Preservation of amplitude modulation coding in the presence of background noise by chinchilla auditory-nerve fibers

Reduction in sound discrimination in noise is related to envelope similarity and not to a decrease in envelope tracking abilities

Humans and animals constantly face challenging acoustic environments, such as various background noises, that impair the detection, discrimination and identification of behaviourally relevant sounds. Here, we disentangled the role of temporal envelope tracking in the reduction in neuronal and behavioural discrimination between communication sounds in situations of acoustic degradations. By collecting neuronal activity from six different levels of the auditory system, from the auditory nerve up to the secondary auditory cortex, in anaesthetized guinea-pigs, we found that tracking of slow changes of the temporal envelope is a general functional property of auditory neurons for encoding communication sounds in quiet conditions and in adverse, challenging conditions. Results from a go/no-go sound discrimination task in mice support the idea that the loss of distinct slow envelope cues in noisy conditions impacted the discrimination performance. Together, these results suggest that envelope tracking is potentially a universal mechanism operating in the central auditory system, which allows the detection of any between-stimulus difference in the slow envelope and thus copes with degraded conditions. KEY POINTS: In quiet conditions, envelope tracking in the low amplitude modulation range (<20 Hz) is correlated with the neuronal discrimination between communication sounds as quantified by mutual information from the cochlear nucleus up to the auditory cortex. At each level of the auditory system, auditory neurons retain their abilities to track the communication sound envelopes in situations of acoustic degradation, such as vocoding and the addition of masking noises up to a signal-to-noise ratio of -10 dB. In noisy conditions, the increase in between-stimulus envelope similarity explains the reduction in both behavioural and neuronal discrimination in the auditory system. Envelope tracking can be viewed as a universal mechanism that allows neural and behavioural discrimination as long as the temporal envelope of communication sounds displays some differences.

Association Analysis of Candidate Gene Polymorphisms and Tinnitus in Young Musicians

INTRODUCTION

Subjective tinnitus, a perception of phantom sound, is a common otological condition that affects almost 15% of the general population. It is known that noise-induced hearing loss (NIHL) and tinnitus exhibit a high level of comorbidity in individuals exposed to intense noise and music. However, the influence of genetic variants associated with NIHL on tinnitus remains elusive. We hypothesized that young musicians carrying genetic variants associated with NIHL would exhibit a higher prevalence of tinnitus than their counterparts.

METHODS

To test this hypothesis, we analyzed the database by Bhatt et al. (2020) (originally developed by Phillips et al., 2015) that investigated the genetic links to NIHL in young college-aged musicians. The present study identified 186 participants (average age = 20.3 yrs, range = 18-25 yrs) with normal tympanometry and otoscopic findings and with no missing data. We included 19 single nucleotide polymorphisms in 13 cochlear genes that were previously associated with NIHL. The candidate genes include: KCNE1, KCNQ1, CDH23, GJB2, GJB4, KCNJ10, CAT, HSP70, PCDH70, MYH14, GRM7, PON2, and ESRRB.

RESULTS

We find that individuals with at least one minor allele of rs163171 (C > T) in KCNQ1 exhibit significantly higher odds of reporting tinnitus compared to individuals carrying the major allele of rs163171. KCNE1 rs2070358 revealed a suggestive association (p = 0.049) with tinnitus, but the FDR corrected p-value did not achieve statistical significance (p < 0.05). A history of ear infection and sound level tolerance showed a statistically significant association with tinnitus. Music exposure showed a suggestive association trend with tinnitus. Biological sex revealed a statistically significant association with distortion product otoacoustic emissions SNR measures.

CONCLUSIONS

We concluded that KCNQ1/KCNE1 voltage-gated potassium ion channel plays a critical role in the pathogenesis of NIHL and tinnitus. Further research is required to construct clinical tools for identifying genetically predisposed individuals well before they acquire NIHL and tinnitus.

Age-Dependent Auditory Processing Deficits after Cochlear Synaptopathy Depend on Auditory Nerve Latency and the Ability of the Brain to Recruit LTP/BDNF

Age-related decoupling of auditory nerve fibers from hair cells (cochlear synaptopathy) has been linked to temporal processing deficits and impaired speech recognition performance. The link between both is elusive. We have previously demonstrated that cochlear synaptopathy, if centrally compensated through enhanced input/output function (neural gain), can prevent age-dependent temporal discrimination loss. It was also found that central neural gain after acoustic trauma was linked to hippocampal long-term potentiation (LTP) and upregulation of brain-derived neurotrophic factor (BDNF). Using middle-aged and old BDNF-live-exon-visualization (BLEV) reporter mice we analyzed the specific recruitment of LTP and the activity-dependent usage of Bdnf exon-IV and -VI promoters relative to cochlear synaptopathy and central (temporal) processing. For both groups, specimens with higher or lower ability to centrally compensate diminished auditory nerve activity were found. Strikingly, low compensating mouse groups differed from high compensators by prolonged auditory nerve latency. Moreover, low compensators exhibited attenuated responses to amplitude-modulated tones, and a reduction of hippocampal LTP and Bdnf transcript levels in comparison to high compensators. These results suggest that latency of auditory nerve processing, recruitment of hippocampal LTP, and Bdnf transcription, are key factors for age-dependent auditory processing deficits, rather than cochlear synaptopathy or aging per se.

Hidden Hearing Loss: A Disorder with Multiple Etiologies and Mechanisms

Hidden hearing loss (HHL), a recently described auditory disorder, has been proposed to affect auditory neural processing and hearing acuity in subjects with normal audiometric thresholds, particularly in noisy environments. In contrast to central auditory processing disorders, HHL is caused by defects in the cochlea, the peripheral auditory organ. Noise exposure, aging, ototoxic drugs, and peripheral neuropathies are some of the known risk factors for HHL. Our knowledge of the causes and mechanisms of HHL are based primarily on animal models. However, recent clinical studies have also shed light on the etiology and prevalence of this cochlear disorder and how it may affect auditory perception in humans. Here, we review the current knowledge regarding the causes and cellular mechanisms of HHL, summarize information on available noninvasive tests for differential diagnosis, and discuss potential therapeutic approaches for treatment of HHL.

The chinchilla animal model for hearing science and noise-induced hearing loss

The chinchilla animal model for noise-induced hearing loss has an extensive history spanning more than 50 years. Many behavioral, anatomical, and physiological characteristics of the chinchilla make it a valuable animal model for hearing science. These include similarities with human hearing frequency and intensity sensitivity, the ability to be trained behaviorally with acoustic stimuli relevant to human hearing, a docile nature that allows many physiological measures to be made in an awake state, physiological robustness that allows for data to be collected from all levels of the auditory system, and the ability to model various types of conductive and sensorineural hearing losses that mimic pathologies observed in humans. Given these attributes, chinchillas have been used repeatedly to study anatomical, physiological, and behavioral effects of continuous and impulse noise exposures that produce either temporary or permanent threshold shifts. Based on the mechanistic insights from noise-exposure studies, chinchillas have also been used in pre-clinical drug studies for the prevention and rescue of noise-induced hearing loss. This review paper highlights the role of the chinchilla model in hearing science, its important contributions, and its advantages and limitations.

Periodotopy in the gerbil inferior colliculus: local clustering rather than a gradient map

Periodicities in sound waveforms are widespread, and shape important perceptual attributes of sound including rhythm and pitch. Previous studies have indicated that, in the inferior colliculus (IC), a key processing stage in the auditory midbrain, neurons tuned to different periodicities might be arranged along a periodotopic axis which runs approximately orthogonal to the tonotopic axis. Here we map out the topography of frequency and periodicity tuning in the IC of gerbils in unprecedented detail, using pure tones and different periodic sounds, including click trains, sinusoidally amplitude modulated (SAM) noise and iterated rippled noise. We found that while the tonotopic map exhibited a clear and highly reproducible gradient across all animals, periodotopic maps varied greatly across different types of periodic sound and from animal to animal. Furthermore, periodotopic gradients typically explained only about 10% of the variance in modulation tuning between recording sites. However, there was a strong local clustering of periodicity tuning at a spatial scale of ca. 0.5 mm, which also differed from animal to animal.

A neural mechanism of phase-locked responses to sinusoidally amplitude-modulated signals in the inferior colliculus

The central nucleus of the inferior colliculus (ICc) is an auditory region that receives convergent inputs from a large number of lower auditory nuclei. ICc neurons phase-lock to low frequencies of sinusoidally amplitude-modulated (SAM) signals but have a different mechanism in the phase-locking from that in neurons of lower nuclei. In the mustached bat, the phase-locking ability in lower nuclei is created by the coincidence of phase-locked excitatory and inhibitory inputs that have slightly different latencies. In contrast, the phase-locking property of ICc neurons is little influenced by the blocking of inhibitory synapses. Moreover, ICc neurons exhibit different characteristics in the spike patterns and synchronicity, classified here by three types of ICc neurons, or sustained, onset, and non-onset phase-locking neurons. However it remains unclear how ICc neurons create the phase-locking ability and the different characteristics. To address this issue, we developed a model of ICc neuronal population. Using this model, we show that the phase-locking ability of ICc neurons to low SAM frequencies is created by an intrinsic membrane property of ICc neuron, limited by inhibitory ion channels. We also show that response characteristics of the three types of neurons arise from the difference in an inhibitory effect sensitive to SAM frequencies. Our model reproduces well the experimental results observed in the mustached bat. These findings provide necessary conditions of how ICc neurons can give rise to the phase-locking ability and characteristic responses to low SAM frequencies.

No longer falling on deaf ears: mechanisms of degeneration and regeneration of cochlear ribbon synapses

Cochlear ribbon synapses are required for the rapid and precise neural transmission of acoustic signals from inner hair cells to the spiral ganglion neurons. Emerging evidence suggests that damage to these synapses represents an important form of cochlear neuropathy that might be highly prevalent in sensorineural hearing loss. In this review, we discuss our current knowledge on how ribbon synapses are damaged by noise and during aging, as well as potential strategies to promote ribbon synapse regeneration for hearing restoration.

Maintaining acoustic communication at a cocktail party: heterospecific masking noise improves signal detection through frequency separation

We examined acoustic masking in a chirping katydid species of the Mecopoda elongata complex due to interference with a sympatric Mecopoda species where males produce continuous trills at high amplitudes. Frequency spectra of both calling songs range from 1 to 80 kHz; the chirper species has more energy in a narrow frequency band at 2 kHz and above 40 kHz. Behaviourally, chirper males successfully phase-locked their chirps to playbacks of conspecific chirps under masking conditions at signal-to-noise ratios (SNRs) of -8 dB. After the 2 kHz band in the chirp had been equalised to the level in the masking trill, the breakdown of phase-locked synchrony occurred at a SNR of +7 dB. The remarkable receiver performance is partially mirrored in the selective response of a first-order auditory interneuron (TN1) to conspecific chirps under these masking conditions. However, the selective response is only maintained for a stimulus including the 2 kHz component, although this frequency band has no influence on the unmasked TN1 response. Remarkably, the addition of masking noise at 65 dB sound pressure level (SPL) to threshold response levels of TN1 for pure tones of 2 kHz enhanced the sensitivity of the response by 10 dB. Thus, the spectral dissimilarity between masker and signal at a rather low frequency appears to be of crucial importance for the ability of the chirping species to communicate under strong masking by the trilling species. We discuss the possible properties underlying the cellular/synaptic mechanisms of the 'novelty detector'.

Efferent synapses return to inner hair cells in the aging cochlea

Efferent innervation of the cochlea undergoes extensive modification early in development, but it is unclear if efferent synapses are modified by age, hearing loss, or both. Structural alterations in the cochlea affecting information transfer from the auditory periphery to the brain may contribute to age-related hearing deficits. We investigated changes to efferent innervation in the vicinity of inner hair cells (IHCs) in young and old C57BL/6 mice using transmission electron microscopy to reveal increased efferent innervation of IHCs in older animals. Efferent contacts on IHCs contained focal presynaptic accumulations of small vesicles. Synaptic vesicle size and shape were heterogeneous. Postsynaptic cisterns were occasionally observed. Increased IHC efferent innervation was associated with a smaller number of afferent synapses per IHC, increased outer hair cell loss, and elevated auditory brainstem response thresholds. Efferent axons also formed synapses on afferent dendrites but with a reduced prevalence in older animals. Age-related reduction of afferent activity may engage signaling pathways that support the return to an immature state of efferent innervation of the cochlea.

Chronic reduction of endocochlear potential reduces auditory nerve activity: further confirmation of an animal model of metabolic presbyacusis

Gerbils aged in quiet show a decline of the endocochlear potential (EP) and elevated auditory nerve compound action potential (CAP) thresholds. However, establishing a direct relationship between an age-related reduction in the EP and changes in the activities of primary auditory neurons is difficult owing to the complexity of age-related histological changes in the cochlea. To address this issue, we developed a young gerbil model of "metabolic" presbyacusis that uses an osmotic pump to deliver furosemide into the round window niche for 7 days, resulting in a chronically reduced EP. In this model, the only major histopathologic changes were restricted to the hook region of the cochlea and consisted of loss of strial intermediate cells and massive edema in the lateral wall. The morphological and physiological evidence suggests that the cochlea can adapt to furosemide application over time. The morphology of spiral ganglion cells and hair cells appeared normal throughout the cochlea. CAP responses and EP values in this model are similar to those of quiet-aged ears. The spontaneous activity of single auditory fibers (n = 188) was assessed in 15 young gerbils treated with furosemide for 7 days. The percentage of recorded low-spontaneous rate (SR) fibers at characteristic frequencies (CFs) > or = 6 kHz was significantly lower in furosemide-treated than in control ears. Recovery function tests of CAP responses after prior stimulation also showed a decline in activity of the low-SR population with CFs > or = 6 kHz in the treated cochleas. A similar loss in the activity of low-SR fiber has been previously shown in quiet-aged gerbils. These results suggest that dysfunction of the cochlear lateral wall and subsequent chronic reduction in the EP can directly affect the activity patterns of primary auditory neurons in a manner similar to that seen in aged gerbils.

Impact of synaptic depression on spike timing at the endbulb of Held

Many synapses show short-term depression, but it is not well understood what functional purpose depression serves and whether its effects are beneficial or detrimental to information processing. We study this issue at the synapse made by auditory-nerve (AN) fibers onto bushy cells (BCs) of the cochlear nucleus, called the "endbulb of Held." AN fibers carry timing information about sounds and converge on BCs, which relay timing information to brain areas responsible for sound localization. Dynamic-clamp recordings of BCs in mouse brain slices indicated that nonphase-locked inputs influenced the contribution of phase-locked inputs when all inputs had equal strength. We evaluated whether this situation depended on activity-dependent synaptic plasticity. Voltage-clamp recordings indicated that the amount of depression varied over the population of endbulbs, but sibling endbulbs terminating on the same BC had similar plasticity. We tested the effects of endbulb depression on BC spiking using dynamic clamp. Under most conditions, increasing depression led to lower probability of BC spiking. However, the effects on spike timing were highly context dependent. When all inputs carried uniform timing information, depression indirectly affected BC spike precision, by determining how many inputs were required to cross threshold. Earlier work has indicated that this interacts with the temporal distribution of inputs to determine BC spike precision. When inputs carried different timing information, depression greatly improved BC precision by suppressing highly active inputs carrying little phase-locked information. These data suggest that endbulbs with different depression characteristics could produce BCs that enhance response probability or timing under different stimulus conditions.

Dynamic encoding of amplitude-modulated sounds at the level of auditory nerve fibers

The relationship between the mean spike frequency of auditory nerve fibers and the levels of tonal stimuli with frequencies equal to the characteristic frequency can be presented as the "input-output" characteristic. In real auditory nerve fibers, the slope of this characteristic increases and its width decreases with increases in the level of spontaneous activity or the ability of fibers to generate spikes in the absence of a stimulus. However, real fibers with low spontaneous activity reproduce the amplitude modulation of sinusoidally amplitude-modulated signal significantly better than fibers with high levels of spontaneous activity. The simulation experiment reported here shows that the reason for the good reproduction of amplitude modulation in auditory nerve fibers is not the static profiles of the "input-output" characteristic but the dynamic properties of fibers which support the tuning (adaptation) of the threshold of the fiber to the level of the stimulus being applied. Because of their steep "input-output" characteristic, auditory nerve fibers with high levels of spontaneous activity can reproduce the modulation of sounds at subthreshold levels when a weak noise is added to the signal, thus demonstrating the property of stochastic resonance.

Age reduces response latency of mouse inferior colliculus neurons to AM sounds

Age and stimulus rise time (RT) effects on response latency were investigated for inferior colliculus (IC) neurons in young-adult and old CBA mice. Single-unit responses were recorded to unmodulated and sinusoidal amplitude modulated (SAM) broadband noise carriers, presented at 35 to 80 dB SPL. Data from 63 young-adult and 76 old phasic units were analyzed to identify the time interval between stimulus onset and driven-response onset (latency). When controlling for stimulus sound level and AM frequency, significant age-related changes in latency were identified. Absolute latency decreased with age at all stimulus AM frequencies, significantly so for equivalent rise times (RT) < or = 12.5 ms. The linear correlation of latency with AM stimulus RT was significant for both young-adult and old units, and increased significantly with age. It is likely that both the decrease in absolute latency and the increase in latency/RT correlation with age are consistent with a reduction of inhibitory drive with age in the IC. These latency changes will result in age-related timing variations in brainstem responses to stimulus onsets, and therefore affect the encoding of complex sounds.

Glutamatergic and GABAergic regulation of neural responses in inferior colliculus to amplitude-modulated sounds

Recordings were made from single neurons in the rat inferior colliculus in response to sinusoidally amplitude-modulated sounds (10-s duration) presented to the contralateral ear. Neural responses were determined for different rates of modulation (0.5 Hz to 1 kHz) at a depth of 100%, and modulation transfer functions were generated based on firing rate (MTFFR) and vector strength (MTFVS). The effects of AMPA, NMDA, and GABAA receptor antagonists were examined by releasing drugs iontophoretically through a multibarrel pipette attached to a single-barrel recording pipette. Both the AMPA receptor antagonist, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX), and the NMDA receptor antagonist, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) resulted in a decrease in firing rate, and the GABAA receptor antagonist, bicuculline, produced an increase in the firing rate in most of the cells examined. In some cases, the shape of the MTFFR was modified slightly by receptor antagonists, but in most cases, the peak firing rate that determined a neuron's best modulation frequency remained the same. Also there were no changes during delivery of either excitatory or inhibitory antagonists in the maximum response synchrony at the peak of the MTFVS although some changes were noticed at off-peak modulation rates particularly with the AMPA receptor antagonist, NBQX.

Non-linear response to amplitude-modulated waves in the apical turn of the guinea pig cochlea

Mechanical vibrations of the Hensen's cells were measured in the apical turn of the cochlea in living guinea pigs, in response to amplitude-modulated (AM) sound. The FFT of the input wave consisted of spectral components at the carrier frequency C and two sidebands (C+/-M) separated from the carrier by the modulation frequency M. The FFT of the velocity response consisted of components at: (i) the modulation frequency M, and harmonics n M; (ii) Carrier frequency C and sidebands (C+/-n M); (iii) harmonics of the carrier frequency and their side bands (2C+/-n M); (3C+/-n M); (4C+/-n M); em leader n=1,2,3, em leader,10. The carrier and the first pair of side bands were broadly tuned and nearly linear. Other components were sharply tuned and highly non-linear, suggesting a different origin. Evidence is presented that these components are generated in the non-linear stereocilia dynamics. An important function of this non-linearity is to demodulate the AM wave to extract information contained in the modulation.

Addition of noise enhances neural synchrony to amplitude-modulated sounds in the frog's midbrain

The ability of 109 single units in the midbrain acoustic centre of frogs (Rana ridibunda, Rana temporaria) to reproduce 10%, 20 Hz sinusoidal amplitude modulation of a long-duration characteristic frequency tone was studied. The sinusoidal modulation was presented either in isolation or summed with a low-frequency (0-50 Hz) noise. Recordings were obtained in the adapted state. The magnitude of the 20 Hz periodic response component was estimated by means of the synchronisation coefficient and the amplitude of the sine modulation of the instantaneous spike rate. In many units, addition of noise modulation produced considerable enhancement of both the mean discharge rate and the discharge rate synchronised to the 20 Hz amplitude modulation. This enhancement phenomenon is interpreted in the context of stochastic resonance theory.

Endocochlear potentials and compound action potential recovery: functions in the C57BL/6J mouse

The C57BL/6J mouse suffers from cochlear degeneration beginning at an early age and has been used as a model of age-related hearing loss (presbyacusis). Here, the endocochlear potential (EP) and compound action potential (CAP) responses were determined in one-, four-, 12- and 24-month-old C57BL/6J mice. CAP measures included thresholds to tone pips, input/output (I/O) functions, and recovery functions to conditioning tones. EP values among the four age groups did not differ significantly (P>0.05) in either the basal or apical turns. CAP thresholds were increased significantly by 10 to 30 dB in the four-month group compared to the one-month controls at 11.3, 16, 20, and 22.6 kHz. CAP I/O functions were shallower in the four-month group compared to controls at all frequencies. In the 12- and 24-month-old mice, CAP responses were absent, despite normal EP values in these animals. Recovery functions after conditioning tones were obtained at 8, 16, 20 and 22.6 kHz; the functions had fast and slow components at all frequencies tested in both the one- and four-month-old groups. The corresponding recovery curves were identical for both age groups, even with significant threshold shifts in the older group. The two component recovery curves provide the first physiological evidence that different spontaneous rate (SR) classes of auditory neurons exist in the C57BL/6J mouse. Moreover, the unchanged recovery functions in the older group suggest that there was no loss of activity of the low-SR fiber population with age under conditions where the EP remains stable, in contrast to the gerbil model of presbyacusis where there is a loss of low-SR fiber activity and EP does decline with age.

Age-related alterations in the neural coding of envelope periodicities

This research was guided by the working hypothesis that the aging auditory system progressively loses its ability to process rapid acoustic transients efficiently, and in elderly listeners, this results in difficulties in speech perception. Neural correlates of age-related deficits in temporal processing were investigated by recording from inferior colliculus (IC) neurons from young adult and old CBA mice. Single-unit responses were recorded to sinusoidally amplitude-modulated (SAM) noise carriers, presented at 65-80 dB SPL, having modulation frequencies (MFs) that ranged from 10 to 800 Hz. Because phasic-type temporal response patterns dominate responses to tone and noise in mammalian IC, we limited our analyses to only phasic units. Modulation transfer functions (MTF) for both rate (rMTF) and synchronization (sMTF) measures were used to derive respective best modulation frequencies (rBMF and sBMF). The main age-related finding was that there was an overall increase in response rate to SAM noise carriers and a decrease in the median upper cutoff frequency in units from old mice. At rBMF, the median spike count from units from old animals was 1.63 times greater, and at the sBMF, the median spike count was 2.29 times greater than the young adult sample. We explored whether the increase in driven activity was due to a change in the transient (first cycle response) or periodic (remaining response) component of the response to SAM noise. Median spike counts of the transient component decreased with increasing MF for both young adult and old units, with median counts consistently greater in the old sample as compared with young. Median spike counts for the periodic response remained relatively constant as a function of MF; however, there was a significantly greater (3 times) response for older units in a restricted range of MFs. The greater median spike counts found for the transient and periodic response was also evident when we analyzed the cycle-by-cycle response. The magnitude of the differences between the young adult and the old spike median responses was greatest at low MFs and then declined as MF increased. Finally, the young adult distribution of rBMFs extends to higher MFs than the old, with 36.0% of units having rBMFs >100 Hz as compared with only 12.5% of the old unit sample. We postulate that this age-related difference in rate coding of SAM noise carriers is consistent with a loss, or imbalance, of excitatory and inhibitory neural mechanisms known to shape encoding of envelope periodicities in the IC.

Effects of noise and cue enhancement on neural responses to speech in auditory midbrain, thalamus and cortex

Speech perception depends on the auditory system's ability to extract relevant acoustic features from competing background noise. Despite widespread acknowledgement that noise exacerbates this process, little is known about the neurophysiologic mechanisms underlying the encoding of speech in noise. Moreover, the relative contribution of different brain nuclei to these processes has not been fully established. To address these issues, aggregate neural responses were recorded from within the inferior colliculus, medial geniculate body and over primary auditory cortex of anesthetized guinea pigs to a synthetic vowel-consonant-vowel syllable /ada/ in quiet and in noise. In noise the onset response to the stop consonant /d/ was reduced or eliminated at each level, to the greatest degree in primary auditory cortex. Acoustic cue enhancements characteristic of 'clear' speech (lengthening the stop gap duration and increasing the intensity of the release burst) improved the neurophysiologic representation of the consonant at each level, especially at the cortex. Finally, the neural encoding of the vowel segment was evident at subcortical levels only, and was more resistant to noise than encoding of the dynamic portion of the consonant (release burst and formant transition). This experiment sheds light on which speech-sound elements are poorly represented in noise and demonstrates how acoustic modifications to the speech signal can improve neural responses in a normal auditory system. Implications for understanding neurophysiologic auditory signal processing in children with perceptual impairments and the design of efficient perceptual training strategies are also discussed.

Subcortical neural coding mechanisms for auditory temporal processing

Biologically relevant sounds such as speech, animal vocalizations and music have distinguishing temporal features that are utilized for effective auditory perception. Common temporal features include sound envelope fluctuations, often modeled in the laboratory by amplitude modulation (AM), and starts and stops in ongoing sounds, which are frequently approximated by hearing researchers as gaps between two sounds or are investigated in forward masking experiments. The auditory system has evolved many neural processing mechanisms for encoding important temporal features of sound. Due to rapid progress made in the field of auditory neuroscience in the past three decades, it is not possible to review all progress in this field in a single article. The goal of the present report is to focus on single-unit mechanisms in the mammalian brainstem auditory system for encoding AM and gaps as illustrative examples of how the system encodes key temporal features of sound. This report, following a systems analysis approach, starts with findings in the auditory nerve and proceeds centrally through the cochlear nucleus, superior olivary complex and inferior colliculus. Some general principles can be seen when reviewing this entire field. For example, as one ascends the central auditory system, a neural encoding shift occurs. An emphasis on synchronous responses for temporal coding exists in the auditory periphery, and more reliance on rate coding occurs as one moves centrally. In addition, for AM, modulation transfer functions become more bandpass as the sound level of the signal is raised, but become more lowpass in shape as background noise is added. In many cases, AM coding can actually increase in the presence of background noise. For gap processing or forward masking, coding for gaps changes from a decrease in spike firing rate for neurons of the peripheral auditory system that have sustained response patterns, to an increase in firing rate for more central neurons with transient responses. Lastly, for gaps and forward masking, as one ascends the auditory system, some suppression effects become quite long (echo suppression), and in some stimulus configurations enhancement to a second sound can take place.

Perceptual and motor effects of morphine and buprenorphine in baboons

The effects of morphine and buprenorphine on auditory perceptual discriminations and response latency ("reaction time") in baboons are compared. The task employed synthetic human vowel sounds that are readily generated in the laboratory, and closely approximate natural baboon "grunt" vocalizations [J. Acoust. Soc. Am. 101 (1997) 2951]. Baboons pressed a lever to produce one repeating "standard" vowel, and released the lever only when one of four other "comparison" vowels occasionally occurred in place of the standard vowel. The percentage of correct detections and median reaction time for each comparison were measured following intramuscular drug administrations of morphine (0.01-1.8 mg/kg) and buprenorphine (0.00032-0.032 mg/kg). Both morphine and buprenorphine impaired vowel discriminability, and greater impairments occurred for those comparison vowels that were more similar in formant structure to the standard vowel. Morphine increased reaction time in all baboons, and buprenorphine increased reaction time in two of three baboons. Morphine's perceptual effects occurred within 20-40 min following drug administration; buprenorphine's perceptual effects occurred 50-100 min following drug administration. Morphine and buprenorphine did not differ in the time course of their maximal reaction time effects. The results demonstrate that both morphine and buprenorphine can impair auditory discriminations involving human vowel sounds in baboons, as well as lengthen reaction times to the stimuli.

Temporal representation of iterated rippled noise as a function of delay and sound level in the ventral cochlear nucleus

The discharge patterns of single units in the ventral cochlear nucleus (VCN) of anesthetized guinea pigs were examined in response to iterated rippled noise (IRN) as a function of the IRN delay (which determines the IRN pitch) and the IRN sound level. Delays were varied over five octaves in half-octave steps, and sound levels were varied over a 30- or 50-dB range in steps of 5 dB. Neural responses were analyzed in terms of first-order and all-order inter-spike intervals (ISIs). The IRN quasi-periodicity was preserved in the all-order ISIs for most units independent of unit type or best frequency (BF). A deterioration of the temporal all-order code was found, however, when the neural response was influenced by inhibition. The IRN quasi-periodicity was also preserved in first-order ISIs for a limited range of IRN delays and levels. Sustained Chopper units (CS) in the VCN responded with very regular ISIs when the IRN delay corresponded to the unit's chopping period; i.e., the unit showed an increased proportion of intervals corresponding to the IRN delay (interval enhancement) relative to an equal-level, white-noise stimulation. This interval enhancement has a band-pass characteristic with a peak corresponding to the chopping period. Moreover, for CS units in rate saturation, the chopping period, and thus the interval enhancement to the IRN, did not vary with level. Units classified as onset-chopper also show a band-pass interval enhancement to the IRN stimuli; however, they show more level-dependent changes than CS units. Primary-like (PL) units also show level-dependent changes in their ability to code the IRN pitch in first-order intervals. The range of delays where PL units showed interval enhancement was broader and extended to shorter delays. Based on these findings, it is suggested that CS units may play an important role in pitch processing in that they transform a higher-order interval code into a first-order interval place code. Their limited dynamic range together with the preservation of the temporal stimulus features in saturation may serve as a physiological basis for the perceived level independence of pitch.

Gamma-aminobutyric acidergic and glycinergic inputs shape coding of amplitude modulation in the chinchilla cochlear nucleus

Amplitude modulation is a prominent acoustic feature of biologically relevant sounds, such as speech and animal vocalizations. Enhanced temporal coding of amplitude modulation signals is found in certain dorsal and posteroventral cochlear nucleus neurons when they are compared to auditory nerve. Although mechanisms underlying this improved temporal selectivity are not known, involvement of inhibition has been suggested. gamma-Aminobutyric acid- and glycine-mediated inhibition have been shown to shape the dorsal cochlear nucleus and posteroventral cochlear nucleus response properties to other acoustic stimuli. In the present study, responses to amplitude modulation tones were obtained from chinchilla dorsal cochlear nucleus and posteroventral cochlear nucleus neurons. The amplitude modulation carrier was set to the neuron's characteristic frequency and the modulating frequency varied from 10 Hz. Rate and temporal modulation transfer functions were compared across neurons. Bandpass temporal modulation transfer functions were observed in 74% of the neurons studied. Most cochlear nucleus neurons (90%) displayed flat or lowpass rate modulation transfer functions to amplitude modulation signals presented at 2540 dB (re: characteristic frequency threshold). The role of inhibition in shaping responses to amplitude modulation stimuli was examined using iontophoretic application of glycine or gamma-aminobutyric acidA receptor agonists and antagonists. Blockade of gamma-aminobutyric acidA or glycine receptors increased stimulus-evoked discharge rates for a majority of neurons tested. Synchronization to the envelope was reduced, particularly at low and middle modulating frequencies, with temporal modulation transfer functions becoming flattened and less bandpass in appearance. Application of glycine, gamma-aminobutyric acid or muscimol increased the modulation gain over the low- and mid-modulation frequencies and reduced the discharge rate across envelope frequencies for most neurons tested. These findings support the hypothesis that glycinergic and gamma-aminobutyric acidergic inputs onto certain dorsal cochlear nucleus and posteroventral cochlear nucleus neurons play a role in shaping responses to amplitude modulation stimuli and may be responsible for the reported preservation of amplitude modulation temporal coding in dorsal cochlear nucleus and posteroventral cochlear nucleus neurons at high stimulus intensities or in background noise.

Processing of sinusoidally amplitude modulated signals in the nuclei of the lateral lemniscus of the big brown bat, Eptesicus fuscus

Changes in amplitude are a characteristic feature of most natural sounds, including the biosonar signals used by bats for echolocation. Previous evidence suggests that the nuclei of the lateral lemniscus play an important role in processing timing information that is essential for target range determination in echolocation. Neurons that respond to unmodulated tones with a sustained discharge are found in the dorsal nucleus (DNLL), intermediate nucleus (INLL) and multipolar cell division of the ventral nucleus (VNLLm). These neurons provide a graded response over a broad dynamic range of intensities, and would be expected to provide information about the amplitude envelope of a modulated signal. Neurons that respond only at the onset of a tone make up a small proportion of cells in DNLL, INLL and VNLLm, but are the only type found in the columnar division of the ventral nucleus (VNLLc). Onset neurons in VNLLc maintain a constant latency across a wide range of stimulus frequencies and intensities, thus providing a precise marker for when a sound begins. To determine how these different functional classes of cells respond to amplitude changes, we presented sinusoidally amplitude modulated (SAM) signals monaurally to awake, restrained bats and recorded the responses of single neurons extracellularly. There were clear differences in the ability of neurons in the different cell groups to respond to SAM. In the VNLLm, INLL and DNLL, 90% of neurons responded to SAM with a synchronous discharge. Neurons in the VNLLc responded poorly or not at all to SAM signals. This finding was unexpected given the precise onset responses of VNLLc neurons to unmodulated tones and their ability to respond synchronously to sinusoidally frequency modulated (SFM) signals. Among neurons that responded synchronously to SAM, synchronization as a function of modulation rate described either a bandpass or a lowpass function, with the majority of bandpass functions in neurons that responded to unmodulated tones with a sustained discharge. The maximal modulation rates that elicited synchronous responses were similar for the different cell groups, ranging from 320 Hz in VNLLm to 230 Hz in DNLL. The range of best modulation rates was greater for SAM than for SFM; this was also true of the range of maximal modulation rates at which synchronous discharge occurred. There was little correlation between a neuron's best modulation rate or maximal modulation rate for SAM signals and those for SFM signals, suggesting that responsiveness to amplitude and frequency modulations depends on different neural processing mechanisms.

Neural encoding of single-formant stimuli in the ventral cochlear nucleus of the chinchilla

Responses of the principal unit types in the ventral cochlear nucleus of the chinchilla were studied with a single-formant stimulus set that covered fundamental frequency (f0) from 100 Hz to 200 Hz and formant center frequency (F1) from 256 to 782 Hz. Temporal coding for f0 and F1 was explored for 95 stimulus combinations of f0 (n = 5) and F1 (n = 19) in primarylike, onset and chopper unit categories. Several analyses that explored temporal coding were employed including: autocorrelation, interspike interval analysis, and synchronization to each harmonic of f0. In general, the representation of f0 is better in onset and chopper units than in primarylike units. Nearly all units in the cochlear nucleus showed a gain in phase locking to the envelope (f0) of the single-formant stimulus relative to the auditory nerve. The fundamental is represented directly in neural discharges of units in the cochlear nucleus with an interval code (also Cariani and Delgutte, 1996; Rhode, 1995). The formant is represented in the temporal domain in primarylike units, though some chopper and onset units also possess the ability to code F1 through discharge synchrony. Onset-I units, which are associated with the octopus cells, exhibited the strongest phase locking to f0 of any unit types studied. The representation of f0 and F1 in the temporal domain is weak or absent in some units. All-order-interspike interval distributions computed for populations of units show preservation of temporal coding for both f0 and F1. Results are in agreement with earlier amplitude modulation studies that showed nearly all cochlear nucleus unit types phase lock to the signal envelope better than auditory nerve fibers over a considerable range of signal amplitudes.

Glycinergic and GABAergic inputs affect short-term suppression in the cochlear nucleus

Most cochlear nucleus (CN) neurons exhibit short-term response suppression to a second stimulus in a paired-pulse (click), forward-masking, paradigm. The magnitude of suppression, which appears to be greater than that observed in acoustic nerve, is dependent on the temporal separation and/or relative intensities of the two stimuli. Recent evidence suggests that inhibitory circuitry ending on CN neurons may mediate this response suppression. Using extracellular recordings from single CN neurons, suppression was evaluated using a forward-masking paradigm. Responses to paired acoustic clicks (i.e., a 'masker' followed by an identical 'probe' click) were measured while the time interval between the masker and probe was varied systematically. The role of inhibitory circuitry in forward-masking in the CN was assessed by pharmacologic manipulation of the GABA(A) and glycine(I) (strychnine-sensitive) receptors. Blockade of glycinergic or GABAergic receptors by iontophoretic application of the antagonists, strychnine and bicuculline methiodide, decreased the effects of forward-masking by shortening recovery times of the probe response in 2/3 of the neurons tested. Conversely, agonist application (glycine, and GABA or muscimol) increased the magnitude of suppression and delayed recovery of the probe response relative to control values. These findings suggest that known circuits releasing glycine and/or GABA mediate short-term response suppression in some CN neurons.