Envelope Following Response Amplitude Research Articles

Neural Envelope Processing at Low Frequencies Predicts Speech Understanding of Children With Hearing Loss in Noise and Reverberation.

Children with hearing loss experience greater difficulty understanding speech in the presence of noise and reverberation relative to their normal hearing peers despite provision of appropriate amplification. The fidelity of fundamental frequency of voice (f0) encoding-a salient temporal cue for understanding speech in noise-could play a significant role in explaining the variance in abilities among children. However, the nature of deficits in f0 encoding and its relationship with speech understanding are poorly understood. To this end, we evaluated the influence of frequency-specific f0 encoding on speech perception abilities of children with and without hearing loss in the presence of noise and/or reverberation. In 14 school-aged children with sensorineural hearing loss fitted with hearing aids and 29 normal hearing peers, envelope following responses (EFRs) were elicited by the vowel /i/, modified to estimate f0 encoding in low (<1.1 kHz) and higher frequencies simultaneously. EFRs to /i/ were elicited in quiet, in the presence of speech-shaped noise at +5 dB signal to noise ratio, with simulated reverberation time of 0.62 sec, as well as both noise and reverberation. EFRs were recorded using single-channel electroencephalogram between the vertex and the nape while children watched a silent movie with captions. Speech discrimination accuracy was measured using the University of Western Ontario Distinctive Features Differences test in each of the four acoustic conditions. Stimuli for EFR recordings and speech discrimination were presented monaurally. Both groups of children demonstrated a frequency-dependent dichotomy in the disruption of f0 encoding, as reflected in EFR amplitude and phase coherence. Greater disruption (i.e., lower EFR amplitudes and phase coherence) was evident in EFRs elicited by low frequencies due to noise and greater disruption was evident in EFRs elicited by higher frequencies due to reverberation. Relative to normal hearing peers, children with hearing loss demonstrated: (a) greater disruption of f0 encoding at low frequencies, particularly in the presence of reverberation, and (b) a positive relationship between f0 encoding at low frequencies and speech discrimination in the hardest listening condition (i.e., when both noise and reverberation were present). Together, these results provide new evidence for the persistence of suprathreshold temporal processing deficits related to f0 encoding in children despite the provision of appropriate amplification to compensate for hearing loss. These objectively measurable deficits may underlie the greater difficulty experienced by children with hearing loss.

Predicting Hearing aid Benefit Using Speech-Evoked Envelope Following Responses in Children With Hearing Loss.

Electroencephalography could serve as an objective tool to evaluate hearing aid benefit in infants who are developmentally unable to participate in hearing tests. We investigated whether speech-evoked envelope following responses (EFRs), a type of electroencephalography-based measure, could predict improved audibility with the use of a hearing aid in children with mild-to-severe permanent, mainly sensorineural, hearing loss. In 18 children, EFRs were elicited by six male-spoken band-limited phonemic stimuli--the first formants of /u/ and /i/, the second and higher formants of /u/ and /i/, and the fricatives /s/ and /∫/--presented together as /su∫i/. EFRs were recorded between the vertex and nape, when /su∫i/ was presented at 55, 65, and 75 dB SPL using insert earphones in unaided conditions and individually fit hearing aids in aided conditions. EFR amplitude and detectability improved with the use of a hearing aid, and the degree of improvement in EFR amplitude was dependent on the extent of change in behavioral thresholds between unaided and aided conditions. EFR detectability was primarily influenced by audibility; higher sensation level stimuli had an increased probability of detection. Overall EFR sensitivity in predicting audibility was significantly higher in aided (82.1%) than unaided conditions (66.5%) and did not vary as a function of stimulus or frequency. EFR specificity in ascertaining inaudibility was 90.8%. Aided improvement in EFR detectability was a significant predictor of hearing aid-facilitated change in speech discrimination accuracy. Results suggest that speech-evoked EFRs could be a useful objective tool in predicting hearing aid benefit in children with hearing loss.

External and middle ear influence on envelope following responses.

Considerable between-subject variability in envelope following response (EFR) amplitude limits its clinical translation. Based on a pattern of lower amplitude and larger variability in the low (<1.2 kHz) and high (>8 kHz), relative to mid (1-3 kHz) frequency carriers, we hypothesized that the between-subject variability in external and middle ear (EM) contribute to between-subject variability in EFR amplitude. It is predicted that equalizing the stimulus reaching the cochlea by accounting for EM differences using forward pressure level (FPL) calibration would at least partially improve response amplitude and reduce between-subject variability. In 21 young normal hearing adults, EFRs of four modulation rates (91, 96, 101, and 106 Hz) were measured concurrently from four frequency bands [low (0.091-1.2 kHz), mid (1-3 kHz), high (4-5.4 kHz), and very high (vHigh; 8-9.4 kHz)], respectively, with 12 harmonics each. The results indicate that FPL calibration in-ear and in a coupler leads to larger EFR amplitudes in the low and vHigh frequency bands relative to conventional coupler root-mean-square calibration. However, improvement in variability was modest with FPL calibration. This lack of a statistically significant improvement in variability suggests that the dominant source of variability in EFR amplitude may arise from cochlear and/or neural processing.

Speech-Evoked Envelope Following Responses in Children and Adults.

Envelope following responses (EFRs) could be useful for objectively evaluating audibility of speech in children who are unable to participate in routine clinical tests. However, relative to adults, the characteristics of EFRs elicited by frequency-specific speech and their utility in predicting audibility in children are unknown. EFRs were elicited by the first (F1) and second and higher formants (F2+) of male-spoken vowels /u/ and /i/ and by fricatives /ʃ/ and /s/ in the token /suʃi/ presented at 15, 35, 55, 65, and 75 dB SPL. The F1, F2+, and fricatives were low-, mid-, and high-frequency dominant, respectively. EFRs were recorded between the vertex and the nape from twenty-three 6- to 17-year-old children and 21 young adults with normal hearing. Sensation levels of stimuli were estimated based on behavioral thresholds. In children, amplitude decreased with age for /ʃ/-elicited EFRs but remained stable for low- and mid-frequency stimuli. As a group, EFR amplitude and phase coherence did not differ from that of adults. EFR sensitivity (proportion of audible stimuli detected) and specificity (proportion of inaudible stimuli not detected) did not vary between children and adults. Consistent with previous work, EFR sensitivity increased with stimulus frequency and level. The type of statistical indicator used for EFR detection did not influence accuracy in children. Adultlike EFRs in 6- to 17-year-old typically developing children suggest mature envelope encoding for low- and mid-frequency stimuli. EFR sensitivity and specificity in children, when considering a wide range of stimulus levels and audibility, are ~77% and ~92%, respectively. https://doi.org/10.23641/asha.21136171.

Variability in the Estimated Amplitude of Vowel-Evoked Envelope Following Responses Caused by Assumed Neurophysiologic Processing Delays.

Vowel-evoked envelope following responses (EFRs) reflect neural encoding of the fundamental frequency of voice (f0). Accurate analysis of EFRs elicited by natural vowels requires the use of methods like the Fourier analyzer (FA) to consider the production-related f0 changes. The FA's accuracy in estimating EFRs is, however, dependent on the assumed neurophysiological processing delay needed to time-align the f0 time course and the recorded electroencephalogram (EEG). For male-spoken vowels (f0 ~ 100Hz), a constant 10-ms delay correction is often assumed. Since processing delays vary with stimulus and physiological factors, we quantified (i) the delay-related variability that would occur in EFR estimation, and (ii) the influence of stimulus frequency, non-f0 related neural activity, and the listener's age on such variability. EFRs were elicited by the low-frequency first formant, and mid-frequency second and higher formants of /u/, /a/, and /i/ in young adults and 6- to 17-year-old children. To time-align with the f0 time course, EEG was shifted by delays between 5 and 25ms to encompass plausible response latencies. The delay-dependent range in EFR amplitude did not vary by stimulus frequency or age and was significantly smaller when interference from low-frequency activity was reduced. On average, the delay-dependent range was < 22% of the maximum variability in EFR amplitude that could be expected by noise. Results suggest that using a constant EEG delay correction in FA analysis does not substantially alter EFR amplitude estimation. In the present study, the lack of substantial variability was likely facilitated by using vowels with small f0 ranges.

The Influence of Male- and Female-Spoken Vowel Acoustics on Envelope-Following Responses.

The influence of male and female vowel characteristics on the envelope-following responses (EFRs) is not well understood. This study explored the role of vowel characteristics on the EFR at the fundamental frequency (f0) in response to the vowel /ε/ (as in "head"). Vowel tokens were spoken by five males and five females and EFRs were measured in 25 young adults (21 females). An auditory model was used to estimate changes in auditory processing that might account for talker effects on EFR amplitude. There were several differences between male and female vowels in relation to the EFR. For male talkers, EFR amplitudes were correlated with the bandwidth and harmonic count of the first formant, and the amplitude of the trough below the second formant. For female talkers, EFR amplitudes were correlated with the range of f0 frequencies and the amplitude of the trough above the second formant. The model suggested that the f0 EFR reflects a wide distribution of energy in speech, with primary contributions from high-frequency harmonics mediated from cochlear regions basal to the peaks of the first and second formants, not from low-frequency harmonics with energy near f0. Vowels produced by female talkers tend to produce lower-amplitude EFR, likely because they depend on higher-frequency harmonics where speech sound levels tend to be lower. This work advances auditory electrophysiology by showing how the EFR evoked by speech relates to the acoustics of speech, for both male and female voices.

Comparison of age-related declines in behavioral auditory responses versus electrophysiological measures of amplitude modulation

Amplitude and frequency modulations are important for speech intelligibility, especially in noise. Neurophysiological responses assessed by envelope following responses (EFRs) are smaller at faster amplitude modulation frequencies (AMF) in older subjects compared to younger subjects. A typical assumption is that a decline in EFRs necessarily results in corresponding perceptual deficits. To test this in an animal model, we investigated the behavioral AMF discrimination of young and aged Fischer-344 rats and compared those abilities to their EFRs. A modified version of prepulse inhibition of the acoustic startle reflex was used to measure behavior. When AMF differences and modulation depths were large, young and aged animals’ behavioral performances were comparable. Aged animals’ discrimination abilities declined as the difference between background and prepulse AMF decreased and as modulation depth decreased. These declines were larger than in younger animals, even compared to young rats with similar peripheral activation (ABR wave I amplitudes), whose EFR amplitudes were smaller than the aged animals. The results revealed larger age-related deficits in behavioral perception compared to EFRs, suggesting additional factors that affect perception in aging.

The Influence of Sensation Level on Speech-Evoked Envelope Following Responses.

To evaluate sensation level (SL)-dependent characteristics of envelope following responses (EFRs) elicited by band-limited speech dominant in low, mid, and high frequencies. In 21 young normal hearing adults, EFRs were elicited by 8 male-spoken speech stimuli-the first formant, and second and higher formants of /u/, /a/ and /i/, and modulated fricatives, /∫/ and /s/. Stimulus SL was computed from behaviorally measured thresholds. At 30 dB SL, the amplitude and phase coherence of fricative-elicited EFRs were ~1.5 to 2 times higher than all vowel-elicited EFRs, whereas fewer and smaller differences were found among vowel-elicited EFRs. For all stimuli, EFR amplitude and phase coherence increased by roughly 50% for every 10 dB increase in SL between ~0 and 50 dB. Stimulus and frequency dependency in EFRs exist despite accounting for differences in audibility of speech sounds. The growth rate of EFR characteristics with SL is independent of stimulus and its frequency.

Fundamental frequency-dependent changes in vowel-evoked envelope following responses

Scalp-recorded envelope following responses (EFRs) provide a non-invasive method to assess the encoding of the fundamental frequency (f0) of voice that is important for speech understanding. It is well-known that EFRs are influenced by voice f0. However, this effect of f0 has not been examined independent of concomitant changes in spectra or neural generators. We evaluated the effect of voice f0 on EFRs while controlling for vowel formant characteristics and potentially avoiding significant changes in dominant neural generators using a small f0 range. EFRs were elicited by a male-spoken vowel /u/ (average f0 = 100.4 Hz) and its lowered f0 version (average f0 = 91.9 Hz) with closely matched formant characteristics. Vowels were presented to each ear of 17 young adults with normal hearing. EFRs were simultaneously recorded between the vertex and the nape, and the vertex and the ipsilateral mastoid—the two most common electrode montages used for EFRs. Our results indicate that when vowel formant characteristics are matched, an increase in f0 by 8.5 Hz reduces EFR amplitude by 25 nV, phase coherence by 0.05 and signal-to-noise ratio by 3.5 dB, on average. The reduction in EFR characteristics was similar across ears of stimulation and the two montages used. These findings will help parse the influence of f0 or stimulus spectra on EFRs when both co-vary.

The Influence of Vowel Identity, Vowel Production Variability, and Consonant Environment on Envelope Following Responses.

The vowel-evoked envelope following response (EFR) is a useful tool for studying brainstem processing of speech in natural consonant-vowel productions. Previous work, however, demonstrates that the amplitude of EFRs is highly variable across vowels. To clarify factors contributing to the variability observed, the objectives of the present study were to evaluate: (1) the influence of vowel identity and the consonant context surrounding each vowel on EFR amplitude and (2) the effect of variations in repeated productions of a vowel on EFR amplitude while controlling for the consonant context. In Experiment 1, EFRs were recorded in response to seven English vowels (/ij/, /Ι/, /ej/, /ε/, /æ/, /u/, and /JOURNAL/earher/04.03/00003446-202105000-00017/inline-graphic1/v/2021-04-30T105427Z/r/image-tiff/) embedded in each of four consonant contexts (/hVd/, /sVt/, /zVf/, and /JOURNAL/earher/04.03/00003446-202105000-00017/inline-graphic2/v/2021-04-30T105427Z/r/image-tiffVv/). In Experiment 2, EFRs were recorded in response to four different variants of one of the four possible vowels (/ij/, /ε/, /æ/, or /JOURNAL/earher/04.03/00003446-202105000-00017/inline-graphic3/v/2021-04-30T105427Z/r/image-tiff/), embedded in the same consonant-vowel-consonant environments used in Experiment 1. All vowels were edited to minimize formant transitions before embedding in a consonant context. Different talkers were used for the two experiments. Data from a total of 30 and 64 (16 listeners/vowel) young adults with normal hearing were included in Experiments 1 and 2, respectively. EFRs were recorded using a single-channel electrode montage between the vertex and nape of the neck while stimuli were presented monaurally. In Experiment 1, vowel identity had a significant effect on EFR amplitude with the vowel /æ/ eliciting the highest amplitude EFRs (170 nV, on average), and the vowel /ej/ eliciting the lowest amplitude EFRs (106 nV, on average). The consonant context surrounding each vowel stimulus had no statistically significant effect on EFR amplitude. Similarly in Experiment 2, consonant context did not influence the amplitude of EFRs elicited by the vowel variants. Vowel identity significantly altered EFR amplitude with /ε/ eliciting the highest amplitude EFRs (104 nV, on average). Significant, albeit small, differences (<21 nV, on average) in EFR amplitude were evident between some variants of /ε/ and /u/. Based on a comprehensive set of naturally produced vowel samples in carefully controlled consonant contexts, the present study provides additional evidence for the sensitivity of EFRs to vowel identity and variations in vowel production. The surrounding consonant context (after removal of formant transitions) has no measurable effect on EFRs, irrespective of vowel identity and variant. The sensitivity of EFRs to nuances in vowel acoustics emphasizes the need for adequate control and evaluation of stimuli proposed for clinical and research purposes.

Characteristics of Speech-Evoked Envelope Following Responses in Infancy.

Envelope following responses (EFRs) may be a useful tool for evaluating the audibility of speech sounds in infants. The present study aimed to evaluate the characteristics of speech-evoked EFRs in infants with normal hearing, relative to adults, and identify age-dependent changes in EFR characteristics during infancy. In 42 infants and 21 young adults, EFRs were elicited by the first (F1) and the second and higher formants (F2+) of the vowels /u/, /a/, and /i/, dominant in low and mid frequencies, respectively, and by amplitude-modulated fricatives /s/ and /∫/, dominant in high frequencies. In a subset of 20 infants, the in-ear stimulus level was adjusted to match that of an average adult ear (65 dB sound pressure level [SPL]). We found that (a) adult–infant differences in EFR amplitude, signal-to-noise ratio, and intertrial phase coherence were larger and spread across the frequency range when in-ear stimulus level was adjusted in infants, (b) adult–infant differences in EFR characteristics were the largest for low-frequency stimuli, (c) infants demonstrated adult-like phase coherence when they received a higher (i.e., unadjusted) stimulus level, and (d) EFR phase coherence and signal-to-noise ratio changed with age in the first year of life for a few F2+ vowel stimuli in a level-specific manner. Together, our findings reveal that development-related changes in EFRs during infancy likely vary by stimulus frequency, with low-frequency stimuli demonstrating the largest adult–infant differences. Consistent with previous research, our findings emphasize the significant role of stimulus level calibration methods while investigating developmental trends in EFRs.

Chirp Analyzer for Estimating Amplitude and Latency of Steady-State Auditory Envelope Following Responses.

The envelope following response (EFR) is a clinically relevant evoked potential, reflecting the synchronization of the auditory pathway to the temporal envelope of sounds. Since there is no standard analysis of this potential, we here aim at contrasting the relative accuracy of known time-frequency methods and new strategies for the reliable estimation of the EFR amplitude and latency. The EFR was estimated using explicit time-frequency methods: the Short-Term Fourier Transform (STFT) and the Morlet Continuous Wavelet Transform (CWT). Furthermore, the Chirp Analyzer (CA) was introduced as a new tool for the reliable estimation of the EFR. The applicability of the methods was tested in animal and human recordings. Using simulated data for comparing the estimation performance by each method, we found that the CA is able to correctly estimate EFR amplitudes, without the typical bias observed in the STFT estimates. The CA is more robust to noise than the CWT method, although with higher sensitivity to the latency of the response. Thus, the estimation of the EFR amplitude with any of the methods, but especially with CA, should be corrected by using the estimated delay. Analysis of real data confirmed these results and showed that all methods offer estimated EFRs similar to those found in previous studies using the classical Fourier Analyzer. The CA is a potential valuable tool for the analysis of the EFR, which could be extended for the estimation of oscillatory evoked potentials of other sensory modalities.

Test-Retest Variability in the Characteristics of Envelope Following Responses Evoked by Speech Stimuli.

The objective of the present study was to evaluate the between-session test-retest variability in the characteristics of envelope following responses (EFRs) evoked by modified natural speech stimuli in young normal hearing adults. EFRs from 22 adults were recorded in two sessions, 1 to 12 days apart. EFRs were evoked by the token /susa∫ i/ (2.05 sec) presented at 65 dB SPL and recorded from the vertex referenced to the neck. The token /susa∫ i/, spoken by a male with an average fundamental frequency [f0] of 98.53 Hz, was of interest because of its potential utility as an objective hearing aid outcome measure. Each vowel was modified to elicit two EFRs simultaneously by lowering the f0 in the first formant while maintaining the original f0 in the higher formants. Fricatives were amplitude-modulated at 93.02 Hz and elicited one EFR each. EFRs evoked by vowels and fricatives were estimated using Fourier analyzer and discrete Fourier transform, respectively. Detection of EFRs was determined by an F-test. Test-retest variability in EFR amplitude and phase coherence were quantified using correlation, repeated-measures analysis of variance, and the repeatability coefficient. The repeatability coefficient, computed as twice the standard deviation (SD) of test-retest differences, represents the ±95% limits of test-retest variation around the mean difference. Test-retest variability of EFR amplitude and phase coherence were compared using the coefficient of variation, a normalized metric, which represents the ratio of the SD of repeat measurements to its mean. Consistency in EFR detection outcomes was assessed using the test of proportions. EFR amplitude and phase coherence did not vary significantly between sessions, and were significantly correlated across repeat measurements. The repeatability coefficient for EFR amplitude ranged from 38.5 nV to 45.6 nV for all stimuli, except for /∫/ (71.6 nV). For any given stimulus, the test-retest differences in EFR amplitude of individual participants were not correlated with their test-retest differences in noise amplitude. However, across stimuli, higher repeatability coefficients of EFR amplitude tended to occur when the group mean noise amplitude and the repeatability coefficient of noise amplitude were higher. The test-retest variability of phase coherence was comparable to that of EFR amplitude in terms of the coefficient of variation, and the repeatability coefficient varied from 0.1 to 0.2, with the highest value of 0.2 for /∫/. Mismatches in EFR detection outcomes occurred in 11 of 176 measurements. For each stimulus, the tests of proportions revealed a significantly higher proportion of matched detection outcomes compared to mismatches. Speech-evoked EFRs demonstrated reasonable repeatability across sessions. Of the eight stimuli, the shortest stimulus /∫/ demonstrated the largest variability in EFR amplitude and phase coherence. The test-retest variability in EFR amplitude could not be explained by test-retest differences in noise amplitude for any of the stimuli. This lack of explanation argues for other sources of variability, one possibility being the modulation of cortical contributions imposed on brainstem-generated EFRs.

Investigating potential interactions between envelope following responses elicited simultaneously by different vowel formants

Envelope following responses (EFRs) evoked by the periodicity of voicing in vowels are elicited at the fundamental frequency of voice (f0), irrespective of the harmonics that initiate it. One approach of improving the frequency specificity of vowel stimuli without increasing test-time is by altering the f0 selectively in one or more formants. The harmonics contributing to an EFR can then be differentiated by the unique f0 at which the EFRs are elicited. The advantages of using such an approach would be increased frequency specificity and efficiency, given that multiple EFRs can be evaluated in a certain test-time. However, multiple EFRs elicited simultaneously could interact and lead to altered amplitudes and outcomes. To this end, the present study aimed to evaluate: (i) if simultaneous recording of two EFRs, one elicited by harmonics in the first formant (F1) and one elicited by harmonics in the second and higher formants (F2+), leads to attenuation or enhancement of EFR amplitude, and (ii) if simultaneous measurement of two EFRs affects its accuracy and anticipated efficiency. In a group of 22 young adults with normal hearing, EFRs were elicited by F1 and F2+ bands of /u/, /a/ and /i/ when F1 and F2+ were presented independently (individual), when F1 and F2+ were presented simultaneously (dual), and when F1 or F2+ was presented with spectrally matched Gaussian noise of the other (noise). Repeated-measures analysis of variance indicated no significant group differences in EFR amplitudes between any of the conditions, suggesting minimal between-EFR interactions. Between-participant variability was evident, however, significant changes were evident only in a third of the participants for the stimulus /u/ F1. For the majority of stimuli, the change between individual and dual conditions was positively correlated with the change between individual and noise conditions, suggesting that interaction-based changes in EFR amplitude, when present, were likely due to the restriction of cochlear regions of excitation in the presence of a competing stimulus. The amplitude of residual noise was significantly higher in the dual or noise relative to the individual conditions, although the mean differences were very small (<3 nV). F-test-based detection of EFRs, commonly used to determine the presence of an EFR, did not vary across conditions. Further, neither the mean reduction in EFR amplitude nor the mean increase in noise amplitude in dual relative to individual conditions was large enough to alter the anticipated gain in efficiency of simultaneous EFR recordings. Together, results suggest that the approach of simultaneously recording two vowel-evoked EFRs from different formants for improved frequency-specificity does not alter test accuracy and is more time-efficient than evaluating EFRs to each formant individually.

Reliability and interrelations of seven proxy measures of cochlear synaptopathy

Investigations of cochlear synaptopathy in living humans rely on proxy measures of auditory nerve function. Numerous procedures have been developed, typically based on the auditory brainstem response (ABR), envelope-following response (EFR), or middle-ear-muscle reflex (MEMR). Validation is challenging, due to the absence of a gold-standard measure in humans. Some metrics correlate with synaptic survival in animal models, but translation between species is not straightforward; measurements in humans are likely to reflect greater error and greater variability from non-synaptopathic sources. The present study assessed the reliability of seven measures, as well as testing for correlations between them. Thirty-one young women with normal audiograms underwent repeated measurements of ABR wave I amplitude, ABR wave I growth, ABR wave V latency shift in noise, EFR amplitude, EFR growth with stimulus modulation depth, MEMR threshold, and an MEMR across-frequency difference measure. Intraclass correlation coefficients for ABR wave I amplitude, EFR amplitude, and MEMR threshold ranged from 0.85 to 0.93, suggesting that such tests can yield highly reliable results, given careful measurement techniques. The ABR and EFR difference measures exhibited only poor-to-moderate reliability. No significant correlations, nor any consistent trends, were observed between the various measures, providing no indication that these metrics reflect the same underlying physiological processes. Findings suggest that many proxy measures of cochlear synaptopathy should be regarded with caution, at least when employed in young adults with normal audiograms.

Phase-locked responses to the vowel envelope vary in scalp-recorded amplitude due to across-frequency response interactions.

Neural encoding of the envelope of sounds like vowels is essential to access temporal information useful for speech recognition. Subcortical responses to envelope periodicity of vowels can be assessed using scalp-recorded envelope following responses (EFRs); however, the amplitude of EFRs vary by vowel spectra and the causal relationship is not well understood. One cause for spectral dependency could be interactions between responses with different phases, initiated by multiple stimulus frequencies. Phase differences can arise from earlier initiation of processing high frequencies relative to low frequencies in the cochlea. This study investigated the presence of such phase interactions by measuring EFRs to two naturally spoken vowels (/ε/ and /u/), while delaying the envelope phase of the second formant band (F2+) relative to the first formant (F1) band in 45° increments. At 0° F2+ phase delay, EFRs elicited by the vowel /ε/ were lower in amplitude than the EFRs elicited by /u/. Using vector computations, we found that the lower amplitude of /ε/-EFRs was caused by linear superposition of F1- and F2+-contributions with larger F1-F2+ phase differences (166°) compared to /u/ (19°). While the variation in amplitude across F2+ phase delays could be modeled with two dominant EFR sources for both vowels, the degree of variation was dependent on F1 and F2+ EFR characteristics. Together, we demonstrate that (a) broadband sounds like vowels elicit independent responses from different stimulus frequencies that may be out-of-phase and affect scalp-based measurements, and (b) delaying higher frequency formants can maximize EFR amplitudes for some vowels.

Reliability and interrelations of seven proxy measures of cochlear synaptopathy

Investigations of cochlear synaptopathy in living humans rely on proxy measures of auditory nerve function. Numerous procedures have been developed, typically based on the auditory brainstem response (ABR), envelope-following response (EFR), or middle-ear muscle reflex (MEMR). Some metrics correlate with synaptic survival in animal models, but translation between species is not straightforward; measurements in humans likely reflect greater error and greater variability from non-synaptopathic sources. The present study assessed the reliability of seven measures, as well as testing for correlations between them. Thirty-one normally hearing young women underwent repeated measurements of ABR wave I amplitude, ABR wave I growth with level, ABR wave V latency shift in noise, EFR amplitude, EFR growth with stimulus modulation depth, MEMR threshold, and an MEMR difference measure. Intraclass correlation coefficients indicated good-to-excellent reliability for the raw ABR and EFR amplitudes, and for both MEMR measures. The ABR and EFR difference measures exhibited poor-to-moderate reliability. No significant correlations, nor any consistent trends, were observed between measures, providing no indication that the between-subject variability in responses are due to the same underlying physiological processes. Findings suggest that proxy measures of cochlear synaptopathy should be regarded with caution, at least when employed in young, normally hearing adults.

Relations between speech perception in noise, high-frequency audiometry, and physiological measures of cochlear synaptopathy

Cochlear synaptopathy, a loss of synapses between inner hair cells and auditory nerve fibers, is associated with age and noise exposure in animal models. However, the functional consequences of synaptopathy for humans are unclear. We pooled data from two recent studies to answer the question: are the common physiological measures of cochlear synaptopathy related to speech-perception-in-noise (SPiN) performance? Eighty-three audiometrically normal participants (ages 18–39) took part. Measures of synaptopathy were as follows: auditory brainstem response (ABR) wave I amplitude (102 dB peSPL click); ABR wave I:V amplitude ratio; envelope following response (EFR) amplitude (4 kHz carrier, 105 Hz modulation frequency); EFR amplitude growth with stimulus modulation depth; and middle ear muscle reflex threshold (1–4 kHz elicitors). We also conducted extended high-frequency (EHF) audiometry (10 and 14 kHz), suggested as a marker for synaptopathy in lower frequency regions. SPiN performance was assessed using the coordinate response measure with spatial maskers. None of the physiological measures of synaptopathy correlated significantly with SPiN. There was a significant correlation between EHF thresholds and SPiN, although it is unclear whether this is due to a direct relation between EHF hearing and SPiN, or whether elevated EHF thresholds are a marker for hidden damage at lower frequencies.

Masking Differentially Affects Envelope-following Responses in Young and Aged Animals

Age-related hearing decline typically includes threshold shifts as well as reduced wave I auditory brainstem response (ABR) amplitudes due to cochlear synaptopathy/neuropathy, which may compromise precise coding of suprathreshold speech envelopes. This is supported by findings with older listeners, who have difficulties in envelope and speech processing, especially in noise. However, separating the effects of threshold elevation, synaptopathy, and degradation by noise on physiological representations may be difficult. In the present study, the effects of notched, low- and high-pass noise on envelope-following responses (EFRs) in aging were compared when sound levels (aged: 85-dB SPL; young: 60- to 80-dB SPL) were matched between groups peripherally, by matching wave I ABR amplitudes, or centrally by matching EFR amplitudes. Low-level notched noise reduced EFRs to sinusoidally amplitude-modulated (SAM) tones in young animals for notch widths up to 2 octaves. High-pass noise above the carrier frequency reduced EFRs. Young animals showed EFR reductions at lower noise levels. Low-pass noise did not reduce EFRs in either young or aged animals. High-pass noise may affect EFR amplitudes in young animals more than aged by reducing the contributions of high-frequency-sensitive inputs. EFRs to SAM tones in modulated noise (NAM) suggest that neurons of young animals can synchronize to NAM at lower sound levels and maintain dual AM representations better than older animals. The overall results show that EFR amplitudes are strongly influenced by aging and the presence of a competing sound that likely reduces or shifts the pool of responsive neurons.

Effects of Early- and Late-Arriving Room Reflections on the Speech-Evoked Auditory Brainstem Response.

Room reverberation alters the acoustical properties of the speech signals reaching our ears, affecting speech understanding. Therefore, it is important to understand the consequences of reverberation on auditory processing. In perceptual studies, the direct sound and early reflections of reverberated speech have been found to constitute useful energy, whereas the late reflections constitute detrimental energy. This study investigated how various components (direct sound versus early reflections versus late reflections) of the reverberated speech are encoded in the auditory system using the speech-evoked auditory brainstem response (ABR). Speech-evoked ABRs were recorded using reverberant stimuli created as a result of the convolution between an ongoing synthetic vowel /a/ and each of the following room impulse response (RIR) components: direct sound, early reflections, late reflections, and full reverberation. Four stimuli were produced: direct component, early component, late component, and full component. Twelve participants with normal hearing participated in this study. Waves V and A amplitudes and latencies as well as envelope-following response (EFR) and fine structure frequency-following response (FFR) amplitudes of the speech-evoked ABR were evaluated separately with one-way repeated measures analysis of variances to determine the effect of stimulus. Post hoc comparisons using Tukey's honestly significant difference test were performed to assess significant differences between pairs of stimulus conditions. For waves V and A amplitudes, a significant difference or trend toward significance was found between direct and late components, between direct and full components, and between early and late components. For waves V and A latencies, significant differences were found between direct and late components, between direct and full components, between early and late components, and between early and full components. For the EFR and FFR amplitudes, a significant difference or trend toward significance was found between direct and late components, and between early and late components. Moreover, eight, three, and one participant reported the early, full, and late stimuli, respectively, to be the most perceptually similar to the direct stimulus. The stimuli that are acoustically most similar (direct and early) result in electrophysiological responses that are not significantly different, whereas the stimuli that are acoustically most different (direct and late, early and late) result in responses that are significantly different across all response measures. These findings provide insights toward the understanding of the effects of the different components of the RIRs on auditory processing of speech.