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ORIGINAL ARTICLE
Year : 2015  |  Volume : 21  |  Issue : 4  |  Page : 260-265

Neurophysiologic measures of auditory brainstem responses to Hindi speech stimulus in typically developing children


1 Department of Audiology, Ali Yavar Jung National Institute for the Hearing Handicapped, Mumbai, Maharashtra, India
2 Technical Director, Dr. S. R. Chandrasekhar Institute of Speech and Hearing, Bengaluru, Karnataka, India

Date of Web Publication16-Oct-2015

Correspondence Address:
Mohammad Shamim Ansari
Department of Audiology, Ali Yavar Jung National Institute for the Hearing Handicapped, K.C. Marg, Bandra (West), Mumbai - 400 050, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-7749.164544

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  Abstract 

Context: Speech evoked auditory brain stem responses (spABR) are used to assess brain stem ability to encode speech. However, brain stem representations of speech sound are affected by acoustic differences of speech, language background and experiences. Hence, linguistically and culturally specific stimulus should be used. Therefore, there is need to investigate the brain stem encoding of Hindi speech sound. Objective: The purpose of the study was to investigate the spABR to Hindi stimulus in typically developing children. Methods and Material: Total 25 atypical children of the mean age of 6.3 years with SD 2.3 years in the age range of 8-12 years were recruited. A 40ms duration Hindi stop phoneme of CV combination |da| was synthesized to gain response of consonant (transient) and vowel (sustained) portion of speech stimulus from all subjects. Results: The spABR latencies and amplitudes of discrete peaks as well as the latency, amplitude, area & slope of V-A complex were measured. The stimulus to response timing and spectral magnitudes of periodic portion were also calculated. The mean, median, standard deviation, minimum, maximum and 95% confidence interval values were calculated. Conclusions: The obtained spABR values suggest faithful representation of acoustic characteristic of speech at brain stem level. These values are found to be similar with previous reports. Hence, spABR can serve as important tools in the diagnostic work up of auditory processing deficits in children to understand speech perception and production abilities.

Keywords: Auditory processing, Frequency following responses, Speech encoding, Speech stimulus, Speech-evoked auditory brainstem response, Transient response


How to cite this article:
Ansari MS, Raghunath R. Neurophysiologic measures of auditory brainstem responses to Hindi speech stimulus in typically developing children. Indian J Otol 2015;21:260-5

How to cite this URL:
Ansari MS, Raghunath R. Neurophysiologic measures of auditory brainstem responses to Hindi speech stimulus in typically developing children. Indian J Otol [serial online] 2015 [cited 2020 Feb 17];21:260-5. Available from: http://www.indianjotol.org/text.asp?2015/21/4/260/164544


  Introduction Top


Auditory brainstem responses (ABRs) are electrical potentials generated by the synchronous activity of populations of neurons in response to an auditory stimulus like click or tone and recorded from disk electrodes attached to the human scalp.[1] The neural responses initiate in the auditory nerve and continue through the cochlear nucleus, superior olivary complex, and lateral lemniscuses up to the inferior colliculus. Hence, these responses are useful to estimate the magnitude of hearing loss and to differentiate among cochlea, cochlear nerve, and brainstem lesions. It is for these reasons the click-evoked ABR has enjoyed wide-scale clinical use as a metric for determining auditory thresholds noninvasively, objectively, and inattentively in children and difficult to test population and detecting auditory neuropathologies.[2],[3]

The ABRs to clicks and tones are instrumental in defining basic response patterns of the brainstem and proven to be a clinically useful tool to assess auditory function. However, their clinical utility to predict speech processing at brainstem is limited, due to its poor physical approximation to speech as it includes both sustained and transient features. Thus, the brainstem ability to process speech is not predictable from the click and tones.[4],[5] Moreover, the majority of persons with auditory disorders primarily have difficulty in perceiving and understanding speech. Hence, it is enviable that speech should be used to obtain auditory system ability to process speech.

Similar to click ABRs, speech stimulus of consonant-vowel (CV) combination particularly stops voiced phoneme are used for eliciting ABRs.[6],[7],[8] The sub-cortical response emerges as a waveform of seven identifiable peaks, labelled as V, A, C, D, E, F, and O. Waves V and A reflect the onset of the response much like click ABR, Wave C the transition region, Waves D, E, and F the periodic region (i.e., the frequency following response), and Wave O the offset of the response.[9] The Waves D, E, and F reflect the periodic features of speech stimulus, named as frequency following response shows periodic response to fundamental frequency and vowel formants, and reflects phase-locking characteristics of neuron to the waveform of the presented stimulus.[8],[9] These spABRs waveform has a characteristic morphology of physiologic representation to speech at brainstem that varies little among individuals with typical development.[6],[7],[8]

Therefore, altered spABR provides a physiologic depiction of poor speech encoding evident in children with language, literacy, reading, and learning deficits.[10] Children with known language-based learning problems exhibited delayed latencies compared to their normal learning peers.[11],[12] In addition, children with poor reading abilities found to have prolonged latencies, poorer waveform morphology, and weaker spectral encoding compared to children with better reading abilities.[10],[13] These studies show a trend that difficulties in language, literacy, reading, and learning affects sub-cortical representation of speech and delayed response latencies in the fraction of few milliseconds between normal and clinical population are associated with these difficulties.

Since the last decade, brainstem encoding of speech sound /da/ has been investigated and reported to reflect some of the acoustic-phonetic characteristics of speech with remarkable precision at sub-cortical level.[6], 8, [14],[15],[16],[17],[18],[19] Therefore, it has been suggested that the spABR can be used for clinical assessment of auditory function. Although, there are some sporadic data to spABRs are available in Indian literature. However, there exists no speech stimulus, especially in Hindi language to record spABRs, which is the first language and spoken by more than 50% of India's population. Hence, the current study proposes to use linguistically and culturally sensitive Hindi stimulus to obtain spABRs in children. This may have clinical significance in the evaluation of auditory language disorders and may also be helpful in designing and devising appropriate therapeutic intervention programs.

Objectives of the study

The purpose of the current study is to record and investigate the Hindi speech stimulus /da/ evoked brainstem potentials in typically developing children.


  Subjects and Methods Top


Research design

This prospective survey was conducted at the Electrophysiological Laboratory of Department of Audiology. The study was accorded necessary ethical clearance from the Institutional Ethical Board. Written and informed consent was obtained from the parents.

Participants

Totally 25 children of both sexes in the age range 8–12 years (mean = 9.3 years, standard deviation [SD] =2.2 years) were recruited for the study. The subjects were right-handed native Hindi speaker with no known history of neurological, otological disease or trauma, and a psychiatric problem. All the children had pure-tone thresholds ≤25 dB HL at octave frequencies from 250 to 8000 Hz, speech identification score ≥90% at 40 dB SL (reference: Pure-tone average of 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz) in both ears and normal middle ear function on immittance evaluation (A type tympanogram with acoustic reflexes at normal sensation levels). Inclusion of children in the study also required Wave V latency to 100 µs click within the range of normative values (5.41–5.96 ms, mean ± 1.5 SDs) presented at 45 dB nHL in rarefaction to the right ear at the rate of 21.1 clicks/s).

Stimulus and recording parameters

To elicit spABRs, Hindi stop voiced phoneme of CV combination /da/ with 5-formants of 40 ms, was synthesized with Praat (Boersma and Weenink 2009) according to methods described in previous publications.[14],[20] The speech sound is comprising an initial noise burst of 10 ms and a formant transition of 30 ms between the consonant and the vowel. The F0 and the first three formants (F1, F2, and F3) change linearly over the duration of the stimulus: F0 from 113 to 147, F1 from 240 to 770, F2 from 1670 to 1350, and F3 from 2680 to 2550 Hz. F4 and F5 remain constant at 3700 and 4600 Hz, respectively.

The spABRs were recorded from Ag-AgCl electrodes, with surface contact impedance of <5 kΩ, positioned centrally on the scalp, at Cz, behind the right mastoid (reference) and on the forehead (ground). Stimuli were presented into the right ear at a rate of 11.1/s at comfortable listening level of 65 dB SL relative to the threshold at 1000 Hz through insert earphones (ER-3, Etymotic Research, USA). The sampling rate was 20,000 Hz and responses were online band passed filtered from 100 to 3000 Hz, 12 dB/octave. Trials with eye-blinks or other motion artifacts greater than ± 35 µV were rejected online. Two traces of 2000 sweeps were collected at alternating polarity. The recording window was 50 ms starting 10 ms prior to stimulus onset. During testing, the children were sited on the reclining chair comfortably. Waveforms were averaged online in Intelligent Hearing Systems Smart EP software (version 2.39).

Result analysis

The obtained waveforms were subjected for peak identification by two experienced observers who had experience with spABR recording and analysis. The stimulus used for recording sub-cortical responses was blinded to observers. The two observers who independently agreed on individuals peaks were considered for analysis and peak at which disagreement occurred was discarded from the data pool.

The individual peaks identified were labelled as V, A, C, D, E, F, and O using nomenclature previously established for the S-ABR (Russo et al. 2004; Johnson et al. 2005). The speech stimulus traces includes a positive Peak V (analogue to the V Wave produced by the click) before 10 ms after onset, followed by a negative peak (Wave A). Waves V and A reflect the onset of the response, Wave C the transition region, Waves D, E, and F the periodic region (i.e. the frequency following response), and Wave O the offset of the response. These waves were analyzed to their absolute latencies and amplitude. V-A complex was analyzed for latency, amplitude and slope (VA amplitude/VA duration).

Data processing was performed using MATLAB version R2010a (The MathWorks, Inc., Natick, Massachusetts, USA). The peak values were subjected to statistical analysis. The mean, median, SD, minimum, maximum, and confidence interval (CI) values were calculated for the sample. In order to determine whether or not there is statistically significant difference between the mean results of current study and mean results of individual peak found in literature data, one-way ANOVA was studied at significance levels of 5% (P < 0.05). The statistical analyses were performed using SPSS 16.0 (SPSS Inc., Chicago, IL, USA).


  Results Top


The current study recorded the neurophysiologic responses of the brainstem to Hindi stop voiced phoneme of CV combination /da/ in atypical children. All 25 participants exhibited speech evoked ABRs. [Figure 1] shows speech evoked ABRs with Hindi stimulus /da/ in one of the subject.
Figure 1: Waveform of Hindi stimulus |da| evoked auditory brainstem responses in one of the subject

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The results indicated that the onset (Waves V and A) and the transition (Peak C) were observed in all the (100%) participants. While, frequency following response (Peaks D, E, and F) was present in 73.3% and offset (Peak O) responses were observed in 89% of the participants. The latencies and amplitudes of discrete peaks [Table 1] and sustained components [Table 2] of the brainstem response to speech stimulus were measured.
Table 1: Mean, SD (in parenthesis), median, minimum and maximum and 95% confidence level values of sub-cortical responses peaks latencies and amplitudes in Hindi speaking persons

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Table 2: Mean, SD of (A) stimulus-to-response correlations (B) spectral amplitude measures of Hindi stimulus in native Hindi speaker

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Discrete peak measures of the Hindi speech stimulus-evoked auditory brainstem responses

The spABR latencies and amplitudes of discrete peaks as well as the latency, amplitude, area, and slope of Waves V and A are depicted in [Table 1]. [Table 1] shows mean, SD (in parenthesis), median, minimum, maximum, and 95% confidence values of sub-cortical responses of individual peak latencies and amplitudes of Hindi-speaking persons.

Sustained portion of the Hindi speech stimulus-auditory brainstem response measures

The response between Waves C and O comprises the sustained portion or FFR (Peaks D, E, and F) that occurs approximately between 10 and 40 ms after the onset response to the voiced portion of the syllable. Responses to formant transitions of stimuli were analyzed using, S-R Correlation, FFT, and RMS measures on the MATLAB routines are depicted in [Table 2]. These measures provide information about the overall magnitude of sustained neural activity and phase-locking capabilities of the neural population in the auditory system.[8],[14]

The stimulus-to-response correlation (SR corr) and the amount of stimulus-to-response delay or shift (SR lag) were calculated to determine timing (latency) of the FFR. Frequency Fourier transform analysis was performed to calculate the magnitude of the neural response to the entire period of stimulus (RMS amp) as well as the magnitude of spectral components in narrow frequency regions surrounding the stimulus fundamental frequency (F0 amp: 113 to 147 Hz), the first formant (F1 amp: 440 to 742 Hz), and a higher frequency region (HF amp: 742 to 1350 Hz). The RMS, F0, F1, and HF amplitudes signify the magnitude of the response. The greatest amount of energy is present in the F0 region of response shown in [Figure 2].
Figure 2: Greatest amount of energy is concentrated around F0 area

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  Discussion Top


This is the first study to the best of our information and knowledge that used Hindi stop voiced phoneme of CV combination /da/ to measure neurophysiological responses of the brainstem in children. The 96% peaks in the waveform of Hindi sound /da/ elicited ABR were identifiable by the peak observers. The two measures, that is, transient and frequency following responses of spABRs are used to describe the brainstem neural activity for temporally and spectrally distributed Hindi speech stimulus.[8]

The neurophysiologic response characteristic of brainstem to transient component of acoustic stimulus were identified. [Figure 1]. For each peak, latency (time relative to stimulus onset) and amplitude measurements were obtained. Inter-peak measurements were also calculated; these include inter-peak amplitude, duration, slope, and area [Table 1]. The latency measures provide information about the precision with which the brainstem nuclei synchronously respond to speech stimuli and amplitude measures provide information about the robustness of the response of the brainstem nuclei for the speech stimulus.

The transient portion of spABR, exhibited presence of an initial Wave V at a mean latency of 6.78 ms analogous to the Wave V elicited by a click stimulus. However, this is more than the documented mean latency between 5 and 6 ms of Peak V for click stimulus.[21],[22] The latencies of the later peaks were calculated with reference to the latency of Wave V. Peak A appeared at negative trough immediately after Wave V at a mean latency of 7.55 ms, Wave C emerged at a mean latency of 18.81 ms, Wave D occurred at a mean latency of 22.78 ms, Wave E crop up at a mean latency of 31.82 ms, Wave F was visible at a mean latency of 39.48 ms, and Wave O was visible at a mean latency of 49.03 ms. The latency obtained for the different peaks of speech evoked ABR in the present study were almost similar to the studies reported earlier publications.[8],[14]

The Fast Fourier analysis of brain-stem neurophysiologic response between 10.4 and 44.4 to speech stimulus has demonstrated a peak at F0 and F1. It can be observed from [Table 3] and [Figure 2] that the amplitude of the F0 is larger as compared to the F1. This could be due to the acoustic characteristics of stimulus and the phase-locking properties of the auditory neurons as the stimulus has a greater energy in the F0 region compared to its harmonics and greater energy stimulus components are represented better at the neuronal level.[9] Also, the F0 has a lower frequency compared to its harmonics, and it is reported that lower frequencies have a better phase locked neuronal responses.
Table 3: Comparisons of mean latency values obtained in the present study with the study by Russo et al. (2004)[8] and Karawani and Banai (2010)[18]

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Thus, it can be contemplated that the F0 region contains greater energy and has better phase-locking characteristics resulting in more robust recordings of low frequencies component of the stimulus. Hence, it can be concluded that the potentials evoked by rapid timing changes of consonant and vowel portion of the Hindi stimulus are faithfully and accurately encoded and preserved at the brainstem level.[17]

Comparison of Hindi stimulus-evoked auditory brainstem responses with the Western normative data

Another goal of this study was to compare the obtained spABRs to Hindi stimulus /da/ with the values reported in the literature.[8],[14],[15],[18] The major findings of comparisons between the Hindi samples and previous literature values are on timing measures. [Table 3] shows mean (SD) along with minimum and maximum bounds at 95% CI of the test sample in the left column, US and Israeli norms in the middle column and right-most column, respectively. The overlap among Hindi, Israeli, and the US CIs is equivalent to an insignificant result of a t-test with P = 0.05.

The brainstem data obtained with Hindi stimulus are statistically indistinguishable from the US and Israeli norms for this age group, entail that the Hindi stimulus /da/ as a nontonal stimulus represented in the brainstems of Hindi speakers in similar fashion as English /da/, in English and other language users. The obtained brainstem response timing data suggest that similar results are expected in the population and languages where this stimulus will be present.

In spite of the stimulus used in this study is a variant of /da/, the nonexistence of statistically significant difference in the evoked timing data to speech stimulus between current study and western reports may be due to the fact that durational aspect (pattern) of the stimulus used in the previous studies are same. Hence, it can be assumed that brainstem may be sensitive to encode durational aspect (pattern) of the stimulus at brainstem level. The subtle differences invariants of /da/ in the current study, may be within the limits of neuronal response pattern at brainstem level. The temporal resolution within the categorical boundaries of the stimulus may be operated by another mechanism or higher order auditory centres. However, more research required to address this point.

Hence, the spABR faithfully reflects many acoustic properties of the speech signal in normally perceiving auditory system and the stimulus timing is accurately and precisely represented at the level of the brainstem. Consequently, the brainstem responses provide a quantifiable measure of an individual's attention-independent neural encoding of speech sounds for understanding the neural bases of normal and deficient auditory function.


  Conclusions Top


The Hindi speech sound /da/ elicited brainstem response timing and magnitude measures about the neural encoding of brief and periodic aspects of the stimulus in the auditory pathway are in accordance with the previous western literature. The latency measures of response provide information about the precision with which the brainstem nuclei synchronously respond to acoustic stimulus while amplitude measures furnish information about robustness of the response of the brainstem nuclei for the acoustic stimulus with extreme accuracy.[17],[18] Thus, differences in neural representation at brainstem level on the order of tenths of a millisecond may be clinically significant. Therefore, alterations in these measures might indicate a difference in conduction speed along the dendrites and axon projections, or a difference in kinetic channels of neurons, or even differences in the synchronization of the response generators in clinical population.

Comparing together, our results of brainstem potentials with western findings suggest that the Hindi stimulus-evoked ABR waveforms morphology reflects some of the acoustic-phonetic characteristics of speech with remarkable precision in atypical children. Further, there exist insignificant differences in the obtained ABR values to Hindi stimulus as compared to the reported values in literature suggesting that the early processing of stop consonants may be similar across the different languages in which this consonant will be present and used.

Therefore, it can be suggested that Hindi stimulus can be used to record spABRs in children. However, we would like to emphasize a point here that acoustic-phonetics variations among the various languages in India may play a crucial role in diagnostic work-up of the clinical population. As a growing body of literature has revealed that brainstem encoding of specific elements of speech sounds in Mandarin speakers encode pitch patterns more robust than English speakers that convey linguistic meaning in Mandarin, but not in English.[16]

Since, India is multilingual and multicultural society, studying encoding of Hindi speech stimulus in different language users may through more light in understanding and underpinning the physiological processes in normal as well as disorder populations. Hence, unlike any other language-based test, appropriate speech stimulus and norms may be more meaningful in assessment and categorization of the clinical population.

Acknowledgments

We would like to express our thanks and gratitude to our participants for their help in this research.

Financial support and sponsorship

Nil.

Conflicts of interest

The corresponding author is doing PhD under the guidence of the second author on the same topic.

 
  References Top

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    Figures

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    Tables

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