Effects of age and noise exposure history on auditory nerve response amplitudes: A systematic review, study, and meta-analysis.

Auditory nerve (AN) function has been hypothesized to deteriorate with age and noise exposure. Here, we perform a systematic review of published studies and find that the evidence for age-related deficits in AN function is largely consistent across the literature, but there are inconsistent findings among studies of noise exposure history. Further, evidence from animal studies suggests that the greatest deficits in AN response amplitudes are found in noise-exposed aged mice, but a test of the interaction between effects of age and noise exposure on AN function has not been conducted in humans. We report a study of our own examining differences in the response amplitude of the compound action potential N1 (CAP N1) between younger and older adults with and without a self-reported history of noise exposure in a large sample of human participants (63 younger adults 18-30 years of age, 103 older adults 50-86 years of age). CAP N1 response amplitudes were smaller in older than younger adults. Noise exposure history did not appear to predict CAP N1 response amplitudes, nor did the effect of noise exposure history interact with age. We then incorporated our results into two meta-analyses of published studies of age and noise exposure history effects on AN response amplitudes in neurotypical human samples. The meta-analyses found that age effects across studies are robust (r = -0.407), but noise exposure effects are weak (r = -0.152). We conclude that noise exposure effects may be highly variable depending on sample characteristics, study design, and statistical approach, and researchers should be cautious when interpreting results. The underlying pathology of age-related and noise-induced changes in AN function are difficult to determine in living humans, creating a need for longitudinal studies of changes in AN function across the lifespan and histological examination of the AN from temporal bones collected post-mortem.

Effects of Age and Noise Exposure History on Auditory Nerve Response Amplitudes: A Systematic Review, Study, and Meta-Analysis.

Auditory nerve (AN) function has been hypothesized to deteriorate with age and noise exposure. Here, we perform a systematic review of published studies and find that the evidence for age-related deficits in AN function is largely consistent across the literature, but there are inconsistent findings among studies of noise exposure history. Further, evidence from animal studies suggests that the greatest deficits in AN response amplitudes are found in noise-exposed aged mice, but a test of the interaction between effects of age and noise exposure on AN function has not been conducted in humans. We report a study of our own examining differences in the response amplitude of the compound action potential N1 (CAP N1) between younger and older adults with and without a self-reported history of noise exposure in a large sample of human participants (63 younger adults 18-30 years of age, 103 older adults 50-86 years of age). CAP N1 response amplitudes were smaller in older than younger adults. Noise exposure history did not appear to predict CAP N1 response amplitudes, nor did the effect of noise exposure history interact with age. We then incorporated our results into two meta-analyses of published studies of age and noise exposure history effects on AN response amplitudes in neurotypical human samples. The meta-analyses found that age effects across studies are robust (r=-0.407), but noise-exposure effects are weak (r=-0.152). We conclude that noise-exposure effects may be highly variable depending on sample characteristics, study design, and statistical approach, and researchers should be cautious when interpreting results. The underlying pathology of age-related and noise-induced changes in AN function are difficult to determine in living humans, creating a need for longitudinal studies of changes in AN function across the lifespan and histological examination of the AN from temporal bones collected post-mortem.

Sensory tetanisation to induce long-term-potentiation-like plasticity: A review and reassessment of the approach.

There is great interest in developing non-invasive approaches for studying cortical plasticity in humans. High-frequency presentation of auditory and visual stimuli, or sensory tetanisation, can induce long-term-potentiation-like (LTP-like) changes in cortical activity. However, contrasting effects across studies suggest that sensory tetanisation may be unreliable. We review these contrasting effects, conduct our own study of auditory and visual tetanisation, and perform meta-analyses to determine the average effect of sensory tetanisation across studies. We measured auditory-evoked amplitude changes in a group of younger (18-29 years of age) and older (55-83 years of age) adults following tetanisation to 1 and 4 kHz tone bursts and following a slow-presentation control. We also measured visual-evoked amplitude changes following tetanisation to horizontal and vertical sign gradients. Auditory and visual response amplitudes decreased following tetanisation, consistent with some studies but contrasting with others finding amplitude increases (i.e. LTP-like changes). Older adults exhibited more modest auditory-evoked amplitude decreases, but visual-evoked amplitude decreases like those of younger adults. Changes in response amplitude were not specific to tetanised stimuli. Importantly, slow presentation of auditory tone bursts produced response amplitude changes approximating those observed following tetanisation in younger adults. Meta-analyses of visual and auditory tetanisation studies found that the overall effect of sensory tetanisation was not significant across studies or study sites. The results suggest that sensory tetanisation may not produce reliable changes in cortical responses and more work is needed to determine the validity of sensory tetanisation as a method for inducing human cortical plasticity in vivo.

Afferent loss, GABA, and Central Gain in older adults: Associations with speech recognition in noise.

Deficits in auditory nerve (AN) function for older adults reduce afferent input to the cortex. The extent to which the cortex in older adults adapts to this loss of afferent input and the mechanisms underlying this adaptation are not well understood. We took a neural systems approach measuring AN and cortical evoked responses within 50 older and 27 younger human adults (59 female) to estimate central gain, or increased cortical activity despite reduced AN activity. Relative to younger adults, older adults' AN response amplitudes were smaller, but cortical responses were not. We used the relationship between AN and cortical response amplitudes in younger adults to predict cortical response amplitudes for older adults from their AN responses. Central gain in older adults was thus defined as the difference between their observed cortical responses and those predicted from the parameter estimates of younger adults. In older adults, decreased afferent input contributed to lower cortical GABA levels, greater central gain, and poorer speech recognition in noise (SIN). These effects on SIN occur in addition to, and independent from, effects attributed to elevated hearing thresholds. Our results are consistent with animal models of central gain and suggest that reduced AN afferent input in some older adults may result in changes in cortical encoding and inhibitory neurotransmission, which contribute to reduced SIN. An advancement in our understanding of the changes that occur throughout the auditory system in response to the gradual loss of input with increasing age may provide potential therapeutic targets for intervention.Significance:Age-related hearing loss is one of the most common chronic conditions of aging, yet little is known about how the cortex adapts to this loss of sensory input. We measured AN and cortical responses to the same stimulus in younger and older adults. In older adults we found hyperexcitability in cortical activity relative to concomitant declines in afferent input that are consistent with central gain Lower levels of cortical GABA, an inhibitory neurotransmitter was associated with greater central gain, which predicted poorer SIN. The results suggest that the cortex in older adults may adapt to attenuated sensory input by reducing inhibition to amplify the cortical response, but this amplification may lead to poorer SIN.

Age-related central gain with degraded neural synchrony in the auditory brainstem of mice and humans.

Aging is associated with auditory nerve (AN) functional deficits and decreased inhibition in the central auditory system, amplifying central responses in a process referred to here as central gain. Although central gain increases response amplitudes, central gain may not restore disrupted response timing. In this translational study, we measured responses putatively generated by the AN and auditory midbrain in younger and older mice and humans. We hypothesized that older mice and humans exhibit increased central gain without an improvement in inter-trial synchrony in the midbrain. Our data demonstrated greater age-related deficits in AN response amplitudes than auditory midbrain response amplitudes, as shown by significant interactions between inferred neural generator and age group, indicating increased central gain in auditory midbrain. However, synchrony decreases with age in both the AN and midbrain responses. These results reveal age-related increases in central gain without concomitant improvements in synchrony, consistent with those predictions based on decreases in inhibition. Persistent decreases in synchrony may contribute to auditory processing deficits in older mice and humans.

Evidence for Loss of Activity in Low-Spontaneous-Rate Auditory Nerve Fibers of Older Adults.

Auditory function declines with age, as evidenced by communication difficulties in challenging listening environments for older adults. Declining auditory function may arise, in part, from an age-related loss and/or inactivity of low-spontaneous-rate (SR) auditory nerve (AN) fibers, a subgroup of neurons important for suprathreshold processing. Compared to high-SR fibers, low-SR fibers take longer to recover from prior stimulation. Taking advantage of this difference, the forward-masked recovery function paradigm estimates the relative proportions of low- and high-SR fibers in the AN by quantifying the time needed for AN responses to recover from prior stimulation (ΔTrecovery). Due to the slower recovery of low-SR fibers, ANs that need more time to fully recover (longer ΔTrecovery) are estimated to have a larger proportion of low-SR fibers than ANs that need less time (shorter ΔTrecovery). To test the hypothesis that low-SR fiber activity is reduced in older humans, the current study assessed recovery functions in 32 older and 16 younger adults using the compound action potential. Results show that ΔTrecovery is shorter for older adults than for younger adults, consistent with a theorized age-related loss and/or inactivity of low-SR fibers. ΔTrecovery did not differ between individuals with and without a prior history of noise exposure as assessed by self-report. This study is the first to successfully assess forward-masked recovery functions in both younger and older adults and provides important insights into the structural and functional changes occurring in the AN with increasing age.

Two distinct types of nodes of Ranvier support auditory nerve function in the mouse cochlea.

The auditory nerve (AN) of the inner ear is the primary conveyor of acoustic information from sensory hair cells to the brainstem. Approximately 95% of peripheral AN fibers are myelinated by glial cells. The integrity of myelin and the glial-associated paranodal structures at the node of Ranvier is critical for normal AN activity and axonal survival and function in the central auditory nervous system. However, little is known about the node of Ranvier's spatiotemporal development in the AN, how the aging process (or injury) affects the activity of myelinating glial cells, and how downstream alterations in myelin and paranodal structure contribute to AN degeneration and sensorineural hearing loss. Here, we characterized two types of Ranvier nodes-the axonal node and the ganglion node-in the mouse peripheral AN, and found that they are distinct in several features of postnatal myelination and age-related degeneration. Cellular, molecular, and structure-function correlations revealed that the two node types are each critical for different aspects of peripheral AN function. Neural processing speed and synchrony is associated with the length of the axonal node, while stimulus level-dependent amplitude growth and action potentials are associated with the ganglion node. Moreover, our data indicate that dysregulation of glial cells (e.g., satellite cells) and degeneration of the ganglion node structure are an important new mechanism of age-related hearing loss.

Neural Presbyacusis in Humans Inferred from Age-Related Differences in Auditory Nerve Function and Structure.

A common complaint of older adults is difficulty understanding speech, particularly in challenging listening conditions. Accumulating evidence suggests that these difficulties may reflect a loss and/or dysfunction of auditory nerve (AN) fibers. We used a novel approach to study age-related changes in AN structure and several measures of AN function, including neural synchrony, in 58 older adults and 42 younger adults. AN activity was measured in response to an auditory click (compound action potential; CAP), presented at stimulus levels ranging from 70 to 110 dB pSPL. Poorer AN function was observed for older than younger adults across CAP measures at higher but not lower stimulus levels. Associations across metrics and stimulus levels were consistent with age-related AN disengagement and AN dyssynchrony. High-resolution T2-weighted structural imaging revealed age-related differences in the density of cranial nerve VIII, with lower density in older adults with poorer neural synchrony. Individual differences in neural synchrony were the strongest predictor of speech recognition, such that poorer synchrony predicted poorer recognition of time-compressed speech and poorer speech recognition in noise for both younger and older adults. These results have broad clinical implications and are consistent with an interpretation that age-related atrophy at the level of the AN contributes to poorer neural synchrony and may explain some of the perceptual difficulties of older adults.SIGNIFICANCE STATEMENT Differences in auditory nerve (AN) pathophysiology may contribute to the large variations in hearing and communication abilities of older adults. However, current diagnostics focus largely on the increase in detection thresholds, which is likely because of the absence of indirect measures of AN function in standard clinical test batteries. Using novel metrics of AN function, combined with estimates of AN structure and auditory function, we identified age-related differences across measures that we interpret to represent age-related reductions in AN engagement and poorer neural synchrony. Structure-function associations are consistent with an explanation of AN deficits that arise from age-related atrophy of the AN. Associations between neural synchrony and speech recognition suggest that individual and age-related deficits in neural synchrony contribute to speech recognition deficits.

Early auditory cortical processing predicts auditory speech in noise identification and lipreading.

Individuals typically exhibit better cross-sensory perception following unisensory loss, demonstrating improved perception of information available from the remaining senses and increased cross-sensory use of neural resources. Even individuals with no sensory loss will exhibit such changes in cross-sensory processing following temporary sensory deprivation, suggesting that the brain's capacity for recruiting cross-sensory sources to compensate for degraded unisensory input is a general characteristic of the perceptual process. Many studies have investigated how auditory and visual neural structures respond to within- and cross-sensory input. However, little attention has been given to how general auditory and visual neural processing relates to within and cross-sensory perception. The current investigation examines the extent to which individual differences in general auditory neural processing accounts for variability in auditory, visual, and audiovisual speech perception in a sample of young healthy adults. Auditory neural processing was assessed using a simple click stimulus. We found that individuals with a smaller P1 peak amplitude in their auditory-evoked potential (AEP) had more difficulty identifying speech sounds in difficult listening conditions, but were better lipreaders. The results suggest that individual differences in the auditory neural processing of healthy adults can account for variability in the perception of information available from the auditory and visual modalities, similar to the cross-sensory perceptual compensation observed in individuals with sensory loss.

Audiovisual speech is more than the sum of its parts: Auditory-visual superadditivity compensates for age-related declines in audible and lipread speech intelligibility.

Multisensory input can improve perception of ambiguous unisensory information. For example, speech heard in noise can be more accurately identified when listeners see a speaker's articulating face. Importantly, these multisensory effects can be superadditive to listeners' ability to process unisensory speech, such that audiovisual speech identification is better than the sum of auditory-only and visual-only speech identification. Age-related declines in auditory and visual speech perception have been hypothesized to be concomitant with stronger cross-sensory influences on audiovisual speech identification, but little evidence exists to support this. Currently, studies do not account for the multisensory superadditive benefit of auditory-visual input in their metrics of the auditory or visual influence on audiovisual speech perception. Here we treat multisensory superadditivity as independent from unisensory auditory and visual processing. In the current investigation, older and younger adults identified auditory, visual, and audiovisual speech in noisy listening conditions. Performance across these conditions was used to compute conventional metrics of the auditory and visual influence on audiovisual speech identification and a metric of auditory-visual superadditivity. Consistent with past work, auditory and visual speech identification declined with age, audiovisual speech identification was preserved, and no age-related differences in the auditory or visual influence on audiovisual speech identification were observed. However, we found that auditory-visual superadditivity improved with age. The novel findings suggest that multisensory superadditivity is independent of unisensory processing. As auditory and visual speech identification decline with age, compensatory changes in multisensory superadditivity may preserve audiovisual speech identification in older adults. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Cross-modal transfer of talker-identity learning.

A speech signal carries information about meaning and about the talker conveying that meaning. It is now known that these two dimensions are related. There is evidence that gaining experience with a particular talker in one modality not only facilitates better phonetic perception in that modality, but also transfers across modalities to allow better phonetic perception in the other. This finding suggests that experience with a talker provides familiarity with some amodal properties of their articulation such that the experience can be shared across modalities. The present study investigates if experience with talker-specific articulatory information can also support cross-modal talker learning. In Experiment 1 we show that participants can learn to identify ten novel talkers from point-light and sinewave speech, expanding on prior work. Point-light and sinewave speech also supported similar talker identification accuracies, and similar patterns of talker confusions were found across stimulus types. Experiment 2 showed these stimuli could also support cross-modal talker matching, further expanding on prior work. Finally, in Experiment 3 we show that learning to identify talkers in one modality (visual-only point-light speech) facilitates learning of those same talkers in another modality (auditory-only sinewave speech). These results suggest that some of the information for talker identity takes a modality-independent form.

A multi-metric approach to characterizing mouse peripheral auditory nerve function using the auditory brainstem response.

BACKGROUND: The auditory brainstem response (ABR), specifically wave I, is widely used to noninvasively measure auditory nerve (AN) function. Recent work in humans has introduced novel electrocochleographic measures to comprehensively characterize AN function that emphasize suprathreshold processing and estimate neural synchrony. NEW METHOD: This study establishes new tools for evaluating AN function in vivo in adult mice using tone-evoked ABRs obtained from young-adult CBA/CaJ mice, adapting the approach previously introduced in humans. Six metrics are obtained from ABR wave I at suprathreshold stimulus levels. RESULTS: Change-point analyses show that the metrics' rate of change with stimulus level differs between moderate and high suprathreshold levels, suggesting that this approach can potentially characterize the presence of heterogeneous AN fiber types. COMPARISON WITH EXISTING METHODS: Traditional ABR approaches focus on response thresholds and averaged amplitudes/latencies. In contrast, our multi-metric approach, which uses single-trial data and suprathreshold stimuli, provides novel information and identifies evidence of neural synchrony deficits and changes in the heterogeneity of AN fibers underlying AN behavior. CONCLUSION: The techniques reported here provide a novel tool to assess changes in AN function in vivo in a commonly used animal model. A benchmark of most current hearing research is the transition from animal to human studies. Here we established a translational objective approach, applying methods that were first developed in humans to animals. This approach enables researchers to identify changes in AN function arising from the animal models with well-characterized pathology, and predict similar pathological changes in human AN dysfunction and hearing loss.

Intra- and interhemispheric white matter tract associations with auditory spatial processing: Distinct normative and aging effects.

Declining auditory spatial processing is hypothesized to contribute to the difficulty older adults have detecting, locating, and selecting a talker from among others in noisy listening environments. Though auditory spatial processing has been associated with several cortical structures, little is known regarding the underlying white matter architecture or how age-related changes in white matter microstructure may affect it. The arcuate fasciculus is a target for understanding age-related differences in auditory spatial attention based on normative spatial attention findings in humans. Similarly, animal and human clinical studies suggest that the corpus callosum plays a role in the cross-hemispheric integration of auditory spatial information important for spatial localization and attention. The current investigation used diffusion imaging to examine the extent to which age-group differences in the identification of spatially cued speech were accounted for by individual differences in the white matter microstructure of the right arcuate fasciculus and the corpus callosum. Higher right arcuate and callosal fractional anisotropy (FA) predicted better segregation and identification of spatially cued speech across younger and older listeners. Further, individual differences in callosal microstructure mediated age-group differences in auditory spatial processing. Follow-up analyses suggested that callosal tracts connecting left and right pre-frontal and posterior parietal cortex are particularly important for auditory spatial processing. The results are consistent with previous work in animals and clinical human samples and provide a cortical mechanism to account for age-related deficits in auditory spatial processing. Further, the results suggest that both intrahemispheric and interhemispheric mechanisms are involved in auditory spatial processing.

Sustained envelope periodicity representations are associated with speech-in-noise performance in difficult listening conditions for younger and older adults.

Temporal modulations are an important part of speech signals. An accurate perception of these time-varying qualities of sound is necessary for successful communication. The current study investigates the relationship between sustained envelope encoding and speech-in-noise perception in a cohort of normal-hearing younger (ages 18-30 yr, n = 22) and older adults (ages 55-90+ yr, n = 35) using the subcortical auditory steady-state response (ASSR). ASSRs were measured in response to the envelope of 400-ms amplitude-modulated (AM) tones with 3,000-Hz carrier frequencies and 80-Hz modulation frequencies. AM tones had modulation depths of 0, -4, and -8 dB relative to m = 1 (m = 1, 0.631, and 0.398, respectively). The robustness, strength at modulation frequency, and synchrony of subcortical envelope encoding were quantified via time-domain correlations, spectral amplitude, and phase-locking value, respectively. Speech-in-noise ability was quantified via the QuickSIN test in the 0- and 5-dB signal-to-noise (SNR) conditions. All ASSR metrics increased with increasing modulation depth and there were no effects of age group. ASSR metrics in response to shallow modulation depths predicted 0-dB speech scores. Results demonstrate that sustained amplitude envelope processing in the brainstem relates to speech-in-noise abilities, but primarily in difficult listening conditions at low SNRs. These findings furthermore highlight the utility of shallow modulation depths for studying temporal processing. The absence of age effects in these data demonstrate that individual differences in the robustness, strength, and specificity of subcortical envelope processing, and not age, predict speech-in-noise performance in the most difficult listening conditions.NEW & NOTEWORTHY Failure to correctly understand speech in the presence of background noise is a significant problem for many normal-hearing adults and may impede healthy communication. The relationship between sustained envelope encoding in the brainstem and speech-in-noise perception remains to be clarified. The present study demonstrates that the strength, specificity, and robustness of the brainstem's representations of sustained stimulus periodicity relates to speech-in-noise perception in older and younger normal-hearing adults, but only in highly challenging listening environments.

Time-Compressed Speech Identification Is Predicted by Auditory Neural Processing, Perceptuomotor Speed, and Executive Functioning in Younger and Older Listeners.

Older adults typically have difficulty identifying speech that is temporally distorted, such as reverberant, accented, time-compressed, or interrupted speech. These difficulties occur even when hearing thresholds fall within a normal range. Auditory neural processing speed, which we have previously found to predict auditory temporal processing (auditory gap detection), may interfere with the ability to recognize phonetic features as they rapidly unfold over time in spoken speech. Further, declines in perceptuomotor processing speed and executive functioning may interfere with the ability to track, access, and process information. The current investigation examined the extent to which age-related differences in time-compressed speech identification were predicted by auditory neural processing speed, perceptuomotor processing speed, and executive functioning. Groups of normal-hearing (up to 3000 Hz) younger and older adults identified 40, 50, and 60 % time-compressed sentences. Auditory neural processing speed was defined as the P1 and N1 latencies of click-induced auditory-evoked potentials. Perceptuomotor processing speed and executive functioning were measured behaviorally using the Connections Test. Compared to younger adults, older adults exhibited poorer time-compressed speech identification and slower perceptuomotor processing. Executive functioning, P1 latency, and N1 latency did not differ between age groups. Time-compressed speech identification was independently predicted by P1 latency, perceptuomotor processing speed, and executive functioning in younger and older listeners. Results of model testing suggested that declines in perceptuomotor processing speed mediated age-group differences in time-compressed speech identification. The current investigation joins a growing body of literature suggesting that the processing of temporally distorted speech is impacted by lower-level auditory neural processing and higher-level perceptuomotor and executive processes.

The role of speech fidelity in the irrelevant sound effect: Insights from noise-vocoded speech backgrounds.

The Irrelevant Sound Effect (ISE) is the finding that background sound impairs accuracy for visually presented serial recall tasks. Among various auditory backgrounds, speech typically acts as the strongest distractor. Based on the changing-state hypothesis, speech is a disruptive background because it is more complex than other nonspeech backgrounds. In the current study, we evaluate an alternative explanation by examining whether the speech-likeness of the background (speech fidelity) contributes, beyond signal complexity, to the ISE. We did this by using noise-vocoded speech as a background. In Experiment 1, we varied the complexity of the background by manipulating the number of vocoding channels. Results indicate that the ISE increases with the number of channels, suggesting that more complex signals produce greater ISEs. In Experiment 2, we varied complexity and speech fidelity independently. At each channel level, we selectively reversed a subset of channels to design a low-fidelity signal that was equated in overall complexity. Experiment 2 results indicated that speech-like noise-vocoded speech produces a larger ISE than selectively reversed noise-vocoded speech. Finally, in Experiment 3, we evaluated the locus of the speech-fidelity effect by assessing the distraction produced by these stimuli in a missing-item task. In this task, even though noise-vocoded speech disrupted task performance relative to silence, neither its complexity nor speech fidelity contributed to this effect. Together, these findings indicate a clear role for speech fidelity of the background beyond its changing-state quality and its attention capture potential.

Reliability of Measures of N1 Peak Amplitude of the Compound Action Potential in Younger and Older Adults.

Purpose: Human auditory nerve (AN) activity estimated from the amplitude of the first prominent negative peak (N1) of the compound action potential (CAP) is typically quantified using either a peak-to-peak measurement or a baseline-corrected measurement. However, the reliability of these 2 common measurement techniques has not been evaluated but is often assumed to be relatively poor, especially for older adults. To address this question, the current study (a) compared test-retest reliability of these 2 methods and (b) tested the extent to which measurement type affected the relationship between N1 amplitude and experimental factors related to the stimulus (higher and lower intensity levels) and participants (younger and older adults). Method: Click-evoked CAPs were recorded in 24 younger (aged 18-30 years) and 20 older (aged 55-85 years) adults with clinically normal audiograms up to 3000 Hz. N1 peak amplitudes were estimated from peak-to-peak measurements (from N1 to P1) and baseline-corrected measurements for 2 stimulus levels (80 and 110 dB pSPL). Baseline-corrected measurements were made with 4 baseline windows. Each stimulus level was presented twice, and test-retest reliability of these 2 measures was assessed using the intraclass correlation coefficient. Linear mixed models were used to evaluate the extent to which age group and click level uniquely predicted N1 amplitude and whether the predictive relationships differed between N1 measurement techniques. Results: Both peak-to-peak and baseline-corrected measurements of N1 amplitude were found to have good-to-excellent reliability, with intraclass correlation coefficient values > 0.60. As expected, N1 amplitudes were significantly larger for younger participants compared with older participants for both measurement types and were significantly larger in response to clicks presented at 110 dB pSPL than at 80 dB pSPL for both measurement types. Furthermore, the choice of baseline window had no significant effect on N1 amplitudes using the baseline-corrected method. Conclusions: Our results suggest that measurements of AN activity can be robustly and reliably recorded in both younger and older adults using either peak-to-peak or baseline-corrected measurements of the N1 of the CAP. Peak-to-peak measurements yield larger N1 response amplitudes and are the default measurement type for many clinical systems, whereas baseline-corrected measurements are computationally simpler. Furthermore, the relationships between AN activity and stimulus- and participant-related variables were not affected by measurement technique, which suggests that these relationships can be compared across studies using different techniques for measuring the CAP N1.

Complementary metrics of human auditory nerve function derived from compound action potentials.

Declines in auditory nerve (AN) function contribute to suprathreshold auditory processing and communication deficits in individuals with normal hearing, hearing loss, hyperacusis, and tinnitus. Procedures to characterize AN loss or dysfunction in humans are limited. We report several novel complementary metrics using the compound action potential (CAP), a direct measure of summated AN activity. Together, these metrics may be used to characterize AN function noninvasively in humans. We examined how these metrics change with stimulus intensity and interpreted these changes within a framework of known physiological properties of the basilar membrane and AN. Our results reveal how neural synchrony and the recruitment of AN fibers with longer first-spike latencies likely contribute to the CAP, affect auditory processing, and differ with noise exposure history in younger adults with normal pure-tone thresholds. Moving forward, this new battery of metrics provides a crucial step toward new diagnostics of AN function in humans. NEW & NOTEWORTHY Loss or inactivity of auditory nerve (AN) fibers is thought to contribute to suprathreshold auditory processing deficits, but evidence-based methods to assess these effects are not available. We describe several novel metrics that together may be used to quantify neural synchrony and characterize AN function in humans.

Influences of selective adaptation on perception of audiovisual speech.

Research suggests that selective adaptation in speech is a low-level process dependent on sensory-specific information shared between the adaptor and test-stimuli. However, previous research has only examined how adaptors shift perception of unimodal test stimuli, either auditory or visual. In the current series of experiments, we investigated whether adaptation to cross-sensory phonetic information can influence perception of integrated audio-visual phonetic information. We examined how selective adaptation to audio and visual adaptors shift perception of speech along an audiovisual test continuum. This test-continuum consisted of nine audio-/ba/-visual-/va/ stimuli, ranging in visual clarity of the mouth. When the mouth was clearly visible, perceivers "heard" the audio-visual stimulus as an integrated "va" percept 93.7% of the time (e.g., McGurk & MacDonald, 1976). As visibility of the mouth became less clear across the nine-item continuum, the audio-visual "va" percept weakened, resulting in a continuum ranging in audio-visual percepts from /va/ to /ba/. Perception of the test-stimuli was tested before and after adaptation. Changes in audiovisual speech perception were observed following adaptation to visual-/va/ and audiovisual-/va/, but not following adaptation to auditory-/va/, auditory-/ba/, or visual-/ba/. Adaptation modulates perception of integrated audio-visual speech by modulating the processing of sensory-specific information. The results suggest that auditory and visual speech information are not completely integrated at the level of selective adaptation.