**2. Assessment**

Speech ABR has an important feature, that is, the specific aspects of acoustic signal are preserved and reflects the neural coding in figure [representation of 40 ms of syllable /da/ (gray) stimulus and responses (black)] [4]. Furthermore, this assessment permits to understand the neural basis of the auditory system, even if it is normal or deficient stimulus and responses (black)] [4] (**Figure 1**).

(ABR) The ABR is a clinical tool to assess the neural functionality of the auditory brainstem [1]. Until recently, assessment using clinical ABR protocols was carried out only with nonverbal stimuli, such as clicks, tone-bursts, and chirps. The ABR responses (i) permit the analysis of the integrityofthe auditorypathways and(ii) canestablishelectrophysiologicalthresholds inorder to identify basic neural abnormalities and to evaluate patients who did not provide reliable

Although the use of the click-evoked ABR has been widely used clinically, it is still necessary to unravel how verbal sounds are coded in the brainstem. Recent technological advances have enabled the inclusion of verbal stimuli in the ABR commercial equipment. The use of verbal stimuli in ABR protocols provided important information of how the speech stimuli are processed by the brainstem structure, which actively participates in the analysis of the complex

The verbal stimulus most widely used in speech ABR is a syllable composed of a consonant and a vowel (CV) [4]. The consonant perception is performed by the distinction between vocal production times and sound of consonant that guarantees the intelligibility in the process of

The perception of speech sounds seems to begin in the brainstem, which has an important role in reading process and phonological acquisition [5–7]. An effective and objective way to investigate this process will be the assessment of speech ABR that allows the identification of fine-grained auditory processing deficits associated with real-world communication, skills which do not appear in click responses, and it also can be used for early identification of auditory processing impairments in very young children [8]. Above all, speech ABR can be used as an objective measure of the hearing function. One of the great advantages of this method is that it is not influenced by environmental issues, which can disrupt the behavioral assessments [2]. Even the best behavioral tests can confound the subject by factors such as attention, motivation, alertness/fatigue, and by co-occurring disorders, such as language

Understanding the neural processing of speech sounds at the brainstem level provides knowledge regarding the central auditory processes involved in normal hearing subjects and also in clinical populations [10]. Moreover, altered responses of speech ABR may be associated with impaired speech perception in noise. These changes can cause a negative impact on communication and serious consequences for academic success [8]. According to Sinha and Basavaraj [11], the major application of speech ABR can be in diagnosing and categorizing children with learning disability in different subgroups, assessing the effects of aging on central auditory processing of speech, assessing the effects of central auditory deficits in hearing aid,

Speech ABR has an important feature, that is, the specific aspects of acoustic signal are preserved and reflects the neural coding in figure [representation of 40 ms of syllable /da/ (gray)

responses in the standard behavioral audiological assessment [2].

human communication and the proper development of language.

impairments, learning impairments, or attention deficits [9].

verbal stimuli [3].

10 Advances in Clinical Audiology

and cochlear implant users.

**2. Assessment**

**Figure 1.** Representation of 40 ms of syllable /da/ (*gray*) stimulus and responses (*black*) [4].

The verbal stimulus used in the speech ABR assessment, normally, is the syllables /ba/, /da/, or /ga/. The verbal assessment provides information about how the speech syllable is encoded by auditory system. The trace of the speech ABR response can be dismembered in two parts: the onset and the frequency following response (FFR). The first part represents the consonant and the second part the vowel [10].

The best-known model used is elicited with the synthesized syllable /da/ provided by a computer software. The use of synthesized speech allows acoustic parameters to be controlled and constant and ensures the quality of the stimulus that will be presented to the listener and/ or the patient [12]. This stimulus modality was developed by the group of Dr. Nina Kraus at Northwestern University. The stimulus consisted of the consonant /d/ (transient portion onset) and the short vowel /a/ (sustained portion—following frequency response). When elicited by the stimulus /da/, the subcortical response emerges as a waveform of seven peaks —V, A, C, D, E, F, and O—wherein the single wave with a positive peak is the complex of wave V. Waves V and A reflect the onset response, wave C the transition area, waves D, E, and F the periodic area (the frequency following response), and wave O the offset response (**Figure 2**) [4, 13, 14]. A typical response is shown in **Figure 2** (electrophysiological response representation of synthesized syllable /da/. Investigator's personal data based on the assessment of a normal hearing, performed with the BioMARKTM software) [13].

**Figure 2.** Electrophysiological response representation of synthesized syllable /da/. Investigator's personal data based on the assessment of a normal hearing, performed with the BioMARKTM software [13].

It is important to describe that the onset component seems to be elicited around 10 ms and is considered the transient portion of sound stimulus reflecting the decoding of fast temporal changes inherent in the consonant [15]. The component FFR is called sustained portion and seems to be elicited around 18–50 ms. This component reflects the encoding of periodic and harmonic structure of vowel sound related to harmonic vowels [11] and is also related to encoding of the elements of fundamental frequency and its modulations (first and second formants) [4, 15].

Another feature of speech ABR responses is that there is no variation between intra and inter subject, maintaining stable the morphological characteristics [16, 17].

The speech sounds are present more frequently in the daily lives of every human being. A long-term of auditory experience can improve the performance of the whole auditory system. Therefore, a subject who has a good processing of speech sounds has better electrophysiological responses for this type of stimulus, showing that auditory experience might modify the basic sensorial coding of the whole auditory pathway [18–21]. On the other hand, a subject who has auditory deprivation may have significant electrophysiological changes in the auditory system, as can be seen in children with history of otitis media.

## **3. Parameters**

There are several searches about the coding processing of verbal sound occurs and to insert speech ABR as part of clinical routine.

The syllable /da/ is commonly used speech for ABR assessment due to it being considered a universal syllable and allows it to be applied in different countries with good clinical assertiveness [4]. However, previous studies show that there is difference response in subjects from different culture [22] since each language has its own characteristics and peculiarities that can contribute or not to a suitable processing of speech sounds.

The majority of the studies was performed with native English speakers, which is explained by the fact that Dr. Kraus, the leading researcher and creator of the speech stimulus, did her work at Northwestern University, USA [1, 4]. However, additional studies have been initiated in numerous languages such as Arabic, Brazilian Portuguese, French, Greek, Hebrew, Indian, Japanese, and Persian [1, 11, 13, 22–32].

In each laboratory and/or institution, researchers choose their own parameters that will be applied on clinical investigation. Below are some items that should be thought about at the time of creation of the assessment parameters.

#### **3.1. Equipment and software**

Sanfins and Colella-Santos analyzed which equipments and software were often used for assessment of speech ABR. Biologic Navigator Pro (Natus) is the most used equipment followed by Neuroscan equipment (Biolink). As regards the software, the *BioMark* (Biological Marker of Auditory Processing) and *Neuroscam Stim 2* are the main packages available [1].

#### **3.2. Electrode montage**

**Figure 2.** Electrophysiological response representation of synthesized syllable /da/. Investigator's personal data based

It is important to describe that the onset component seems to be elicited around 10 ms and is considered the transient portion of sound stimulus reflecting the decoding of fast temporal changes inherent in the consonant [15]. The component FFR is called sustained portion and seems to be elicited around 18–50 ms. This component reflects the encoding of periodic and harmonic structure of vowel sound related to harmonic vowels [11] and is also related to encoding of the elements of fundamental frequency and its modulations (first and second

Another feature of speech ABR responses is that there is no variation between intra and inter

The speech sounds are present more frequently in the daily lives of every human being. A long-term of auditory experience can improve the performance of the whole auditory system. Therefore, a subject who has a good processing of speech sounds has better electrophysiological responses for this type of stimulus, showing that auditory experience might modify the basic sensorial coding of the whole auditory pathway [18–21]. On the other hand, a subject who has auditory deprivation may have significant electrophysiological changes in the

There are several searches about the coding processing of verbal sound occurs and to insert

The syllable /da/ is commonly used speech for ABR assessment due to it being considered a universal syllable and allows it to be applied in different countries with good clinical asser-

on the assessment of a normal hearing, performed with the BioMARKTM software [13].

subject, maintaining stable the morphological characteristics [16, 17].

auditory system, as can be seen in children with history of otitis media.

formants) [4, 15].

12 Advances in Clinical Audiology

**3. Parameters**

speech ABR as part of clinical routine.

The position of the electrodes follows the traditional ABR assessment (click ABR). Neurophysiological responses can be recorded with an active electrode positioned on the vertex (Cz), the reference electrodes on the ipsilateral mastoid, and the ground on the contralateral mastoid, using one channel with surface electrodes fixed, according to the 10–20 system [33]. Automatic switching function of the reference signals and the amplifier ground based on the stimulated ear should be activated on the equipment. The electrode on the left ear can be connected to input 2/channel 1, and the electrode on the right ear can be connected to ground connection cable. During the recording session, impedance should be maintained at below 5 kΩ and interelectrode impedance below 3 kΩ [22].

#### **3.3. Stimulated ear**

Research shows that there is an asymmetry for the auditory processing of verbal sounds that occur in the brainstem and extend to auditory cortex when evaluating the differences between the responses obtained from the presentation of acoustic stimuli on the right and left ears [34, 35].

Regarding the stimulated ear, the great majority of studies performed the assessment of ABR with speech stimuli elicited only on the right ear, which can be explained by the advantage of right ear in encoding speech by contralateral projection to the left hemisphere [24, 26, 29, 31, 32, 36–44].

However, some researchers have written that stimulus presentation can be performed on the ear with better threshold confirmed by pure tone audiometry [45]. In a systematic review about the applicability of speech ABR [1], it was possible to see that in 14.3% of articles, stimulation was performed monaurally; however, between the left ear and right ear stimulation, there is scientific evidence that even if there is a proven right ear advantage in the processing of speech, the left ear can participate in this process, but with less intense electrophysiological responses [28, 36, 46]. Therefore, an analysis of responses from both ears could help in the diagnosis process as well as therapeutic monitoring.

Importantly, there is a tutorial about ABR of complex sounds that notify that the monaural stimulation is preferred for children, while the binaural stimulation is more realistic than monoaural [4].

Ahadi et al. [25] presented the sound stimulus on three conditions: monaural right, monaural, and binaural left. They showed that the magnitude and strength of speech ABR responses depend on the stimulus presentation mode, and the binaural presentation of speech syllable enables better visualization of the response, however,

#### **3.4. Stimulus**

The speech ABR assessment allows to apply different types of sound stimulus. The syllable / da/ is most well known and applied more often in studies [11, 13, 15, 22, 25, 28, 29, 32, 36, 39, 45, 47]. However, there are researchers who used disyllables as /baba/ [27] or even other syllables composed by consonant-vowel as /ba/ [23, 30, 31].

The presentation rate parameter is related to the duration of the sound stimulus; in the case of speech ABR, it is related to the size of the sound stimulus speech. The frequent value found in the studies is 10.9/s, however, no reports of the use of 11.1/s. In a study of literature review, it is noted that in about 19% of the previous studies on the assessment of speech ABR, this parameter is not described by the researchers [1].

Considering the length parameter, it is observed that the most frequently found values were 40 and 170 ms [1]. There is a relation between the presentation rate and duration, so the higher, the shorter will be the presentation rate [45, 48]. Song et al. [16] used both acoustic stimuli and concluded that short (40 ms) and long stimulus (170 ms) reflect the coding of speech in the brainstem in a reliable way, thus enabling that neural changes can be monitored through an objective electrophysiological measure.

The type of polarity of the sound stimulus is one of the most consistent parameters across studies on the assessment of speech ABR. Approximately, 90.5% of the previous studies have used alternating polarity [13, 22, 23, 28, 29, 39, 45, 47, 49, 50]. The choice for this type of polarity should be the reduction of artifacts and cochlear microphonic [51].

Regarding the intensity used in the assessment of speech, ABR suggests the use of 60–85 dB SPL [4, 15]. It is noted that, as it is an assessment process, the sound should be applied in an audible and comfortable intensity to the patient. The majority of studies has used the intensity of 80 dB SPL [1].

The speech stimulus requires approximately 4000 and 6000 sweeps in order to get a robust and replicable response, differently, the click stimulus or tone burst that needs around 2000 sweeps

to get a good quality of response [4]. The number of sweeps is one of the most diverse parameters across studies [1]; however, the majority of researches used two blocks of 3000 free sweeps artifacts [13, 22, 25, 28, 36, 39–41, 47, 49]. Both trials were averaged to create a calculated wave of 6000 sweeps. The traces of both recordings were added, and the responses of the resultant waves were identified and analyzed in **Figure 3** (electrophysiological response representation of two blocks of 3000 sweeps and calculated wave of 6000 sweeps. Investigator's personal data based on a subject's assessment performed with BioMARKTM software).

**Figure 3.** Electrophysiological response representation of two blocks of 3000 sweeps and calculated wave of 6000 sweeps. Investigator's personal data based on a subject's assessment performed with BioMARKTM software.

#### **3.5. Transducer**

the applicability of speech ABR [1], it was possible to see that in 14.3% of articles, stimulation was performed monaurally; however, between the left ear and right ear stimulation, there is scientific evidence that even if there is a proven right ear advantage in the processing of speech, the left ear can participate in this process, but with less intense electrophysiological responses [28, 36, 46]. Therefore, an analysis of responses from both ears could help in the diagnosis

Importantly, there is a tutorial about ABR of complex sounds that notify that the monaural stimulation is preferred for children, while the binaural stimulation is more realistic than

Ahadi et al. [25] presented the sound stimulus on three conditions: monaural right, monaural, and binaural left. They showed that the magnitude and strength of speech ABR responses depend on the stimulus presentation mode, and the binaural presentation of speech syllable

The speech ABR assessment allows to apply different types of sound stimulus. The syllable / da/ is most well known and applied more often in studies [11, 13, 15, 22, 25, 28, 29, 32, 36, 39, 45, 47]. However, there are researchers who used disyllables as /baba/ [27] or even other

The presentation rate parameter is related to the duration of the sound stimulus; in the case of speech ABR, it is related to the size of the sound stimulus speech. The frequent value found in the studies is 10.9/s, however, no reports of the use of 11.1/s. In a study of literature review, it is noted that in about 19% of the previous studies on the assessment of speech ABR, this

Considering the length parameter, it is observed that the most frequently found values were 40 and 170 ms [1]. There is a relation between the presentation rate and duration, so the higher, the shorter will be the presentation rate [45, 48]. Song et al. [16] used both acoustic stimuli and concluded that short (40 ms) and long stimulus (170 ms) reflect the coding of speech in the brainstem in a reliable way, thus enabling that neural changes can be monitored through an

The type of polarity of the sound stimulus is one of the most consistent parameters across studies on the assessment of speech ABR. Approximately, 90.5% of the previous studies have used alternating polarity [13, 22, 23, 28, 29, 39, 45, 47, 49, 50]. The choice for this type of polarity

Regarding the intensity used in the assessment of speech, ABR suggests the use of 60–85 dB SPL [4, 15]. It is noted that, as it is an assessment process, the sound should be applied in an audible and comfortable intensity to the patient. The majority of studies has used the intensity

The speech stimulus requires approximately 4000 and 6000 sweeps in order to get a robust and replicable response, differently, the click stimulus or tone burst that needs around 2000 sweeps

process as well as therapeutic monitoring.

enables better visualization of the response, however,

syllables composed by consonant-vowel as /ba/ [23, 30, 31].

should be the reduction of artifacts and cochlear microphonic [51].

parameter is not described by the researchers [1].

objective electrophysiological measure.

of 80 dB SPL [1].

monoaural [4].

14 Advances in Clinical Audiology

**3.4. Stimulus**

The literature recommends that earphones are not to be used once this device can increase the chances of artifacts. Thus, the recommendation is to use the insert earphones. In cases of insert earphones are not possible to be used, there is the possibility to do the test with loudspeakers. It is important to consider that the responses are not so reliable as ones with insert earphones. The evaluator should be very careful in positioning the patient and the loudspeakers, and these loudspeakers should be equidistant between the right and left ears [4]. In addition, previous study has presented speech stimulus through individual hearing aid with excellent results with free and high-quality artifact [23].

#### **3.6. Assessment of condition**

As the traditional ABR assessment, the patients are instructed to keep their bodies relaxed with no movements in order to minimize the myogenic artifacts. [24]

Researchers reinforce that the attention can influence the FFR portion of speech sounds [52]. Therefore, the majority of researches has allowed the patient to watch a movie with reduced sound intensity or with subtitle [16, 23, 40, 41, 50], which seems that it keeps them quiet and relaxed during the assessment. Other researchers allow the patient to choose between watching a movie or sleep during the assessment process [24, 45].

Different parameters are being used. The parameters most cited in the literature about the assessment of speech ABR and with good clinical results are presented below in **Chart 1** (Speech ABR parameters). Note that there is a well-written tutorial by Skoe and Kraus [4] with detailed, clear, and objective information about the functioning and clinical application of speech ABR. This tutorial can be a material support to those interested in unraveling this new and effective electrophysiological assessment method.


**Chart 1.** Speech ABR parameters.
