**6. Conclusions**

42 Hearing Loss

Fig. 7. DPOAE suppression frequency response curves (Weibull regressions) for f2 values between 3.1 and 7.7 kHz, plotted on a normalized suppression scale (data from figs 4 and 6).

This study demonstrates that suppression of DPOAEs by contralateral pure tones can be detected in the chinchilla with real time recording. DPOAE suppression is greatest when using contralateral stimulation tones close to primary tone f2. This tonotopic response is consistent with other investigations of frequency specificity in the MOCS pathway (Chery-Croze, Moulin, & Collet, 1993; Cody & Johnstone, 1982; M. C. Liberman, 1989; Murata et al., 1980; Robertson, 1984; Robertson & Gummer, 1985; Veuillet et al., 1991; Warren III & Liberman, 1989a; Warren III & Liberman, 1989b). Unlike observations in human subjects, we did not observe any dips in fine structure DPOAEs to account for differences in the magnitude of suppression at different values of f2 or between chinchillas (Wagner,

Measurement of contralateral frequency tuning of MOCS fibers has revealed narrow band tuning equivalent in sharpness to cochlear afferent neurons (Brown, 1989; M. Liberman & Brown, 1986; Robertson, 1984). The final, divergent innervation pattern of MOCS fibers at the OHC level appears to degrade this cochleotopicity (or frequency tuning) by a factor of 4- 5 from 0.33 octaves (the approximate bandwidth of auditory afferents) to about 1.7 octaves for f2 = 3.1kHz and 1.3 octaves for f2 = 7.7kHz. The difference in tuning likely rests with the divergent OHC innervation by the MOCS fibers. Neural tracing studies in guinea pig have shown MOCS fibers innervating 15 -61 OHCs (Brown, 1989). In the cat, individual cochlear efferents contact 23 – 84 OHCs spanning 0.55-2.8mm (M. Liberman & Brown, 1986). Thus although tuning in the efferent fibers themselves appears to be as sharp as afferent tuning,

the effect of individual fibers on the organ of Corti will be much less precise.

**5.1.3 Discussion** 

Heppelmann, Müller, Janssen, & Zenner, 2007).

Objective tests such as OAE and ABR are widely used in hearing screening programs and have lead to great advances in the early detection and rehabilitation of neonatal hearing

Contralateral Suppression of Otoacoustic Emissions:

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679-682.

Working Towards a Simple Objective Frequency Specific Test for Hearing Screening 45

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loss. However these tests do not provide a quick and easy means for assessing hearing threshold at different frequencies, indeed the presence of OAE does not even guarantee the presence of normal hearing. An objective frequency specific test of hearing ability would have widespread advantages, not just for neonatal testing but in many circumstances in all age groups.

In the present study we have demonstrated frequency specificity in contralateral suppression using a chinchilla model. The majority of studies shedding light onto the function of the MOCS have been derived from animal experiments. However, there is enough data in human studies to suggest that the human efferent system is qualitatively similar (Guinan Jr, 2006; James, 2006). We have shown previously that contralateral suppression of DPOAE can be assessed in real time in babies and adults (James et al., 2005) and can be used to test hearing very effectively in neonates (James, 2011). We have shown that this technique can distinguish between middle ear muscle reflexes and the OCR in an animal model (Wolter, Harrison, & James, 2011) and here show that it can be used to assess hearing threshold in a frequency specific manner. We envisage many clinical applications of this technique including the diagnosis and assessment of ANSD and more accurate hearing screening in neonatal and elderly populations.

#### **7. References**


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**1. Introduction**

**2.1 Location**

was about 21 dB(A).

**0**

**3**

*Japan*

Takahiro Tamesue

**A Prediction Method for Speech Audibility**

Securing good transmission characteristics for speech information and achieving a comfortable sound environment in buildings used by a great variety of people in public city spaces, to say nothing of spaces used for intellectual or mental work such as schools and offices, comprise two of the most important problems of environmental design. A common method for evaluating listening scores and psychological impressions for audio signal has been discussed previously (Tamesue T. et al., 2003). However, this research paid attention only to subjects in their twenties with normal hearing, and as a result the relationships between the frequency characteristics of hearing loss due to factors such as aging and the listening scores for audio signals and the psychological impressions related to speech audibility were not considered. Taking this into consideration, this chapter considers how the listening scores of audio signals and the psychological impressions for speech audibility change while taking into account the effects of hearing loss due to factors such as aging. Specifically, frequency filters for simulating hearing loss are first prepared. Next, psychological listening experiments are conducted in which both the audio signal and the noise passing through the above-mentioned filters are transmitted to subjects with normal hearing. Using the observed experimental data, the relationships between the weight-mean spectral distance (Tamesue T. et al., 2003) and the listening scores of the audio signals and psychological impressions with respect to speech audibility are investigated. Next, based on these relationships, problems associated with the prediction of listening scores and

psychological impressions with respect to speech audibility are discussed.

Psychological listening experiment I was conducted to establish the regression models of the listening scores of the audio signal and the psychological impressions related to speech

The experiment was conducted in a simple soundproof room on campus having the following dimensions: length 5.1 m, width 3.3 m, and height 2.2 m. The sound pressure level of the background noise was about 37 dB. The sound pressure level in this chapter is the value measured by a sound level meter with FLAT response. The A-weighted sound pressure level

**2. Outline of psychological listening experiment I**

audibility. The outline of the indoor experiment is as follows.

**Taking Account of Hearing Loss Due to**

**Aging Under Meaningless Noise**

*Organization for Academic Information, Yamaguchi University*

