve Music Perception in Cochlear Implantees 69

There are many factors that can influence functional outcomes post-cochlear implantation including surgical techniques, variability of array placement, device coding strategies, intensity of rehabilitation and pathology of hearing loss (Wilson & Dorman, 2008a, 2008b). In addition to the variability of functional outcomes, music appreciation in cochlear implant recipients is also variable, presumably for similar reasons. While it is not possible to differentiate between all of these, technological developments of cochlear implants aim to maximize an individual's ability to reach their maximum potential. As cochlear implant candidacy is expanded with improvements in technology, individuals with increasing levels of residual low-frequency hearing (e.g those with steeply sloping severe-profound hearing loss) fall within the candidacy range (Gantz et al., 2006). In this population, where hearing is retained after surgery, combined electric and acoustic stimulation can be used which may provide access to finer spectral resolution and temporal fine structure, enhancing music perception (Gantz et al., 2005; Kong et al., 2004). Techniques aimed to preserve residual hearing include the insertion of a short electrode array (Gantz et al., 2006; Gfeller et al., 2005) or partial or full insertion of a standard electrode array combined with a soft surgery technique to minimize intracochlear trauma (Fraysse et al., 2006;

Short electrode arrays, including the research 10mm Iowa/Nucleus Hybrid-S Cochlear Implant, have been designed to facilitate electric and acoustic stimulation in individuals with residual hearing by only entering the descending cochlear basal turn (Gantz et al., 2005). Results reported as part of the multi-centre FDA clinical trial in 47 patients with the Nucleus Hybrid implant (Gantz et al., 2006) showed hearing preservation in 45 immediately after implantation, with hearing within 10dB of pre-operative thresholds maintained in 25 and within 30dB maintained in 22 for up to 3 years in some patients. Within this study, comparisons between long-term Hybrid-S users and long-term long array users who were matched on word understanding in quiet showed a difference in speech perception in noise (using both multi-talker babble and steady-state noise), suggesting that the Hybrid-S users perform better within a more realistic listening environment. Despite these benefits, Briggs and colleagues (Briggs et al., 2006) identified the possibility that shortening of the electrode array to 10mm may cause a place-frequency mismatch because only the basal portion of the cochlea will be stimulated, causing a disproportionately higher frequency percept than with a standard array. Further, should hearing not be preserved, then concerns have been raised that speech perception outcomes will be impaired for individuals who only receive electrical stimulation in such a limited region of the cochlea (Gstoettner et al., 2009). Nonetheless, in the clinical trial of the commercially-available 16mm Hybrid-L24, Lenarz et al. (Lenarz et al., 2009) showed good post-operative hearing preservation in 24 recipients implanted with a

A standard commercially available electrode array has also been used for hearing preservation with full or partial insertion of the array using an atraumatic surgical

**3. Part II: Other factors related to the improvement of music perception in** 

**3.1 Effect of residual hearing preservation on music perception in cochlear** 

**cochlear implantees** 

Gstoettner et al., 2004).

round-window surgical approach.

**implantees** 

sensitive period (≤6 y/o) would be beneficial for development of pitch sensations (Baharloo et al., 2000).

#### **2.4.4 Limits of this study**

While pitch ranking was assessed, testing intervals used in this research may be too small for the evaluation of real-world music appreciation. It has been reported that postlingual cochlear implantees were generally less accurate in identification of formerly well-known music pieces than normal-hearing subjects (Gfeller et al., 2005). Further study using larger intervals/musical extracts is thus necessary to see if improvement of pitch discrimination could result in a better music perception in prelingual cochlear implantees.

Though loudness was monitored to avoid the possible effect of intensity variation in this study, it is clear that loudness matching of different tones from a piano cannot be as precise as that of computerized sounds. Since musical training could improve loudness discrimination in normal-hearing subjects (Plath, 1968), the training might also improve pitch differentiation by advancing use of available loudness differences created unintentionally by cochlear implant programming. Future research using computerized tones with a more precise matching of loudness and analyzing how the results relate to MAPs will be helpful to separate tone discrimination from loudness differences.

#### **2.5 Conclusion**

In summary, the ability to discriminate sounds was improved with musical experience in prelingually deafened children with cochlear implants. Implanted children attending music classes revealed significant differences compared with those without musical training. We suggest that structured training on music perception should begin early in life and be included in the post-operative rehabilitation program for prelingually deafened children with cochlear implants. Since auditory plasticity might play an important role in the enhancement of pitch perception, our research invites further studies on a larger group of implanted children to correlate neuroelectrical changes over time from cochlear implantation and music performance. A longitudinal study is also needed to show whether such neuroelectrical responses change with improvement of music performance in prelingually deafened children with a cochlear implant.

#### **2.6 Acknowledgment**

This study was funded by Cheng Hsin General Hospital (9522, 9631, 9739) of Taiwan. We declare that we have no conflict of interest or financial relationships with this manuscript. Special thanks to Ms Meei-Ling Kuan, Wen-Chen Chiu, Meng-Ju Lien, and Hsiu-Wen Chang for audiological assistance.

#### **2.7 Annotations**


sensitive period (≤6 y/o) would be beneficial for development of pitch sensations (Baharloo

While pitch ranking was assessed, testing intervals used in this research may be too small for the evaluation of real-world music appreciation. It has been reported that postlingual cochlear implantees were generally less accurate in identification of formerly well-known music pieces than normal-hearing subjects (Gfeller et al., 2005). Further study using larger intervals/musical extracts is thus necessary to see if improvement of pitch discrimination

Though loudness was monitored to avoid the possible effect of intensity variation in this study, it is clear that loudness matching of different tones from a piano cannot be as precise as that of computerized sounds. Since musical training could improve loudness discrimination in normal-hearing subjects (Plath, 1968), the training might also improve pitch differentiation by advancing use of available loudness differences created unintentionally by cochlear implant programming. Future research using computerized tones with a more precise matching of loudness and analyzing how the results relate to

In summary, the ability to discriminate sounds was improved with musical experience in prelingually deafened children with cochlear implants. Implanted children attending music classes revealed significant differences compared with those without musical training. We suggest that structured training on music perception should begin early in life and be included in the post-operative rehabilitation program for prelingually deafened children with cochlear implants. Since auditory plasticity might play an important role in the enhancement of pitch perception, our research invites further studies on a larger group of implanted children to correlate neuroelectrical changes over time from cochlear implantation and music performance. A longitudinal study is also needed to show whether such neuroelectrical responses change with improvement of music performance in

This study was funded by Cheng Hsin General Hospital (9522, 9631, 9739) of Taiwan. We declare that we have no conflict of interest or financial relationships with this manuscript. Special thanks to Ms Meei-Ling Kuan, Wen-Chen Chiu, Meng-Ju Lien, and Hsiu-Wen

2. Section 2.2.3: Normal distribution of data was confirmed by using Kolmogorov-

3. Section 2.3.1: The power was 0.35 for the boys/girls comparison and 0.32 for the

1. Section 2.2.3: The type of ANOVA used was repeated measure ANOVA.

could result in a better music perception in prelingual cochlear implantees.

MAPs will be helpful to separate tone discrimination from loudness differences.

prelingually deafened children with a cochlear implant.

subjects >6 yr/subjects ≤6 yr comparison.

et al., 2000).

**2.5 Conclusion** 

**2.6 Acknowledgment** 

**2.7 Annotations** 

Smirnov test.

Chang for audiological assistance.

**2.4.4 Limits of this study** 

#### **3. Part II: Other factors related to the improvement of music perception in cochlear implantees**

#### **3.1 Effect of residual hearing preservation on music perception in cochlear implantees**

There are many factors that can influence functional outcomes post-cochlear implantation including surgical techniques, variability of array placement, device coding strategies, intensity of rehabilitation and pathology of hearing loss (Wilson & Dorman, 2008a, 2008b). In addition to the variability of functional outcomes, music appreciation in cochlear implant recipients is also variable, presumably for similar reasons. While it is not possible to differentiate between all of these, technological developments of cochlear implants aim to maximize an individual's ability to reach their maximum potential. As cochlear implant candidacy is expanded with improvements in technology, individuals with increasing levels of residual low-frequency hearing (e.g those with steeply sloping severe-profound hearing loss) fall within the candidacy range (Gantz et al., 2006). In this population, where hearing is retained after surgery, combined electric and acoustic stimulation can be used which may provide access to finer spectral resolution and temporal fine structure, enhancing music perception (Gantz et al., 2005; Kong et al., 2004). Techniques aimed to preserve residual hearing include the insertion of a short electrode array (Gantz et al., 2006; Gfeller et al., 2005) or partial or full insertion of a standard electrode array combined with a soft surgery technique to minimize intracochlear trauma (Fraysse et al., 2006; Gstoettner et al., 2004).

Short electrode arrays, including the research 10mm Iowa/Nucleus Hybrid-S Cochlear Implant, have been designed to facilitate electric and acoustic stimulation in individuals with residual hearing by only entering the descending cochlear basal turn (Gantz et al., 2005). Results reported as part of the multi-centre FDA clinical trial in 47 patients with the Nucleus Hybrid implant (Gantz et al., 2006) showed hearing preservation in 45 immediately after implantation, with hearing within 10dB of pre-operative thresholds maintained in 25 and within 30dB maintained in 22 for up to 3 years in some patients. Within this study, comparisons between long-term Hybrid-S users and long-term long array users who were matched on word understanding in quiet showed a difference in speech perception in noise (using both multi-talker babble and steady-state noise), suggesting that the Hybrid-S users perform better within a more realistic listening environment. Despite these benefits, Briggs and colleagues (Briggs et al., 2006) identified the possibility that shortening of the electrode array to 10mm may cause a place-frequency mismatch because only the basal portion of the cochlea will be stimulated, causing a disproportionately higher frequency percept than with a standard array. Further, should hearing not be preserved, then concerns have been raised that speech perception outcomes will be impaired for individuals who only receive electrical stimulation in such a limited region of the cochlea (Gstoettner et al., 2009). Nonetheless, in the clinical trial of the commercially-available 16mm Hybrid-L24, Lenarz et al. (Lenarz et al., 2009) showed good post-operative hearing preservation in 24 recipients implanted with a round-window surgical approach.

A standard commercially available electrode array has also been used for hearing preservation with full or partial insertion of the array using an atraumatic surgical

Strategies to Improve Music Perception in Cochlear Implantees 71

perception, bimodal hearing was also revealed to be superior to unimodal hearing for prelingually deafened children in one of our previous studies (Chen et al., in submission). Scores for pitch differentiation were generally higher for the condition of "simultaneous use of both hearing aid as well as cochlear implant" than that of "utilization of cochlear implant only" in the same subject, although the differences were not statistically significant enough which could possibly be ascribed to the small sample size. The performance of pitch-interval differentiation was furthermore shown to be superior in subjects with longer duration of hearing aids use and longer duration of hearing aids use

Our study was congruent with one recent research in which bimodal hearing was noted to be better than hearing with bilateral cochlear implantation regarding music perception in patients with post-lingual deafness (Cullington & Zeng, 2011). The mechanisms underlying the superior effect of bimodal hearing on music perception over unimodal hearing and hearing with bilateral cochlear implantation remained unknown. One possibility is that the low-frequency cues inherent in hearing aids can compensate for the insufficiency of lowfrequency cues built-in in the contemporary systems of cochlear implant in terms of pitch discrimination (Cullington & Zeng, 2011). Another more plausible explanation is that the auditory signals transmitted by hearing aids are analog in format (Chen et al., in submission). The acoustic information enclosed is thus much more abundant than that conveyed via the "digital" devices of cochlear implant, which in turn could sound more like

A usable high-frequency hearing gain by using hearing aids sometimes leads to a longer duration of hearing aids use prior to the cochlear implantation (Chen et al., in submission). The implanted ear will continue to benefit the implantees with a good high-frequency hearing gain even after the cochlear implantation. Since the neuronal architects serving auditory perception are hardwired to fine-tune to subtle differences in the auditory environment (Illing & Reisch, 2006), longer duration of hearing aids use will enable our subjects to become more familiar with the presented tone pairs, which would consecutively

In current commercially available cochlear implant systems, four main sound coding strategies are utilized (Wilson & Dorman, 2008a, 2008b). These are: (i) SPEAK (spectral peak strategy) (ii) CIS (continuous interleaved sampling); (iii) ACE (advanced combination encoder), which extracts both spectral and temporal cues; and (iv) *n of m* (number of maxima spectral speech extractor). However, it is proposed by some researchers that two main limitations affect music perception: (1) low-frequency fine structure information is poorly represented by envelope–based strategies; and (2) insufficient numbers of independent effective channels exist to deliver fine structure due to current spread and electrode interactions (conventional arrays have been 12-22 channels). More recently, considerable attention has been focused on the development of *novel* strategies to address this. These include the development of virtual channels through current steering (Firszt et al., 2007), and fine structure processing which intends to increase access to spectral and temporal fine

prior to the cochlear implantation.

that a normal-hearing subject would percept.

**3.3 Coding strategies** 

lead to a better capability of pitch-interval differentiation.

technique (Roland, 2005). Using a prospective multicenter study, Fraysse et al. (Fraysse et al., 2006) compared changes in hearing threshold levels after 27 patients were implanted with the Nucleus 24 Contour Advance perimodiolar electrode array. Of these, 12 were implanted with a soft surgery technique using a 17mm insertion depth. The authors demonstrated that preservation of hearing thresholds was more successful when the soft surgery technique was used with median changes in average hearing thresholds between 250-500Hz measured at 40dB for the entire group and 23dB for the soft surgery group. Success in hearing preservation has also been reported using partial insertion of other electrode arrays, including the MED-EL C40+ implant (Gstoettner et al., 2004; Skarzynski et al., 2007). However delayed loss of residual hearing has been reported in some instances even when an atraumatic surgical technique is used (Fraysse et al., 2006; Gstoettner et al*.*, 2006).

In contrast to the standard length electrode array, Skarzynski and Podskarbi-Fayette (Skarzynski & Podskarbi-Fayette, 2010) reported on the Nucleus® Straight Research Array (Cochlear Ltd), an atraumatic electrode array. The main characteristics of this array that are different from the usual straight or Contour Advance arrays are that it is thinner and smoother which aim to reduce intracochlear trauma and kinking of the proximal end during insertion with a 20mm insertion. This study showed that of nine patients who had lowfrequency residual hearing ≤50dBHL at 500Hz, the mean increase in thresholds at this frequency was 19dB. Similarly, Gstoettner and colleagues (Gstoettner et al., 2009) reported on the outcomes of 9 patients implanted with the MED-EL Flex EAS (with increased flexibility of the array) showing that 4 patients had full hearing preservation and 5 showed partial preservation. However, Baumgartner et al (Baumgartner et al., 2007) reported hearing preservation in 10 of 16 patients fitted with the MED-EL Flex*soft* at 1 month postimplantation but this declined to only 4 patients at 6 months post-implantation, suggesting variable outcomes which may or may not reflect the array *per se,* or the surgical technique or the underlying pathology or combination of the above.

To date, only limited evidence exists to support the possibility that any of these techniques result in improved music perception for implant recipients. Gfeller and colleagues (Gfeller, 2005, 2006) compared music perception in 17 normally-hearing adults, 39 with a conventional long array (from Cochlear Ltd, Advanced Bionics and Ineraid) and 4 patients with a Hybrid-S Cochlear Implant (Cochlear Ltd). The results showed that Hybrid-S recipients and NH listeners performed significantly better than those with a standard-electrode array on recognizing real-world songs with no lyrics and instrument recognition (with no significant difference observed with device or processing strategy for the standard-electrode array group). Nonetheless, it does indicate the possibility of combined electrical and acoustic stimulation for improved musical recognition in cochlear implant recipients.

#### **3.2 Effect of bimodal hearing and/or bilateral implantation on music perception in cochlear implantees**

It has long been known that bimodal hearing is better than unimodal hearing for patients with hearing impairment in terms of speech/language perception. With respect to music

technique (Roland, 2005). Using a prospective multicenter study, Fraysse et al. (Fraysse et al., 2006) compared changes in hearing threshold levels after 27 patients were implanted with the Nucleus 24 Contour Advance perimodiolar electrode array. Of these, 12 were implanted with a soft surgery technique using a 17mm insertion depth. The authors demonstrated that preservation of hearing thresholds was more successful when the soft surgery technique was used with median changes in average hearing thresholds between 250-500Hz measured at 40dB for the entire group and 23dB for the soft surgery group. Success in hearing preservation has also been reported using partial insertion of other electrode arrays, including the MED-EL C40+ implant (Gstoettner et al., 2004; Skarzynski et al., 2007). However delayed loss of residual hearing has been reported in some instances even when an atraumatic surgical technique is used (Fraysse et al., 2006;

In contrast to the standard length electrode array, Skarzynski and Podskarbi-Fayette (Skarzynski & Podskarbi-Fayette, 2010) reported on the Nucleus® Straight Research Array (Cochlear Ltd), an atraumatic electrode array. The main characteristics of this array that are different from the usual straight or Contour Advance arrays are that it is thinner and smoother which aim to reduce intracochlear trauma and kinking of the proximal end during insertion with a 20mm insertion. This study showed that of nine patients who had lowfrequency residual hearing ≤50dBHL at 500Hz, the mean increase in thresholds at this frequency was 19dB. Similarly, Gstoettner and colleagues (Gstoettner et al., 2009) reported on the outcomes of 9 patients implanted with the MED-EL Flex EAS (with increased flexibility of the array) showing that 4 patients had full hearing preservation and 5 showed partial preservation. However, Baumgartner et al (Baumgartner et al., 2007) reported hearing preservation in 10 of 16 patients fitted with the MED-EL Flex*soft* at 1 month postimplantation but this declined to only 4 patients at 6 months post-implantation, suggesting variable outcomes which may or may not reflect the array *per se,* or the surgical technique or

To date, only limited evidence exists to support the possibility that any of these techniques result in improved music perception for implant recipients. Gfeller and colleagues (Gfeller, 2005, 2006) compared music perception in 17 normally-hearing adults, 39 with a conventional long array (from Cochlear Ltd, Advanced Bionics and Ineraid) and 4 patients with a Hybrid-S Cochlear Implant (Cochlear Ltd). The results showed that Hybrid-S recipients and NH listeners performed significantly better than those with a standard-electrode array on recognizing real-world songs with no lyrics and instrument recognition (with no significant difference observed with device or processing strategy for the standard-electrode array group). Nonetheless, it does indicate the possibility of combined electrical and acoustic stimulation for improved musical recognition in cochlear

**3.2 Effect of bimodal hearing and/or bilateral implantation on music perception in** 

It has long been known that bimodal hearing is better than unimodal hearing for patients with hearing impairment in terms of speech/language perception. With respect to music

Gstoettner et al*.*, 2006).

implant recipients.

**cochlear implantees** 

the underlying pathology or combination of the above.

perception, bimodal hearing was also revealed to be superior to unimodal hearing for prelingually deafened children in one of our previous studies (Chen et al., in submission). Scores for pitch differentiation were generally higher for the condition of "simultaneous use of both hearing aid as well as cochlear implant" than that of "utilization of cochlear implant only" in the same subject, although the differences were not statistically significant enough which could possibly be ascribed to the small sample size. The performance of pitch-interval differentiation was furthermore shown to be superior in subjects with longer duration of hearing aids use and longer duration of hearing aids use prior to the cochlear implantation.

Our study was congruent with one recent research in which bimodal hearing was noted to be better than hearing with bilateral cochlear implantation regarding music perception in patients with post-lingual deafness (Cullington & Zeng, 2011). The mechanisms underlying the superior effect of bimodal hearing on music perception over unimodal hearing and hearing with bilateral cochlear implantation remained unknown. One possibility is that the low-frequency cues inherent in hearing aids can compensate for the insufficiency of lowfrequency cues built-in in the contemporary systems of cochlear implant in terms of pitch discrimination (Cullington & Zeng, 2011). Another more plausible explanation is that the auditory signals transmitted by hearing aids are analog in format (Chen et al., in submission). The acoustic information enclosed is thus much more abundant than that conveyed via the "digital" devices of cochlear implant, which in turn could sound more like that a normal-hearing subject would percept.

A usable high-frequency hearing gain by using hearing aids sometimes leads to a longer duration of hearing aids use prior to the cochlear implantation (Chen et al., in submission). The implanted ear will continue to benefit the implantees with a good high-frequency hearing gain even after the cochlear implantation. Since the neuronal architects serving auditory perception are hardwired to fine-tune to subtle differences in the auditory environment (Illing & Reisch, 2006), longer duration of hearing aids use will enable our subjects to become more familiar with the presented tone pairs, which would consecutively lead to a better capability of pitch-interval differentiation.

#### **3.3 Coding strategies**

In current commercially available cochlear implant systems, four main sound coding strategies are utilized (Wilson & Dorman, 2008a, 2008b). These are: (i) SPEAK (spectral peak strategy) (ii) CIS (continuous interleaved sampling); (iii) ACE (advanced combination encoder), which extracts both spectral and temporal cues; and (iv) *n of m* (number of maxima spectral speech extractor). However, it is proposed by some researchers that two main limitations affect music perception: (1) low-frequency fine structure information is poorly represented by envelope–based strategies; and (2) insufficient numbers of independent effective channels exist to deliver fine structure due to current spread and electrode interactions (conventional arrays have been 12-22 channels). More recently, considerable attention has been focused on the development of *novel* strategies to address this. These include the development of virtual channels through current steering (Firszt et al., 2007), and fine structure processing which intends to increase access to spectral and temporal fine

Strategies to Improve Music Perception in Cochlear Implantees 73

Gfeller, K., Witt, S., Adamek, M., Mehr, M., Rogers, J., Stordahl, J., & Ringgenberg, S. (2002).

Guiraud, J., Besle, J., Arnold, L., Boyle, P., Giard, M.H., Bertrand, O., Norena, A., Truy, E., &

Harrison, R.V., Ibrahim, D., & Mount, R.J. (1998). Plasticity of tonotopic maps in auditory

Huang, C.Y., Yang, H.M., Sher, Y.J., Lin, Y.H., & Wu, J.L. (2005) Speech intelligibility of

Huffman, K.J. & Cramer, K.S. (2007). EphA4 misexpression alters tonotopic projections in the auditory brainstem. *Developmental Neurobiology*, Vol. 67, pp. 1655-1668 Illing, R.B. & Reisch, A. (2006). Specific plasticity responses to unilaterally decreased or

Koelsch, S., Wittfoth, M., Wolf, A., Muller, J., & Hahne, A. (2004). Music perception in

Lee, K.Y. & van Hasselt, C.A. (2005). Spoken word recognition in children with cochlear

Li, L.P, Shiao, A.S., Chen, L.F., Niddam, D.M., Chang, S.Y., Lien, C.F., Lee, S.K., & Hsieh, J.C.

Li, L.P, Shiao, A.S., Lin, Y.Y., Chen, L.F., Niddam, D.M., Chang, S.Y., Lien, C.F., Chou, N.S.,

responses in sudden hearing loss. *Annals of Neurology*, Vol. 53, pp. 810-815 McDermott, H.J. (2004). Music perception with cochlear implants: a review. *Trends in* 

Nardo, W.D., Cantore, I., Cianfrone, F., Melillo, P., Fetoni, A.R., & Paludetti, G. (2007).

Pantev, C., Dinnesen, A., Ross, B., Wollbrink, A., & Knief, A. (2006). Dynamics of auditory

Pantev, C., Oostenveld, R., Engelien, A., Ross, B., Roberts, L.E., & Hoke, M. (1998). Increased auditory cortical representation in musicians. *Nature*, Vol. 392, pp. 811-814

pitch perception. *Acta Otolaryngology*, Vol. 127, pp. 370-377

cochlear implant users. *Journal of Neuroscience*, Vol. 27, pp. 7838-7846 Hamzavi, J. & Arnoldner, C. (2006). Effect of deep insertion of the cochlear implant electrode

*Experimental Brain Research*, Vol. 123, pp. 449-460

*Pediatric Otorhinolaryngology*, Vol. 69, pp. 505-511

*Research*, Vol. 216-217, pp. 189-197

Vol. 115, pp. 966-972

26, pp. 30S-37S

Vol. 24, pp. 937-946

pp. 31-36

*Amplification*, Vol. 8, pp. 49-82

pp. 132-145

1182-1187

Effects of training on timbre recognition and appraisal by postlingually deafened cochlear implant recipients. *Journal of the American Academy of Audiology*, Vol. 13,

Collet, L. (2007). Evidence of a tonotopic organization of the auditory cortex in

array on pitch estimation and speech perception. *Acta Otolaryngolagy*, Vol. 126, pp.

midbrain following partial cochlear damage in the developing chinchilla.

Mandarin-speaking deaf children with cochlear implants. *International Journal of* 

increased hearing intensity in the adult cochlear nucleus and beyond. *Hearing* 

cochlear implant users: an event-related potential study. *Clinical Neurophysiology*,

implants: a five-year study on speakers of a tonal language. *Ear and Hearing*, Vol.

(2006). Healthy-side dominance of middle- and long-latency neuromagnetic fields in idiopathic sudden sensorineural hearing loss. *European Journal of Neuroscience*,

Ho, L.T., & Hsieh, J.C. (2003). Healthy-side dominance of cortical neuromagnetic

Differences between electrode-assigned frequencies and cochlear implant recipient

plasticity after cochlear implantation: a longitudinal study. *Cerebral Cortex*, Vol. 16,

structure (Hochmair et al., 2006). While such strategies are continually being improved to facilitate improved music perception and appreciation, limited empirical evidence currently exists to support the role of virtual channels or fine structure coding at this stage (Berenstein et al., 2008; Firszt et al., 2007). Nonetheless, they continue to represent possibilities for the future.

#### **3.4 Conclusion**

In summary, only limited evidence exists to support the possibility that factors such as residual hearing, bimodal hearing, and coding strategies result in improved music perception for implant recipients to date. However, they continue to represent opportunities for the future. The importance of techniques aimed to preserve residual hearing thus cannot be overemphasized in cochlear implantation. Further studies are also needed to show the longitudinal effect of bimodal hearing and newly developed coding strategies to benefit music performance in cochlear implantees.

#### **4. References**

#### **4.1 References - Part I**


structure (Hochmair et al., 2006). While such strategies are continually being improved to facilitate improved music perception and appreciation, limited empirical evidence currently exists to support the role of virtual channels or fine structure coding at this stage (Berenstein et al., 2008; Firszt et al., 2007). Nonetheless, they continue to represent possibilities for the

In summary, only limited evidence exists to support the possibility that factors such as residual hearing, bimodal hearing, and coding strategies result in improved music perception for implant recipients to date. However, they continue to represent opportunities for the future. The importance of techniques aimed to preserve residual hearing thus cannot be overemphasized in cochlear implantation. Further studies are also needed to show the longitudinal effect of bimodal hearing and newly developed coding strategies to benefit

Baharloo, S., Service, S.K., Risch, N., Gitschier, J., & Freimer, N.B. (2000). Familial

Busby, P.A. & Plant, K.L. (2005). Dual electrode stimulation using the nucleus CI24RE

Chen, J.K., Chuang, A.Y., McMahon, C., Hsieh, J.C., Tung, T.H., & Li, L.P. (2010). Music

Donaldson, G.S., Kreft, H.A., & Litvak, L. (2005). Place-pitch discrimination of single- versus

Durham, D., Park, D.L., & Girod, D.A. (2000). Central nervous system plasticity during hair

Firszt, J.B., Koch, D.B., Downing, M., & Litvak, L. (2007). Current steering creates additional

Galvin, J.J., 3rd, Fu, Q.J., & Nogaki, G. (2007). Melodic contour identification by cochlear

Gfeller, K. & Lansing, C.R. (1991). Melodic, rhythmic, and timbral perception of adult

Gfeller, K., Olszewski, C., Rychener, M., Sena, K., Knutson, J.F., Witt, S., & Macpherson, B.

cell loss and regeneration. *Hearing Research*, Vol. 147, pp. 145-159

and normal-hearing adults. *Ear and Hearing*, Vol. 26, pp. 237-250

aggregation of absolute pitch. *American Journal of Human Genetics*, Vol. 67, pp. 755-

cochlear implant: electrode impedance and pitch ranking studies. *Ear and Hearing*,

training improves pitch perception in prelingually deafened children with cochlear

dual-electrode stimuli by cochlear implant users (L). *Journal of Acoustical Society of* 

pitch percepts in adult cochlear implant recipients. *Otology Neurotology*, Vol. 28, pp.

(2005). Recognition of "real-world" musical excerpts by cochlear implant recipients

future.

**3.4 Conclusion** 

**4. References** 

**4.1 References - Part I** 

758

629-636

music performance in cochlear implantees.

Vol. 26, pp. 504-511

implants. *Pediatrics*, Vol. 125, pp. e793-800

implant listeners. *Ear and Hearing*, Vol. 28, 302-319

cochlear implant users. *J Speech Hear Res*, 34, 916-920

*America*, Vol. 118, pp. 623-626


Strategies to Improve Music Perception in Cochlear Implantees 75

Gantz, B.J., Turner, C., & Gfeller, K.E. (2006). Acoustic plus electric speech processing:

Gantz, B.J., Turner, C., Gfeller, K.E., & Lowder, M.W. (2005). Preservation of hearing in

Gfeller, K., Olszewski, C., Rychener, M., Sena, K., Knutson, J.F., Witt, S., & Macpherson, B.

Gfeller, K.E., Olszewski, C., Turner, C., Gantz, B., & Oleson, J. (2006). Music perception with

Gstoettner, W., Helbig, S., Settevendemie, C., Baumann, U., Wagenblast, J., & Arnoldner, C.

Gstoettner, W., Kiefer, J., Baumgartner, W.D., Pok, S., Peters, S., & Adunka, O. (2004).

Gstoettner, W.K., Helbig, S., Maier, N., Kiefer, J., Radeloff, A. & Adunka, O.F. (2006).

Illing, R.B. & Reisch, A. (2006). Specific plasticity responses to unilaterally decreased or

Kong, Y.Y., Cruz, R., Jones, J.A., & Zeng, F.G. (2004). Music perception with temporal cues in acoustic and electric hearing. *Ear and Hearing*, Vol. 25, pp. 173-185 Lenarz, T., Stover, T., Buechner, A., Lesinski-Schiedat, A., Patrick, J., & Pesch, J. (2009).

Roland, J.T., Jr. (2005). A model for cochlear implant electrode insertion and force

Skarzynski, H., Lorens, A., Piotrowska, A., & Anderson, I. (2007). Preservation of low

Skarzynski, H. & Podskarbi-Fayette, R. (2010). A new cochlear implant electrode design for

window surgical approach. *Acta Otolaryngology*, Vol. 127, pp. 41-48

hearing preservation. *Audiology Neurootology*, Vol. 11 Suppl 1, pp. 49-56 Hochmair, I., Nopp, P., Jolly, C., Schmidt, M., Schosser, H., Garnham, C., & Anderson, I.

advance cochlear implant. *Otology Neurotology*, Vol. 27, pp. 624-633

implant. *Audiology Neurootology*, Vol. 11 Suppl 1, pp. 63-68

and normal-hearing adults. *Ear and Hearing*, Vol. 26, pp. 237-250

first clinical results. *Acta Otolaryngology*, Vol. 129, pp. 372-379

processing. *Laryngoscope*, Vol. 115, pp. 796-802

*Otolaryngology*, Vol. 124, pp. 348-352

*Trends in Amplification*, Vol. 10, pp. 201-219

*Research*, Vol. 216-217, pp. 189-197

*Laryngoscope*, Vol. 115, pp. 1325-1339.

Suppl 1, pp. 22-31

130, pp. 435-442

12-15

hearing conservation and electroacoustic stimulation with the nucleus 24 contour

preliminary results of a multicenter clinical trial of the Iowa/Nucleus Hybrid

cochlear implant surgery: advantages of combined electrical and acoustical speech

(2005). Recognition of "real-world" musical excerpts by cochlear implant recipients

cochlear implants and residual hearing. *Audiology Neurootology*, Vol. 11 Suppl 1, pp.

(2009). A new electrode for residual hearing preservation in cochlear implantation:

Hearing preservation in cochlear implantation for electric acoustic stimulation. *Acta* 

Ipsilateral electric acoustic stimulation of the auditory system: results of long-term

(2006). MED-EL Cochlear implants: state of the art and a glimpse into the future.

increased hearing intensity in the adult cochlear nucleus and beyond. *Hearing* 

Hearing conservation surgery using the Hybrid-L electrode. Results from the first clinical trial at the Medical University of Hannover. *Audiology Neurootology*, Vol. 14

evaluation: results with a new electrode design and insertion technique.

frequency hearing in partial deafness cochlear implantation (PDCI) using the round

preservation of residual hearing: a temporal bone study. *Acta Otolaryngology*, Vol.


#### **4.2 References - Part II**


Plath, P. (1968). [Influence of training effect on the loudness differentiation-threshold in

Reiss, L.A., Turner, C.W., Erenberg, S.R., & Gantz, B.J. (2007) Changes in pitch with a

Singh, S., Kong, Y.Y., & Zeng, F.G. (2009). Cochlear Implant Melody Recognition as a

Sucher, C.M. & McDermott, H.J. (2007). Pitch ranking of complex tones by normally

Yu, X., Sanes, D.H., Aristizabal, O., Wadghiri, Y.Z., & Turnbull, D.H. (2007). Large-scale

Zhang, Y., Dyck, R.H., Hamilton, S.E., Nathanson, N.M., & Yan, J. (2005). Disrupted

Baumgartner, W.D., Jappel, A., Morera, C., Gstottner, W., Muller, J., Kiefer, J., Van De

Chen, J.K., Chuang, A.Y., McMahon, C., Hsieh, J.C., Tung, T.H., & Li, L.P. Concomittant

Cullington, H.E., & Zeng, F.G. (2011). Comparison of bimodal and bilateral cochlear

Firszt, J.B., Koch, D.B., Downing, M., & Litvak, L. (2007). Current steering creates additional

Fraysse, B., Macias, A.R., Sterkers, O., Burdo, S., Ramsden, R., Deguine, O., Klenzner, T.,

with the FLEXsoft electrode. *Acta Otolaryngology*, Vol. 127, pp. 579-586 Berenstein, C.K., Mens, L.H., Mulder, J.J., & Vanpoucke, F.J. (2008). Current steering and

*Kehlkopfheilkunde*, Vol. 190, pp. 286-290

*and Hearing*, Vol. 30, pp. 160-168

receptor. *Hearing Research*, Vol. 201, pp. 145-155

pp. 241-257

87

12198

**4.2 References - Part II** 

Suppl 1, pp. 42-48.

32, pp. 16-30

629-636

cochlear implant. In submission.

normal hearing persons]. *Archiv fur Klinische und Experimentelle Ohren- Nasen- und* 

cochlear implant over time. *Journal of Associated Research in Otolaryngology*, Vol. 8,

Function of Melody Frequency Range, Harmonicity, and Number of Electrodes. *Ear* 

hearing subjects and cochlear implant users. *Hearing Research*, Vol. 230, pp. 80-

reorganization of the tonotopic map in mouse auditory midbrain revealed by MRI. *Proceedings of the National Academy of Sciences USA*, Vol. 104, pp. 12193-

tonotopy of the auditory cortex in mice lacking M1 muscarinic acetylcholine

Heyning, P., Anderson, I., & Nielsen, S.B. (2007). Outcomes in adults implanted

current focusing in cochlear implants: comparison of monopolar, tripolar, and virtual channel electrode configurations. *Ear and Hearing*, Vol. 29, pp. 250-260 Briggs, R.J., Tykocinski, M., Xu, J., Risi, F., Svehla, M., Cowan, R., Stover, T., Erfurt, P., &

Lenarz, T. (2006). Comparison of round window and cochleostomy approaches with a prototype hearing preservation electrode. *Audiology Neurootology*, Vol. 11

hearing aids use improving pitch perception in prelingually deafened children with

implant users on speech recognition with competing talker, music perception, affective prosody discrimination, and talker identification. *Ear and Hearing*, Vol.

pitch percepts in adult cochlear implant recipients. *Otology Neurotology*, Vol. 28,

Lenarz, T., Rodriguez, M.M., Von Wallenberg, E., & James, C. (2006). Residual

hearing conservation and electroacoustic stimulation with the nucleus 24 contour advance cochlear implant. *Otology Neurotology*, Vol. 27, pp. 624-633


**5** 

**A Review of Stimulating Strategies** 

Many animals use sound to communicate with each other, and hearing is particularly important for survival and reproduction. In species that use sound as a primary means of communication, their hearing is typically most acute for the range of pitches produced in calls and speech. Human is one such species and Fig. 1 shows a human ear consisting of the outer, middle and inner ear. The eardrum of an ear converts incoming acoustic pressure waves through the middle ear to the inner ear. In the inner ear the distribution of vibrations along the length of the basilar membrane is detected by hair cells. The location and intensity of these vibrations are transmitted to the brain by the auditory nerves. If the hair cells are damaged (as shown in Fig. 2(b)), the auditory system is unable to convert acoustic pressure waves to neural impulses, which results in hearing impairment. Damaged hair cells can subsequently lead to the degeneration of adjacent auditory neurons. If a large number of hair cells or auditory neurons are damaged or missing, the condition is called *profound* 

Fig. 1. A diagram of the anatomy of the human ear. (Chittka, L. & Brockmann, A. (2005))

**1. Introduction** 

*hearing impairment* (Yost, 2000).

**for Cochlear Implants** 

*1National Chiao Tung University,* 

*Taiwan, R.O.C.* 

Charles T. M. Choi1 and Yi-Hsuan Lee2

*2National Taichung University of Education,* 


## **A Review of Stimulating Strategies for Cochlear Implants**

Charles T. M. Choi1 and Yi-Hsuan Lee2 *1National Chiao Tung University, 2National Taichung University of Education,* 

*Taiwan, R.O.C.* 

#### **1. Introduction**

76 Cochlear Implant Research Updates

Wilson, B.S. & Dorman, M.F. (2008a). Cochlear implants: a remarkable past and a brilliant

Wilson, B.S. & Dorman, M.F. (2008b) Cochlear implants: current designs and future possibilities. *Journal of Rehabilitation Research and Development*, Vol. 45, pp. 695-730

future. *Hearing Research*, Vol. 242, pp. 3-21.

Many animals use sound to communicate with each other, and hearing is particularly important for survival and reproduction. In species that use sound as a primary means of communication, their hearing is typically most acute for the range of pitches produced in calls and speech. Human is one such species and Fig. 1 shows a human ear consisting of the outer, middle and inner ear. The eardrum of an ear converts incoming acoustic pressure waves through the middle ear to the inner ear. In the inner ear the distribution of vibrations along the length of the basilar membrane is detected by hair cells. The location and intensity of these vibrations are transmitted to the brain by the auditory nerves. If the hair cells are damaged (as shown in Fig. 2(b)), the auditory system is unable to convert acoustic pressure waves to neural impulses, which results in hearing impairment. Damaged hair cells can subsequently lead to the degeneration of adjacent auditory neurons. If a large number of hair cells or auditory neurons are damaged or missing, the condition is called *profound hearing impairment* (Yost, 2000).

Fig. 1. A diagram of the anatomy of the human ear. (Chittka, L. & Brockmann, A. (2005))

A Review of Stimulating Strategies for Cochlear Implants 79

2004). One cause of these problems is that the spectral resolution perceived by CI user is not good enough. If a novel stimulating strategy is developed to increase spectral resolution, these problems can to some extent be relieved. In this literature review we first introduce basic stimulating strategies used in commercial cochlear implant systems. We then discuss a new *hybrid stimulating strategy*, and some experimental results of normal hearing tests are

In the cochlear implant system, the stimulating strategy plays an extremely important role in generating the sounds heard by users (Wilson et al., 1991; Kiefer et al., 2001; Koch et al., 2004; Wilson & Dorman, 2008). It functions to convert sounds into a series of electric impulses which determines which electrodes should be activated in each cycle. A complete

3. The number of consecutive clock cycles required to deliver selected channels,; and

Many stimulating strategies have been developed over the past two decades. An ideal stimulating strategy is one that closely reproduces the original sound spectrum and allows a CI user to hear clear sounds. In the following we briefly describe and compare the *advanced combinational encoder* (*ACE*), *continuous interleaved sampling* (*CIS*), and *HiRes*120 strategies,

The ACE strategy (Fig. 4), used in the Nucleus implant, is based on a so-called N of M principle. This system uses 22 implanted electrodes which can be activated to generate 22 fixed channels. The signal is processed into 22 frequency bands for each frame of recorded sound. After the envelope information for every frequency band is extracted, 8–10 (set by the audiologist) frequency bands with the largest amplitudes will be stimulated. Electrodes corresponding to the selected channels are then activated. Thus in the ACE strategy, a channel is generated by one implanted electrode, and the original spectrum is reproduced

CIS is a strategy used in the speech processors of all major cochlear implant manufacturers. For Advanced Bionics implants, which have 16 implanted electrodes, a diagram of the strategy is shown in Fig. 5. For each frame of sound, the signal is applied through 16 band-pass filters, and the envelopes of these frequency bands are extracted by full-wave rectifying and low-pass filtering (with 200–400Hz cutoff frequency). Unlike ACE, all 16 frequency bands are then stimulated in sequence. Trains of balanced biphasic pulses modulated with extracted signal envelopes are delivered to each electrode at a constant rate in a non-overlapping sequence. The stimulation rate of each channel is relatively high, and the overlap across channels can also be

presented to compare the performance achieved by different stimulating strategies.

1. The number of channels selected to reproduce the original spectrum;

2. The number of electrodes activated to generate each channel;

which are frequently used in today's commercial cochlear implants.

**2.1.1 Advanced Combinational Encoder (ACE) (Kiefer et al., 2001)** 

**2.1.2 Continuous Interleaved Sampling (CIS) (Wilson et al., 1991)** 

4. The scheduling of the activating sequence of electrodes.

**2. Stimulating strategy review** 

stimulating strategy should address the following:

**2.1 Stimulating strategy using fixed channel** 

by 8–10 fixed channels.

Fig. 2. (a) Normal human ear; (b) Profound hearing impairment. (Loizou, P.C. (1999))

*Cochlear implants* (*CI*) have been commercially available for nearly thirty years. Today cochlear implants still provide the only opportunity for people with profound hearing impairment to recover partial hearing through electrical stimulation of the auditory nerves (Loizou, 1998; Loizou, 1999; Spelman 1999; Wilson & Dorman, 2008). Fig. 3 is a diagram of a cochlear implant. The external part consists of microphones, a speech processor and a transmitter. Internally an array of up to 22 electrodes is inserted through the cochlea, and a receiver is secured to the bone beneath the skin. The microphones pick up sounds and the speech processor converts the sounds into electrical signals based on a stimulating strategy. The electrical signals, which determine the sequence in which the electrodes are activated, are then converted into electric impulses and sent to the implanted electrodes by the transmitter (Girzon, 1987; Suesserman & Spelman, 1993). The simulating strategy plays an extremely important role in maximizing a user's overall communicative potential.

Fig. 3. A cochlear implant.

Until recently the main thrust in cochlear implant research has been to improve the hearing ability of CI users in a quiet environment (Dorman & Loizou, 1997; Loizou et al., 1999). There have been numerous improvements in the current generation of cochlear prosthesis, including the development of completely implantable cochlear implants. However, today CI users still have difficulty in listening to music and tonal languages, and in hearing in a noisy environment (Fu et al., 1998; Friesen et al., 2001; Xu et al., 2002; Kong et al., 2004; Lan et al., 2004). One cause of these problems is that the spectral resolution perceived by CI user is not good enough. If a novel stimulating strategy is developed to increase spectral resolution, these problems can to some extent be relieved. In this literature review we first introduce basic stimulating strategies used in commercial cochlear implant systems. We then discuss a new *hybrid stimulating strategy*, and some experimental results of normal hearing tests are presented to compare the performance achieved by different stimulating strategies.

#### **2. Stimulating strategy review**

78 Cochlear Implant Research Updates

(a) (b)

Fig. 2. (a) Normal human ear; (b) Profound hearing impairment. (Loizou, P.C. (1999))

extremely important role in maximizing a user's overall communicative potential.

Until recently the main thrust in cochlear implant research has been to improve the hearing ability of CI users in a quiet environment (Dorman & Loizou, 1997; Loizou et al., 1999). There have been numerous improvements in the current generation of cochlear prosthesis, including the development of completely implantable cochlear implants. However, today CI users still have difficulty in listening to music and tonal languages, and in hearing in a noisy environment (Fu et al., 1998; Friesen et al., 2001; Xu et al., 2002; Kong et al., 2004; Lan et al.,

Fig. 3. A cochlear implant.

*Cochlear implants* (*CI*) have been commercially available for nearly thirty years. Today cochlear implants still provide the only opportunity for people with profound hearing impairment to recover partial hearing through electrical stimulation of the auditory nerves (Loizou, 1998; Loizou, 1999; Spelman 1999; Wilson & Dorman, 2008). Fig. 3 is a diagram of a cochlear implant. The external part consists of microphones, a speech processor and a transmitter. Internally an array of up to 22 electrodes is inserted through the cochlea, and a receiver is secured to the bone beneath the skin. The microphones pick up sounds and the speech processor converts the sounds into electrical signals based on a stimulating strategy. The electrical signals, which determine the sequence in which the electrodes are activated, are then converted into electric impulses and sent to the implanted electrodes by the transmitter (Girzon, 1987; Suesserman & Spelman, 1993). The simulating strategy plays an In the cochlear implant system, the stimulating strategy plays an extremely important role in generating the sounds heard by users (Wilson et al., 1991; Kiefer et al., 2001; Koch et al., 2004; Wilson & Dorman, 2008). It functions to convert sounds into a series of electric impulses which determines which electrodes should be activated in each cycle. A complete stimulating strategy should address the following:


Many stimulating strategies have been developed over the past two decades. An ideal stimulating strategy is one that closely reproduces the original sound spectrum and allows a CI user to hear clear sounds. In the following we briefly describe and compare the *advanced combinational encoder* (*ACE*), *continuous interleaved sampling* (*CIS*), and *HiRes*120 strategies, which are frequently used in today's commercial cochlear implants.

#### **2.1 Stimulating strategy using fixed channel**

#### **2.1.1 Advanced Combinational Encoder (ACE) (Kiefer et al., 2001)**

The ACE strategy (Fig. 4), used in the Nucleus implant, is based on a so-called N of M principle. This system uses 22 implanted electrodes which can be activated to generate 22 fixed channels. The signal is processed into 22 frequency bands for each frame of recorded sound. After the envelope information for every frequency band is extracted, 8–10 (set by the audiologist) frequency bands with the largest amplitudes will be stimulated. Electrodes corresponding to the selected channels are then activated. Thus in the ACE strategy, a channel is generated by one implanted electrode, and the original spectrum is reproduced by 8–10 fixed channels.

#### **2.1.2 Continuous Interleaved Sampling (CIS) (Wilson et al., 1991)**

CIS is a strategy used in the speech processors of all major cochlear implant manufacturers. For Advanced Bionics implants, which have 16 implanted electrodes, a diagram of the strategy is shown in Fig. 5. For each frame of sound, the signal is applied through 16 band-pass filters, and the envelopes of these frequency bands are extracted by full-wave rectifying and low-pass filtering (with 200–400Hz cutoff frequency). Unlike ACE, all 16 frequency bands are then stimulated in sequence. Trains of balanced biphasic pulses modulated with extracted signal envelopes are delivered to each electrode at a constant rate in a non-overlapping sequence. The stimulation rate of each channel is relatively high, and the overlap across channels can also be

A Review of Stimulating Strategies for Cochlear Implants 81

As described above, both ACE and CIS strategies only use fixed channels to reproduce the original sound spectrum. There are, however, around 30,000 auditory nerve fibers in a human ear, but only 16–22 electrodes can currently be implanted into a CI user's ear to generate 16–22 fixed channels. Due to the limitation of the electrode design, the electrode has limited stimulation selectivity. Thus, these electrodes can only excite a small number of specific auditory nerve fibers, thus restricts the resolution and information received by a CI

One possible way to achieve better spectral resolution is by increasing the number of electrodes. If, however, the number of implanted electrodes is limited and fixed, an alternative is to use the *virtual channel* technique (Donaldson et al., 2005; Koch et al., 2007). This technique uses *current steering* to control the electrical interaction. When two (or more) neighboring electrodes are stimulated in a suitable manner, intermediated channels, also known as virtual channels, are created between the electrodes. These virtual channels can enable CI users to perceive different frequencies between two fixed channels (Koch et al., 2004; Choi & Hsu, 2009). Using the virtual channel technique not only allows for more stimulating space, but also improves the reproduction of the original spectrum. This is Illustrated by the example in Fig. 6. Fig. 6(a) shows a sample original spectrum, and Figs. 6(b) and (c) show spectrums generated using fixed and virtual channel techniques, respectively. There appears to be much similarity between Fig. 6(a) and Fig. 6(c), but Fig. 6(b) is distorted from the original. Therefore, in order to better reproduce the original spectrum and increase the perceptual quality of CI users, it would be beneficial to apply the

virtual channel technique to the stimulating strategies of cochlear implants.

Fig. 6. A comparison of spectrums generated using different stimulating strategies. (a) A sample original spectrum; (b) A spectrum generated using fixed channels; (c) A spectrum

**2.2 Stimulating strategy using virtual channel** 

user is thus restricted.

**2.2.1 Virtual channel technique** 

generated using virtual channels.

eliminated. So, in the CIS strategy a channel is still generated by one implanted electrode. The original spectrum is reproduced by 16 fixed channels, and all electrodes are turned on in a predefined sequence within 16 consecutive clock cycles.

Fig. 4. A block diagram of the ACE stimulating strategy.

Fig. 5. A block diagram of the CIS stimulating strategy.

#### **2.2 Stimulating strategy using virtual channel**

As described above, both ACE and CIS strategies only use fixed channels to reproduce the original sound spectrum. There are, however, around 30,000 auditory nerve fibers in a human ear, but only 16–22 electrodes can currently be implanted into a CI user's ear to generate 16–22 fixed channels. Due to the limitation of the electrode design, the electrode has limited stimulation selectivity. Thus, these electrodes can only excite a small number of specific auditory nerve fibers, thus restricts the resolution and information received by a CI user is thus restricted.

#### **2.2.1 Virtual channel technique**

80 Cochlear Implant Research Updates

eliminated. So, in the CIS strategy a channel is still generated by one implanted electrode. The original spectrum is reproduced by 16 fixed channels, and all electrodes are turned on in a

> Envelope detection

> Envelope detection

> > ‧ ‧ ‧

‧ amplitude

predefined sequence within 16 consecutive clock cycles.

BPF1

BPF2

Amplifier

‧ ‧

Fig. 4. A block diagram of the ACE stimulating strategy.

BPF*<sup>n</sup>*

‧ ‧ ‧

BPF Rectifier LPF Nonlinear

Envelope detection

BPF Rectifier LPF Nonlinear

BPF Rectifier LPF Nonlinear

mapping

Select bands with largest

mapping

‧ ‧ ‧

mapping

El-1

Mapping

El-2

El-*n*

Fig. 5. A block diagram of the CIS stimulating strategy.

One possible way to achieve better spectral resolution is by increasing the number of electrodes. If, however, the number of implanted electrodes is limited and fixed, an alternative is to use the *virtual channel* technique (Donaldson et al., 2005; Koch et al., 2007). This technique uses *current steering* to control the electrical interaction. When two (or more) neighboring electrodes are stimulated in a suitable manner, intermediated channels, also known as virtual channels, are created between the electrodes. These virtual channels can enable CI users to perceive different frequencies between two fixed channels (Koch et al., 2004; Choi & Hsu, 2009). Using the virtual channel technique not only allows for more stimulating space, but also improves the reproduction of the original spectrum. This is Illustrated by the example in Fig. 6. Fig. 6(a) shows a sample original spectrum, and Figs. 6(b) and (c) show spectrums generated using fixed and virtual channel techniques, respectively. There appears to be much similarity between Fig. 6(a) and Fig. 6(c), but Fig. 6(b) is distorted from the original. Therefore, in order to better reproduce the original spectrum and increase the perceptual quality of CI users, it would be beneficial to apply the virtual channel technique to the stimulating strategies of cochlear implants.

Fig. 6. A comparison of spectrums generated using different stimulating strategies. (a) A sample original spectrum; (b) A spectrum generated using fixed channels; (c) A spectrum generated using virtual channels.

A Review of Stimulating Strategies for Cochlear Implants 83

CI users with the HiRes120 strategy devices usually have better hearing performance compared to those with the CIS strategy devices (Koch et al., 2004; Wilson & Dorman, 2008), indicating that applying virtual channel technique does improve the perceptual quality of CI users. However, since the HiRes120 strategy only adjusts the current level ratio of two neighboring electrodes, its spectral resolution is actually not high enough due to the relatively wider stimulation region of the immediate channels. For a channel with a wider stimulation region, more auditory nerve fibers are excited. This increases the difficulty of CI users to discriminate between different channels, and limits the total number of immediate channels that can be generated. In the HiRes120 strategy only seven virtual channels are

If more adjacent electrodes are used to steer the current, it will narrow the stimulation region to focus on firing specific auditory nerve fibers. *Four-electrode current steering schemes* (*FECSS*) is a current steering technique developed to control four adjacent electrodes simultaneously (Choi & Hsu, reviewing; Choi & Hsu, 2009). As shown in Fig. 8, the locus of stimulation is focused between the middle electrode pair, and the stimulation region is apparently narrower. This indicates that applying FECSS to stimulating strategies can help the CI user hear sounds with more specific frequencies, thus improving the perceptual

**3.1 Four-Electrode Current Steering Schemes (FECSS) (Choi & Hsu, reviewing)** 

**3. Hybrid stimulating strategy** 

generated between two electrodes.

quality and number of discriminable virtual channels.

(a) (b)

**3.2 Hybrid stimulating strategy (Choi et al., reviewing)** 

Fig. 8. Current steering technique. (a) Virtual channels generated using two adjacent electrodes; (b) Virtual channels generated using four adjacent electrodes (FECSS).

Although FECSS has the potential to achieve better hearing performance for CI users, it is primarily an algorithm to control the electrical current spread spatially and does not consider the activating sequence of the electrodes. Furthermore all current commercial stimulating strategies are highly inflexible, because the number of electrodes used to

#### **2.2.2 HiRes120 (Koch et al., 2004)**

To apply the virtual channel technique to a cochlear implant system, each electrode must have an independent power source to allow the current to be delivered simultaneously to more than one electrode. Theoretically, with a fine control over the current level ratio of neighboring electrodes, the locus of stimulation is steered between electrodes to create virtual channels. The HiRes120 strategy, used in the Advanced Bionics implant, is the first commercial stimulating strategy that uses the virtual channel technique. Virtual channels are created by adjusting the current level ratio of two neighboring electrodes. Since the Advanced Bionics implant has 16 implanted electrodes, there are 15 electrode pairs that can be used to steer the focus of the electrical stimulation. Fig. 7 is a diagrammatic representation of the HiRes120 strategy. For each frame of sound, the signal is divided by 15 band-pass filters and the envelope is extracted for every frequency band. In addition, 15 spectral peaks, which indicate the most important frequency within each frequency band, are also derived using the *Fast Fourier Transform* (*FFT*). These spectral peaks are then steered by corresponding electrode pairs based on the virtual channel technique. The HiRes120 strategy also delivers channels in sequence with a high stimulation rate similar to that of the CIS. Thus, in the HiRes120 strategy a channel is generated by two neighboring electrodes. The original spectrum is reproduced by 15 virtual channels, and all electrode pairs are turned on in a predefined sequence within 15 consecutive clock cycles.

Fig. 7. A block diagram of the HiRes120 stimulating strategy.

#### **3. Hybrid stimulating strategy**

82 Cochlear Implant Research Updates

To apply the virtual channel technique to a cochlear implant system, each electrode must have an independent power source to allow the current to be delivered simultaneously to more than one electrode. Theoretically, with a fine control over the current level ratio of neighboring electrodes, the locus of stimulation is steered between electrodes to create virtual channels. The HiRes120 strategy, used in the Advanced Bionics implant, is the first commercial stimulating strategy that uses the virtual channel technique. Virtual channels are created by adjusting the current level ratio of two neighboring electrodes. Since the Advanced Bionics implant has 16 implanted electrodes, there are 15 electrode pairs that can be used to steer the focus of the electrical stimulation. Fig. 7 is a diagrammatic representation of the HiRes120 strategy. For each frame of sound, the signal is divided by 15 band-pass filters and the envelope is extracted for every frequency band. In addition, 15 spectral peaks, which indicate the most important frequency within each frequency band, are also derived using the *Fast Fourier Transform* (*FFT*). These spectral peaks are then steered by corresponding electrode pairs based on the virtual channel technique. The HiRes120 strategy also delivers channels in sequence with a high stimulation rate similar to that of the CIS. Thus, in the HiRes120 strategy a channel is generated by two neighboring electrodes. The original spectrum is reproduced by 15 virtual channels, and all electrode pairs are

> Carrier synthesis

> > ‧ ‧ ‧

Carrier synthesis Mapping

El-1, El-2

El-*n*, El-(*n*+1)

Navigator

Mapping

Navigator

turned on in a predefined sequence within 15 consecutive clock cycles.

AGC

Hilbert envelope

Spectral peak locator

Hilbert envelope

Band1

Band*<sup>n</sup>*

Fig. 7. A block diagram of the HiRes120 stimulating strategy.

Spectral peak locator

**2.2.2 HiRes120 (Koch et al., 2004)** 

#### **3.1 Four-Electrode Current Steering Schemes (FECSS) (Choi & Hsu, reviewing)**

CI users with the HiRes120 strategy devices usually have better hearing performance compared to those with the CIS strategy devices (Koch et al., 2004; Wilson & Dorman, 2008), indicating that applying virtual channel technique does improve the perceptual quality of CI users. However, since the HiRes120 strategy only adjusts the current level ratio of two neighboring electrodes, its spectral resolution is actually not high enough due to the relatively wider stimulation region of the immediate channels. For a channel with a wider stimulation region, more auditory nerve fibers are excited. This increases the difficulty of CI users to discriminate between different channels, and limits the total number of immediate channels that can be generated. In the HiRes120 strategy only seven virtual channels are generated between two electrodes.

If more adjacent electrodes are used to steer the current, it will narrow the stimulation region to focus on firing specific auditory nerve fibers. *Four-electrode current steering schemes* (*FECSS*) is a current steering technique developed to control four adjacent electrodes simultaneously (Choi & Hsu, reviewing; Choi & Hsu, 2009). As shown in Fig. 8, the locus of stimulation is focused between the middle electrode pair, and the stimulation region is apparently narrower. This indicates that applying FECSS to stimulating strategies can help the CI user hear sounds with more specific frequencies, thus improving the perceptual quality and number of discriminable virtual channels.

Fig. 8. Current steering technique. (a) Virtual channels generated using two adjacent electrodes; (b) Virtual channels generated using four adjacent electrodes (FECSS).

#### **3.2 Hybrid stimulating strategy (Choi et al., reviewing)**

Although FECSS has the potential to achieve better hearing performance for CI users, it is primarily an algorithm to control the electrical current spread spatially and does not consider the activating sequence of the electrodes. Furthermore all current commercial stimulating strategies are highly inflexible, because the number of electrodes used to

A Review of Stimulating Strategies for Cochlear Implants 85

Fig. 9 shows a flowchart of the hybrid stimulating strategy. For each frame of sound, the signal is divided by *m* band-pass filters. After the envelope information for every frequency band is extracted, *n* (*n m*) spectral peaks are derived using the FFT. A combination of TECSS and FECSS is used to duplicate these *n* spectral peaks within *k* clock cycles. Each selected spectral peak is generated by at most *u* adjacent electrodes (two or four electrodes) and is scheduled to be generated within 8 to 15 clock cycles, without causing temporal and spatial interactions. Notice that not all *n* spectral peaks will be selected at a time. Table 1 lists the characteristics of stimulating strategies including hybrid, HiRes120, CIS, and ACE.

**3.3 Hybrid stimulating strategy with psychoacoustic model (Choi et al., reviewing)** 

Fastl, 2008).

Fig. 10. An audio masking graph.

Fig. 11. A block diagram of the psychoacoustic model.

Hearing is not a purely mechanical wave propagation phenomenon, but also a sensory and perceptual event. In the phenomenon called *masking*, as shown on Fig. 10, a weaker sound is masked if it is made inaudible in the presence of a louder sound (Hellman, 1972; Zwicher &

The psychoacoustic model is a computation model developed to detect the less perceptually important components of audio signals. It has been successfully used in the field of audio coding in order to reduce bandwidth requirements. Authors of the hybrid stimulating strategy also incorporate their strategy with a psycho-acoustic model (Zwicher & Fastl, 2008), and the implementation steps are shown in Fig. 11. After incorporating

generate a channel and the number of channels delivered in every clock cycle are both fixed, which makes it difficult to closely reproduce the original sound spectrum.

In (Choi et al., reviewing) a flexible *hybrid stimulating strategy* is proposed to overcome the limitations mentioned above. This strategy utilizes a combination of the two-electrode current steering scheme (TECSS) and FECSS to reproduce the original sound spectrum. In FECSS it has been shown that it is possible to generate a sharper spectral peak by using 4 electrode stimulation (Choi & Hsu, reviewing). Hence, in the hybrid stimulating strategy algorithm, TECSS and FECSS are used to generate wider and narrower spectral peaks, respectively. The entire spectrum is delivered within eight to fifteen clock cycles, and a number of spectral peaks are delivered in each clock cycle.

Fig. 9. A flowchart of the hybrid stimulating strategy.


Table 1. The characteristics of the stimulating strategy of cochlear implant system including ACE, CIS, HiRes120, and hybrid.

generate a channel and the number of channels delivered in every clock cycle are both fixed,

In (Choi et al., reviewing) a flexible *hybrid stimulating strategy* is proposed to overcome the limitations mentioned above. This strategy utilizes a combination of the two-electrode current steering scheme (TECSS) and FECSS to reproduce the original sound spectrum. In FECSS it has been shown that it is possible to generate a sharper spectral peak by using 4 electrode stimulation (Choi & Hsu, reviewing). Hence, in the hybrid stimulating strategy algorithm, TECSS and FECSS are used to generate wider and narrower spectral peaks, respectively. The entire spectrum is delivered within eight to fifteen clock cycles, and a

Select spectral peaks

Select an undelivered spectral peak with maximum energy

Find suitable electrodes and clock cycle to deliver the selected spectral

All spectral peaks are considered? N Y

Hybrid HiRes120 CIS ACE

Advanced

Bionics Cochlear

No No

(adjustable)

(adjustable)

Bionics

Yes (120 channels)

peaks <sup>~</sup>*n* (adaptive) 15 16 8~10 (fixed)

(adaptive) 2 1 1

adaptive 1 1 1

(adjustable) 15 16 8~10 (fixed)

which makes it difficult to closely reproduce the original sound spectrum.

number of spectral peaks are delivered in each clock cycle.

Fig. 9. A flowchart of the hybrid stimulating strategy.

No. of spectral

No. of electrodes to generate a channel

No. of spectral peaks per clock cycle

No. of clock cycle *<sup>k</sup>* (fixed)

Virtual channel technique

ACE, CIS, HiRes120, and hybrid.

To reproduce a signal frame

Manufacture \*\*\* Advanced

No. of implanted electrodes *m* 16 16 22

Yes (>300 channels)

at most *u*

Table 1. The characteristics of the stimulating strategy of cochlear implant system including

Fig. 9 shows a flowchart of the hybrid stimulating strategy. For each frame of sound, the signal is divided by *m* band-pass filters. After the envelope information for every frequency band is extracted, *n* (*n m*) spectral peaks are derived using the FFT. A combination of TECSS and FECSS is used to duplicate these *n* spectral peaks within *k* clock cycles. Each selected spectral peak is generated by at most *u* adjacent electrodes (two or four electrodes) and is scheduled to be generated within 8 to 15 clock cycles, without causing temporal and spatial interactions. Notice that not all *n* spectral peaks will be selected at a time. Table 1 lists the characteristics of stimulating strategies including hybrid, HiRes120, CIS, and ACE.

#### **3.3 Hybrid stimulating strategy with psychoacoustic model (Choi et al., reviewing)**

Hearing is not a purely mechanical wave propagation phenomenon, but also a sensory and perceptual event. In the phenomenon called *masking*, as shown on Fig. 10, a weaker sound is masked if it is made inaudible in the presence of a louder sound (Hellman, 1972; Zwicher & Fastl, 2008).

Fig. 10. An audio masking graph.

The psychoacoustic model is a computation model developed to detect the less perceptually important components of audio signals. It has been successfully used in the field of audio coding in order to reduce bandwidth requirements. Authors of the hybrid stimulating strategy also incorporate their strategy with a psycho-acoustic model (Zwicher & Fastl, 2008), and the implementation steps are shown in Fig. 11. After incorporating

Fig. 11. A block diagram of the psychoacoustic model.

A Review of Stimulating Strategies for Cochlear Implants 87

**4.3 Performance comparison of different stimulating strategies (Choi et al., reviewing)**  Fig. 13 shows the results of the normal hearing tests for different stimulating strategies. The mean recognition % and standard deviation are both presented in Fig. 13, and the higher recognition % indicates more Chinese sentences can be correctly recognized. In general, the

The recognition % achieved in different SNRs is as follows. An SNR of -5 dB is considered a relatively noisy environment. The hybrid strategy achieved a recognition % of 50–70% at -5 dB SNR, a performance approaching that of people with normal hearing. The HiRes120 and CIS only achieved recognition % of 40–50% and < 20%, respectively. These results indicate that in a noisy environment the hybrid strategy has noticeable advantages compared to the HiRes120 and CIS. With an SNR of 0 dB, the recognition % of the hybrid strategy was 80%– 85%, compared to 70%–85% for HiRes120 and <50% for CIS. With an SNR of 5 dB, a relative quiet environment, both hybrid and HiRes120 strategies had a recognition % of >85%. The

CIS strategy also improved the recognition % to >50% when SNR was 5 dB.

(a) (b)

HiRes120, and hybrid. (a) Multi-talker babble; (b) White noise.

between them.

**model (Choi et al., reviewing)** 

Fig. 13. Results of the normal hearing tests: a comparison among stimulating strategies CIS,

Two-way ANOVA and Post Hoc tests were used to further analyze the results obtained as shown in Fig. 13. ANOVA indicated a significant main effect for SNR and the strategies. The Post Hoc test indicated that there was always a statistically significant difference between the hybrid and the CIS. Between the hybrid and the HiRes120, statistically significant differences only existed at an SNR of -5 dB. When SNR was equal to 5 dB, the hybrid performed similarly to the HiRes120 and no statistically significant difference existed

**4.4 Performance comparison of hybrid strategy with and without psychoacoustic** 

Fig. 14 shows the results of the normal hearing tests for the hybrid strategy with and without the psychoacoustic model, showing that the recognition % before and after incorporating the psychoacoustic model are almost the same. ANOVA also indicated a significant main effect for SNR, but no statistically significant difference existed between the hybrid strategy with and without the psychoacoustic model. These results indicate that

hybrid strategy showed a better performance.

the psychoacoustic model, the number of activated electrodes is reduced compared to basic hybrid strategy, but more power is saved and the hearing performance of CI users is retained.

Fig. 12. A block diagram of the acoustic cochlear implant model.

#### **4. Experimental results**

#### **4.1 Acoustic cochlear implant model**

When a new stimulating strategy for cochlear implants is developed, it is impractical to apply it directly to a speech processor for testing by CI users. Researchers normally implement the strategy in an acoustic cochlear implant model (also called a vocoder), to simulate the sounds heard by CI users for conducting normal hearing tests with normal hearing subjects first.

As shown in Fig. 12, an acoustic cochlear implant model was implemented using LabVIEW (Choi et al., 2008), which contained two main paths. The *spectrum processing* path was used to derive spectral peaks using FFT, and the temporal envelope information for every frequency band was extracted in the *level processing* path. Both white noise and pure tones were used as carriers to synthesize the sounds as heard by CI users. The stimulating strategies implemented included the CIS, HiRes120, and hybrid, with and without a psychoacoustic model.

#### **4.2 Subjects and materials**

Normal hearing tests were conducted to evaluate the performance of a hybrid stimulating strategy (Choi et al., reviewing). Chinese sentences were used as test material (Tsai & Chen, 2002), and subjects were asked to listen to each sentence and recognize the final word. All sentences were mixed by multi-talker babble and white noise in 0, 5, or -5 dB SNR (signal-to-noise ratio). All normal hearing tests were conducted in a quiet room. The test subjects were 25 adults between 25 and 30 years old. SENNHEISER HD-380 PRO headphones were used.

the psychoacoustic model, the number of activated electrodes is reduced compared to basic hybrid strategy, but more power is saved and the hearing performance of CI users is

When a new stimulating strategy for cochlear implants is developed, it is impractical to apply it directly to a speech processor for testing by CI users. Researchers normally implement the strategy in an acoustic cochlear implant model (also called a vocoder), to simulate the sounds heard by CI users for conducting normal hearing tests with normal

As shown in Fig. 12, an acoustic cochlear implant model was implemented using LabVIEW (Choi et al., 2008), which contained two main paths. The *spectrum processing* path was used to derive spectral peaks using FFT, and the temporal envelope information for every frequency band was extracted in the *level processing* path. Both white noise and pure tones were used as carriers to synthesize the sounds as heard by CI users. The stimulating strategies implemented included the CIS, HiRes120, and hybrid, with and without a

Normal hearing tests were conducted to evaluate the performance of a hybrid stimulating strategy (Choi et al., reviewing). Chinese sentences were used as test material (Tsai & Chen, 2002), and subjects were asked to listen to each sentence and recognize the final word. All sentences were mixed by multi-talker babble and white noise in 0, 5, or -5 dB SNR (signal-to-noise ratio). All normal hearing tests were conducted in a quiet room. The test subjects were 25 adults between 25 and 30 years old. SENNHEISER HD-380 PRO

Fig. 12. A block diagram of the acoustic cochlear implant model.

**4. Experimental results** 

hearing subjects first.

psychoacoustic model.

headphones were used.

**4.2 Subjects and materials** 

**4.1 Acoustic cochlear implant model** 

retained.

#### **4.3 Performance comparison of different stimulating strategies (Choi et al., reviewing)**

Fig. 13 shows the results of the normal hearing tests for different stimulating strategies. The mean recognition % and standard deviation are both presented in Fig. 13, and the higher recognition % indicates more Chinese sentences can be correctly recognized. In general, the hybrid strategy showed a better performance.

The recognition % achieved in different SNRs is as follows. An SNR of -5 dB is considered a relatively noisy environment. The hybrid strategy achieved a recognition % of 50–70% at -5 dB SNR, a performance approaching that of people with normal hearing. The HiRes120 and CIS only achieved recognition % of 40–50% and < 20%, respectively. These results indicate that in a noisy environment the hybrid strategy has noticeable advantages compared to the HiRes120 and CIS. With an SNR of 0 dB, the recognition % of the hybrid strategy was 80%– 85%, compared to 70%–85% for HiRes120 and <50% for CIS. With an SNR of 5 dB, a relative quiet environment, both hybrid and HiRes120 strategies had a recognition % of >85%. The CIS strategy also improved the recognition % to >50% when SNR was 5 dB.

Fig. 13. Results of the normal hearing tests: a comparison among stimulating strategies CIS, HiRes120, and hybrid. (a) Multi-talker babble; (b) White noise.

Two-way ANOVA and Post Hoc tests were used to further analyze the results obtained as shown in Fig. 13. ANOVA indicated a significant main effect for SNR and the strategies. The Post Hoc test indicated that there was always a statistically significant difference between the hybrid and the CIS. Between the hybrid and the HiRes120, statistically significant differences only existed at an SNR of -5 dB. When SNR was equal to 5 dB, the hybrid performed similarly to the HiRes120 and no statistically significant difference existed between them.

#### **4.4 Performance comparison of hybrid strategy with and without psychoacoustic model (Choi et al., reviewing)**

Fig. 14 shows the results of the normal hearing tests for the hybrid strategy with and without the psychoacoustic model, showing that the recognition % before and after incorporating the psychoacoustic model are almost the same. ANOVA also indicated a significant main effect for SNR, but no statistically significant difference existed between the hybrid strategy with and without the psychoacoustic model. These results indicate that

A Review of Stimulating Strategies for Cochlear Implants 89

Choi, C. T. M.; Tsai, W. Y. & Lee, Y. H. (reviewing). A Novel Cochlear Implant Stimulating

Donaldson, G. S.; Kreft, H. A. & Litvak, L. (2005). Place-Pitch Eiscrimination of Single-

Friesen, L. M.; Shannon, R. V.; Baskent, D. & Wang, X. (2001). Speech Recognition in Noise

Fu, Q. J.; Shannon, R., V. & Wang, X. (1998). Effect of Noise and Number of Channels on Vowel

Kiefer, J.; Hohl, S.; Sturzebecher, E.; Pfennigdorff, T. & Gstoettner, W. (2001). Comparison of

Koch, D. B.; Downing, M.; Osberger, M. J. & Litvak, L. (2007). Using Current Steering to

Koch, D. B.; Osberger, M. J.; Segal, P. & Kessler, D. (2004). HiResolution and Conventional

Kong, Y. Y.; Cruz, R.; Jones, J. A. & Zeng, F. G. (2004). Music Perception with Temporal Cues

Lan, N.; Nie, K. B.; Gao, S. K. & Zeng, F. G. (2004). A Novel Speech-Processing Strategy

Loizou, P. C. (1998). Mimicking the Human Ear, *IEEE Signal Processing Magazine*, Vol.15,

Loizou, P. C.; Dorman, M. and Tu, Z. (1999). On the Number of Channels Needed to

*America*, Vol.104, No.6, (December 1998), pp. 3586-3596, ISSN 0001-4966 Girzon, G. (1987). Investigation of Current Flow in the Inner Ear during Electrical Stimulation of Intracochlear Electrodes, MS Thesis in EE&CS, MIT, Cambridge, Massachusetts Hellman, R. P. (1972). Asymmetry of Masking between Noise and Tone, *Attention,* 

No.1, (January/February 2001), pp. 32-42, ISSN 0020-6091

No.4, (July/August 2004), pp. 241-223, ISSN 1420-3030

No.5, (September 1998), pp. 101-130, ISSN 1053-5888

1999), pp. 2097-2103, ISSN 0001-4966

No.2, (April 2007), pp. 38S-41S, ISSN 0196-0202

173-185, ISSN 0196-0202

*of America*, Vol.118, No.22, (August 2005), pp. 623-626, ISSN 0001-4966 Dorman, M. F. & and Loizou, P. C. (1997). Speech Intelligibility as a Function of the Number

Model, reviewing in *Journal of Acoustic Society of America*

1997), ppS113-S114, ISSN 0196-0709

2011), pp. 1150-1163, ISSN 0001-4966

Strategy Incorporating a Hybrid Current Steering Scheme and a Psychoacoustic

versus Dual-Electrode Stimuli by Cochlear Implant Users, *Journal of Acoustic Society* 

of Channels of Stimulation for Normal-Hearing Listeners and Patients with Cochlear Implants, *American Journal of Otolaryngology*, Vol.18, No.6, (December

as a Function of the Number of Spectral Channels: Comparison of Acoustic hearing and Cochlear Implants, *Journal of Acoustic Society of America*, Vol.110, No.2, (August

and Consonant Recognition: Acoustic and Electric Hearing, *Journal of Acoustic Society of* 

*Perception, and Psychophysics*, Vol.11, No.3, (May 1972), pp. 241-246, ISSN 1943-3921

Speech Recognition with Different Speech Coding Strategies (SPEAK, CIS, and ACE) and Their Relationship to Telemetric Measures of Compound Action Potentials in the Nucleus CI 24M Cochlear Implant System, *Audiology*, Vol. 40,

Increase Spectral Resolution in CII and HiRes 90K Users, *Ear and Hearing*, Vol.28,

Sound Processing in the HiResolution Bionic Ear: Using Appropriate Outcome Measures to Assess Speech Recognition Ability, *Audiology and Neurotology*, Vol.9,

in Acoustic and Electric Hearing, *Ear and Hearing*, Vol.25, No.2, (April 2004), pp.

Incorporating Tonal Information for Cochlear Implants, *IEEE Transactions on Biomedical Engineering*, Vol.51, No.5, (May 2004), pp. 752-760, ISSN 0018-9294 Loizou, P.C. (1999). Introduction to Cochlear Implant, *IEEE Engineering in Medical and* 

*Biology Magazine*, Vol.18, No.1, (January/February 1999), pp. 32-42, ISSN 0739-5175

Understand Speech, *Journal of Acoustic Society of America*, Vol.106, No.4, (October

incorporating the hybrid stimulating strategy with the psychoacoustic model is a feasible concept. The number of activated electrodes is reduced for power saving, and the hearing performance can be successfully retained.

Fig. 14. Results of normal the hearing tests: the comparison between hybrid strategy with and without a psychoacoustic model. (a) Multi-talker babble; (b) White noise.

#### **5. Conclusions**

In this chapter we considered the most challenging problems currently facing CI research and demonstrated the importance of the stimulating strategy in cochlear implant systems. Some basic stimulating strategies used in commercial systems were reviewed, and a new hybrid stimulating strategy based on the virtual channel technique was introduced. The hybrid strategy can activate implanted electrodes in a more flexible way to reproduce the original sound spectrum. The results from the normal hearing experiments show the hybrid stimulating strategy achieves a better hearing performance when compared with the results from commercial stimulating strategies. The hybrid strategy can also be incorporated with a psychoacoustic model for power saving and load reduction on the stimulating cycles, without compromising the hearing performance. We therefore believe that developing a new stimulating strategy is a possible alternative to improving the hearing ability of CI users.

#### **6. References**


incorporating the hybrid stimulating strategy with the psychoacoustic model is a feasible concept. The number of activated electrodes is reduced for power saving, and the hearing

(a) (b)

and without a psychoacoustic model. (a) Multi-talker babble; (b) White noise.

Fig. 14. Results of normal the hearing tests: the comparison between hybrid strategy with

In this chapter we considered the most challenging problems currently facing CI research and demonstrated the importance of the stimulating strategy in cochlear implant systems. Some basic stimulating strategies used in commercial systems were reviewed, and a new hybrid stimulating strategy based on the virtual channel technique was introduced. The hybrid strategy can activate implanted electrodes in a more flexible way to reproduce the original sound spectrum. The results from the normal hearing experiments show the hybrid stimulating strategy achieves a better hearing performance when compared with the results from commercial stimulating strategies. The hybrid strategy can also be incorporated with a psychoacoustic model for power saving and load reduction on the stimulating cycles, without compromising the hearing performance. We therefore believe that developing a new stimulating strategy is a possible alternative to improving the hearing ability of CI users.

Chittka, L. & Brockmann, A. (2005). Perception Space – The Final Frontier, *PLoS Biology*, Vol.3, No.4, (April 2005), pp. e137 doi:10.1371/journal.pbio.0030137 Choi, C. T. M. & Hsu, C. H. (2009). Conditions for Generating Virtual Channels in Cochlear

Choi, C. T. M.; Hsu, C. H.; Tsai, W. Y. & Lee, Y. H. (2008). A Vocoder for a Novel Cochlear

13*th ICBME*, ISBN 978-3-540-92840-9, Singapore, December 3-6, 2008 Choi, C. T. M. & Hsu, C. H. (reviewing). Novel Current Steering Schemes for Cochlear Prosthesis Systems, reviewing in *IEEE Transactions on Biomedical Engineering*

Prosthesis Systems, *Annals of Biomedical Engineering*, Vol.37, No.3, (March 2009), pp.

Implant Stimulating Strategy based on Virtual Channel Technology, *Proceedings of* 

performance can be successfully retained.

**5. Conclusions** 

**6. References** 

614-624, ISSN 0090-6964


**6** 

**A Fine Structure Stimulation Strategy** 

The auditory system provides a natural frequency-to-place mapping which is designated as *tonotopic organisation* of the cochlea. In the normal-hearing system, the acoustic signal causes a fluid pressure wave which propagates into the cochlea. At particular positions within the cochlea, most of the energy of the wave is absorbed causing mechanical oscillations of the basilar membrane. The oscillations are transduced into electrical signals (action potentials) in neurons by the action of the inner hair cells. Waves caused by low input frequencies travel further into the cochlea than those caused by high frequencies. Thus, each position of the basilar membrane can be associated with a particular frequency of the input signal (Greenwood, 1990). This natural form of frequency-to-place mapping, together with the fact that the positioning and fixation of an intrascalar electrode array is comparatively simple, is likely one of the most important factors for the success of cochlear implants as compared to

In the late 1980s, Wilson and colleagues introduced a coding strategy for cochlear implants designated as "Continuous Interleaved Sampling" (CIS) strategy (Wilson et al., 1991). Supporting significantly better speech perception in comparison to all other coding strategies at the time, CIS became and still is the de-facto standard among CI coding strategy. CIS signal processing involves splitting up of the audio frequency range into spectral bands by means of a filter bank, envelope detection of each filter output signal, and

According to the tonotopic principle of the cochlea, each stimulation electrode in the scala tympani is associated with a band pass filter of the external filter bank. High-frequency bands are associated with electrodes positioned more closely to the base, and low-frequency bands to electrodes positioned more deeply in the direction of the apex. For stimulation, charge-balanced current pulses - usually biphasic symmetrical pulses - are applied. The amplitudes of the stimulation pulses are directly derived from the compressed envelope signals. These signals are sampled sequentially, and, as the characteristic CIS paradigm, the stimulation pulses are applied in a strictly non-overlapping way in time. Typically, the pulse

**1. Introduction** 

other sensory neural prostheses.

**1.1 The "Continuous Interleaved Sampling" stimulation strategy** 

instantaneous nonlinear compression of the envelope signals (map law).

sampling rate per channel is within the range of 0.8-1.5 kpulses/sec.

**and Related Concepts** 

*University of Innsbruck* 

 *Austria* 

Clemens Zierhofer and Reinhold Schatzer *C. Doppler Laboratory for Active Implantable Systems* 

