Perfidious synaptic transmission in the guinea-pig auditory brainstem.

The presence of 'giant' synapses in the auditory brainstem is thought to be a specialization designed to encode temporal information to support perception of pitch, frequency, and sound-source localisation. These 'giant' synapses have been found in the ventral cochlear nucleus, the medial nucleus of the trapezoid body and the ventral nucleus of the lateral lemniscus. An interpretation of these synapses as simple relays has, however, been challenged by the observation in the gerbil that the action potential frequently fails in the ventral cochlear nucleus. Given the prominence of these synapses it is important to establish whether this phenomenon is unique to the gerbil or can be observed in other species. Here we examine the responses of units, thought to be the output of neurons in receipt of 'giant' synaptic endings, in the ventral cochlear nucleus and the medial nucleus of the trapezoid body in the guinea pig. We found that failure of the action-potential component, recorded from cells in the ventral cochlear nucleus, occurred in ~60% of spike waveforms when recording spontaneous activity. In the medial nucleus of the trapezoid body, we did not find evidence for action-potential failure. In the ventral cochlear nucleus action-potential failures transform the receptive field between input and output of bushy cells. Additionally, the action-potential failures result in "non-primary-like" temporal-adaptation patterns. This is important for computational models of the auditory system, which commonly assume the responses of ventral cochlear nucleus bushy cells are very similar to their "primary like" auditory-nerve-fibre inputs.

Dual Coding of Frequency Modulation in the Ventral Cochlear Nucleus.

Frequency modulation (FM) is a common acoustic feature of natural sounds and is known to play a role in robust sound source recognition. Auditory neurons show precise stimulus-synchronized discharge patterns that may be used for the representation of low-rate FM. However, it remains unclear whether this representation is based on synchronization to slow temporal envelope (ENV) cues resulting from cochlear filtering or phase locking to faster temporal fine structure (TFS) cues. To investigate the plausibility of those encoding schemes, single units of the ventral cochlear nucleus of guinea pigs of either sex were recorded in response to sine FM tones centered at the unit's best frequency (BF). The results show that, in contrast to high-BF units, for modulation depths within the receptive field, low-BF units (<4 kHz) demonstrate good phase locking to TFS. For modulation depths extending beyond the receptive field, the discharge patterns follow the ENV and fluctuate at the modulation rate. The receptive field proved to be a good predictor of the ENV responses for most primary-like and chopper units. The current in vivo data also reveal a high level of diversity in responses across unit types. TFS cues are mainly conveyed by low-frequency and primary-like units and ENV cues by chopper and onset units. The diversity of responses exhibited by cochlear nucleus neurons provides a neural basis for a dual-coding scheme of FM in the brainstem based on both ENV and TFS cues.SIGNIFICANCE STATEMENT Natural sounds, including speech, convey informative temporal modulations in frequency. Understanding how the auditory system represents those frequency modulations (FM) has important implications as robust sound source recognition depends crucially on the reception of low-rate FM cues. Here, we recorded 115 single-unit responses from the ventral cochlear nucleus in response to FM and provide the first physiological evidence of a dual-coding mechanism of FM via synchronization to temporal envelope cues and phase locking to temporal fine structure cues. We also demonstrate a diversity of neural responses with different coding specializations. These results support the dual-coding scheme proposed by psychophysicists to account for FM sensitivity in humans and provide new insights on how this might be implemented in the early stages of the auditory pathway.

Modelling firing regularity in the ventral cochlear nucleus: Mechanisms, and effects of stimulus level and synaptopathy.

The auditory system processes temporal information at multiple scales, and disruptions to this temporal processing may lead to deficits in auditory tasks such as detecting and discriminating sounds in a noisy environment. Here, a modelling approach is used to study the temporal regularity of firing by chopper cells in the ventral cochlear nucleus, in both the normal and impaired auditory system. Chopper cells, which have a strikingly regular firing response, divide into two classes, sustained and transient, based on the time course of this regularity. Several hypotheses have been proposed to explain the behaviour of chopper cells, and the difference between sustained and transient cells in particular. However, there is no conclusive evidence so far. Here, a reduced mathematical model is developed and used to compare and test a wide range of hypotheses with a limited number of parameters. Simulation results show a continuum of cell types and behaviours: chopper-like behaviour arises for a wide range of parameters, suggesting that multiple mechanisms may underlie this behaviour. The model accounts for systematic trends in regularity as a function of stimulus level that have previously only been reported anecdotally. Finally, the model is used to predict the effects of a reduction in the number of auditory nerve fibres (deafferentation due to, for example, cochlear synaptopathy). An interactive version of this paper in which all the model parameters can be changed is available online.

Neural Segregation of Concurrent Speech: Effects of Background Noise and Reverberation on Auditory Scene Analysis in the Ventral Cochlear Nucleus.

Concurrent complex sounds (e.g., two voices speaking at once) are perceptually disentangled into separate "auditory objects". This neural processing often occurs in the presence of acoustic-signal distortions from noise and reverberation (e.g., in a busy restaurant). A difference in periodicity between sounds is a strong segregation cue under quiet, anechoic conditions. However, noise and reverberation exert differential effects on speech intelligibility under "cocktail-party" listening conditions. Previous neurophysiological studies have concentrated on understanding auditory scene analysis under ideal listening conditions. Here, we examine the effects of noise and reverberation on periodicity-based neural segregation of concurrent vowels /a/ and /i/, in the responses of single units in the guinea-pig ventral cochlear nucleus (VCN): the first processing station of the auditory brain stem. In line with human psychoacoustic data, we find reverberation significantly impairs segregation when vowels have an intonated pitch contour, but not when they are spoken on a monotone. In contrast, noise impairs segregation independent of intonation pattern. These results are informative for models of speech processing under ecologically valid listening conditions, where noise and reverberation abound.

Enhancement of forward suppression begins in the ventral cochlear nucleus.

A neuron׳s response to a sound can be suppressed by the presentation of a preceding sound. It has been suggested that this suppression is a direct correlate of the psychophysical phenomenon of forward masking, however, forward suppression, as measured in the responses of the auditory nerve, was insufficient to account for behavioural performance. In contrast the neural suppression seen in the inferior colliculus and auditory cortex was much closer to psychophysical performance. In anaesthetised guinea-pigs, using a physiological two-interval forced-choice threshold tracking algorithm to estimate suppressed (masked) thresholds, we examine whether the enhancement of suppression can occur at an earlier stage of the auditory pathway, the ventral cochlear nucleus (VCN). We also compare these responses with the responses from the central nucleus of the inferior colliculus (ICc) using the same preparation. In both nuclei, onset-type neurons showed the greatest amounts of suppression (16.9-33.5dB) and, in the VCN, these recovered with the fastest time constants (14.1-19.9ms). Neurons with sustained discharge demonstrated reduced masking (8.9-12.1dB) and recovery time constants of 27.2-55.6ms. In the VCN the decrease in growth of suppression with increasing suppressor level was largest for chopper units and smallest for onset-type units. The threshold elevations recorded for most unit types are insufficient to account for the magnitude of forward masking as measured behaviourally, however, onset responders, in both the cochlear nucleus and inferior colliculus demonstrate a wide dynamic range of suppression, similar to that observed in human psychophysics.

Reverberation impairs brainstem temporal representations of voiced vowel sounds: challenging "periodicity-tagged" segregation of competing speech in rooms.

The auditory system typically processes information from concurrently active sound sources (e.g., two voices speaking at once), in the presence of multiple delayed, attenuated and distorted sound-wave reflections (reverberation). Brainstem circuits help segregate these complex acoustic mixtures into "auditory objects." Psychophysical studies demonstrate a strong interaction between reverberation and fundamental-frequency (F0) modulation, leading to impaired segregation of competing vowels when segregation is on the basis of F0 differences. Neurophysiological studies of complex-sound segregation have concentrated on sounds with steady F0s, in anechoic environments. However, F0 modulation and reverberation are quasi-ubiquitous. We examine the ability of 129 single units in the ventral cochlear nucleus (VCN) of the anesthetized guinea pig to segregate the concurrent synthetic vowel sounds /a/ and /i/, based on temporal discharge patterns under closed-field conditions. We address the effects of added real-room reverberation, F0 modulation, and the interaction of these two factors, on brainstem neural segregation of voiced speech sounds. A firing-rate representation of single-vowels' spectral envelopes is robust to the combination of F0 modulation and reverberation: local firing-rate maxima and minima across the tonotopic array code vowel-formant structure. However, single-vowel F0-related periodicity information in shuffled inter-spike interval distributions is significantly degraded in the combined presence of reverberation and F0 modulation. Hence, segregation of double-vowels' spectral energy into two streams (corresponding to the two vowels), on the basis of temporal discharge patterns, is impaired by reverberation; specifically when F0 is modulated. All unit types (primary-like, chopper, onset) are similarly affected. These results offer neurophysiological insights to perceptual organization of complex acoustic scenes under realistically challenging listening conditions.

Can comodulation masking release occur when frequency changes could promote perceptual segregation of the on-frequency and flanking bands?

A common characteristic of natural sounds is that the level fluctuations in different frequency regions are coherent. The ability of the auditory system to use this comodulation is shown when a sinusoidal signal is masked by a masker centred at the signal frequency (on-frequency masker, OFM) and one or more off-frequency components, commonly referred to as flanking bands (FBs). In general, the threshold of the signal masked by comodulated masker components is lower than when masked by masker components with uncorrelated envelopes or in the presence of the OFM only. This effect is commonly referred to as comodulation masking release (CMR). The present study investigates if CMR is also observed for a sinusoidal signal embedded in the OFM when the centre frequencies of the FBs are swept over time with a sweep rate of one octave per second. Both a common change of different frequencies and comodulation could serve as cues to indicate which of the stimulus components originate from one source. If the common fate of frequency components is the stronger binding cue, the sweeping FBs and the OFM with a fixed centre frequency should no longer form one auditory object and the CMR should be abolished. However, psychoacoustical results with normal-hearing listeners show that a CMR is also observed with sweeping components. The results are consistent with the hypothesis of wideband inhibition as the underlying physiological mechanism, as the CMR should only depend on the spectral position of the flanking bands relative to the inhibitory areas (as seen in physiological recordings using stationary flanking bands). Preliminary physiological results in the cochlear nucleus of the Guinea pig show that a correlate of CMR can also be found at this level of the auditory pathway with sweeping flanking bands.

Neurometric amplitude-modulation detection threshold in the guinea-pig ventral cochlear nucleus.

Amplitude modulation (AM) is a pervasive feature of natural sounds. Neural detection and processing of modulation cues is behaviourally important across species. Although most ecologically relevant sounds are not fully modulated, physiological studies have usually concentrated on fully modulated (100% modulation depth) signals. Psychoacoustic experiments mainly operate at low modulation depths, around detection threshold (∼5% AM). We presented sinusoidal amplitude-modulated tones, systematically varying modulation depth between zero and 100%, at a range of modulation frequencies, to anaesthetised guinea-pigs while recording spikes from neurons in the ventral cochlear nucleus (VCN). The cochlear nucleus is the site of the first synapse in the central auditory system. At this locus significant signal processing occurs with respect to representation of AM signals. Spike trains were analysed in terms of the vector strength of spike synchrony to the amplitude envelope. Neurons showed either low-pass or band-pass temporal modulation transfer functions, with the proportion of band-pass responses increasing with increasing sound level. The proportion of units showing a band-pass response varies with unit type: sustained chopper (CS) > transient chopper (CT) > primary-like (PL). Spike synchrony increased with increasing modulation depth. At the lowest modulation depth (6%), significant spike synchrony was only observed near to the unit's best modulation frequency for all unit types tested. Modulation tuning therefore became sharper with decreasing modulation depth. AM detection threshold was calculated for each individual unit as a function of modulation frequency. Chopper units have significantly better AM detection thresholds than do primary-like units. AM detection threshold is significantly worse at 40 dB vs. 10 dB above pure-tone spike rate threshold. Mean modulation detection thresholds for sounds 10 dB above pure-tone spike rate threshold at best modulation frequency are (95% CI) 11.6% (10.0-13.1) for PL units, 9.8% (8.2-11.5) for CT units, and 10.8% (8.4-13.2) for CS units. The most sensitive guinea-pig VCN single unit AM detection thresholds are similar to human psychophysical performance (∼3% AM), while the mean neurometric thresholds approach whole animal behavioural performance (∼10% AM).

Equivalent-rectangular bandwidth of single units in the anaesthetized guinea-pig ventral cochlear nucleus.

Frequency-tuning is a fundamental property of auditory neurons. The filter bandwidth of peripheral auditory neurons determines the frequency resolution of an animal's auditory system. Behavioural studies in animals and humans have defined frequency-tuning in terms of the "equivalent-rectangular bandwidth" (ERB) of peripheral filters. In contrast, most physiological studies report the Q [best frequency/bandwidth] of frequency-tuning curves. This study aims to accurately describe the ERB of primary-like and chopper units in the ventral cochlear nucleus, the first brainstem processing station of the central auditory system. Recordings were made from 1020 isolated single units in the ventral cochlear nucleus of anesthetized guinea pigs in response to pure-tone stimuli which varied in frequency and in sound level. Frequency-threshold tuning curves were constructed for each unit and estimates of the ERB determined using methods previously described for auditory-nerve-fibre data in the same species. Primary-like, primary-notch, and sustained- and transient-chopper units showed frequency selectivity almost identical to that recorded in the auditory nerve. Their tuning at pure-tone threshold can be described as a function of best frequency (BF) by ERB = 0.31 * BF(0.5).

The role of auditory nerve innervation and dendritic filtering in shaping onset responses in the ventral cochlear nucleus.

Neurons in the ventral cochlear nucleus (VCN) that respond primarily at the onset of a pure tone stimulus show diversity in terms of peri-stimulus-time-histograms (PSTHs), rate-level functions, frequency tuning, and also their responses to broad band noise. A number of different mechanisms have been proposed as contributing to the onset characteristic: e.g. coincidence, depolarisation block, and low-threshold potassium currents. We show that a simple point neuron receiving convergent inputs from high-spontaneous rate auditory nerve (AN) fibers, with no special currents and no peri-stimulatory shifts in firing threshold, is sufficient to produce much of the diversity seen experimentally. Three sub-classes of onset PSTHs: onset-ideal (OI), onset-chopper (OC) and onset-locker (OL) are reproduced by variations in innervation patterns and dendritic filtering. The factors shaping responses were explored by systematically varying key parameters. An OI response in this model requires a narrow range of AN input best frequencies (BF) which only produce supra-threshold depolarizations during the stimulus onset. For OC and OL responses, receptive fields were wider. Considerable low pass filtering of AN inputs away from BF results in an OL, whilst relatively unfiltered inputs produce an OC response. Rate-level functions in response to pure tones can be sloping, or plateau. These can be also reproduced in the model by the manipulation of the AN innervation. The model supports the coincidence detection hypothesis, and suggests that differences in excitatory innervation and dendritic filtering constant are important factors to consider when accounting for the variation in response characteristics seen in VCN onset units.

Ambiguous pitch and the temporal representation of inharmonic iterated rippled noise in the ventral cochlear nucleus.

Neural coding of the pitch of complex sounds is vital for animals' ability to communicate and to perceptually organize natural acoustic scenes. Harmonic complex sounds typically have a well defined pitch corresponding to their fundamental frequency, whereas inharmonic sounds can exhibit pitch ambiguity: their pitch can have more than one value. Iterated rippled noise (IRN), a common "pitch stimulus," is generated from broadband noise by a cascade of delay-and-add steps, with the delayed noise phase-shifted by varphi degrees. By varying varphi, the (in)harmonicity, and therefore the pitch ambiguity, of IRN can be manipulated. Recordings were made from single-units in the ventral cochlear nucleus of anesthetized guinea pigs in response to IRN and complex tones, systematically varying the inharmonicity. In their all-order interspike interval distributions, primary-like and chopper units tuned within the phase-locking range of best frequencies represent the waveform temporal fine structure (which varies with varphi). In contrast, those units tuned to higher frequencies represent the temporal-envelope modulation (independent of varphi). We show a temporal representation of ambiguous pitch for IRN and complex tones based on responses to the stimulus fine structure. Within the dominance region for pitch this representation follows the predictions of classic human behavioral experiments and provides a unifying contribution to possible neuro-temporal explanations for the pitch shift and pitch ambiguity associated with many inharmonic sounds.

Perceptual organization of sound begins in the auditory periphery.

Segmenting the complex acoustic mixture that makes a typical auditory scene into relevant perceptual objects is one of the main challenges of the auditory system [1], for both human and nonhuman species. Several recent studies indicate that perceptual auditory object formation, or "streaming," may be based on neural activity within the auditory cortex and beyond [2, 3]. Here, we find that scene analysis starts much earlier in the auditory pathways. Single units were recorded from a peripheral structure of the mammalian auditory brainstem, the cochlear nucleus. Peripheral responses were similar to cortical responses and displayed all of the functional properties required for streaming, including multisecond adaptation. Behavioral streaming was also measured in human listeners. Neurometric functions derived from the peripheral responses predicted accurately behavioral streaming. This reveals that subcortical structures may already contribute to the analysis of auditory scenes. This finding is consistent with the observation that species lacking a neocortex can still achieve and benefit from behavioral streaming [4]. For humans, we argue that auditory scene analysis of complex scenes is probably based on interactions between subcortical and cortical neural processes, with the relative contribution of each stage depending on the nature of the acoustic cues forming the streams.

Reverberation challenges the temporal representation of the pitch of complex sounds.

Accurate neural coding of the pitch of complex sounds is an essential part of auditory scene analysis; differences in pitch help segregate concurrent sounds, while similarities in pitch can help group sounds from a common source. In quiet, nonreverberant backgrounds, pitch can be derived from timing information in broadband high-frequency auditory channels and/or from frequency and timing information carried in narrowband low-frequency auditory channels. Recording from single neurons in the cochlear nucleus of anesthetized guinea pigs, we show that the neural representation of pitch based on timing information is severely degraded in the presence of reverberation. This degradation increases with both increasing reverberation strength and channel bandwidth. In a parallel human psychophysical pitch-discrimination task, reverberation impaired the ability to distinguish a high-pass harmonic sound from noise. Together, these findings explain the origin of perceptual difficulties experienced by both normal-hearing and hearing-impaired listeners in reverberant spaces.

Behavioral and physiological correlates of temporal pitch perception in electric and acoustic hearing.

In the "4-6" condition of experiment 1, normal-hearing (NH) listeners compared the pitch of a bandpass-filtered pulse train, whose inter-pulse intervals (IPIs) alternated between 4 and 6 ms, to that of isochronous pulse trains. Consistent with previous results obtained at a lower signal level, the pitch of the 4-6 stimulus corresponded to that of an isochronous pulse train having a period of 5.7 ms-longer than the mean IPI of 5 ms. In other conditions the IPI alternated between 3.5-5.5 and 4.5-6.5 ms. Experiment 2 was similar but presented electric pulse trains to one channel of a cochlear implant. In both cases, as overall IPI increased, the pitch of the alternating-interval stimulus approached that of an isochronous train having a period equal to the mean IPI. Experiment 3 measured compound action potentials (CAPs) to alternating-interval stimuli in guinea pigs and in NH listeners. The CAPs to pulses occurring after 4-ms intervals were smaller than responses to pulses occurring after 6-ms intervals, resulting in a modulated pattern that was independent of overall level. The results are compared to the predictions of a simple model incorporating auditory-nerve (AN) refractoriness, and where pitch is estimated from first-order intervals in the AN response.

Contralateral inhibitory and excitatory frequency response maps in the mammalian cochlear nucleus.

There is increasing evidence that the responses of single units in the mammalian cochlear nucleus can be altered by the presentation of contralateral stimuli, although the functional significance of this binaural responsiveness is unknown. To further our understanding of this phenomenon we recorded single-unit (n = 110) response maps from the cochlear nucleus (ventral and dorsal divisions) of the anaesthetized guinea pig in response to presentation of ipsilateral and contralateral pure tones. Many neurones showed no evidence of input from the contralateral ear (n = 41) but other neurones from both ventral and dorsal cochlear nucleus showed clear evidence of contralateral inhibitory input (n = 61). Inhibitory response patterns were divided into two groups. In 36 neurones, contralateral tone-evoked inhibition was closely aligned with the ipsilateral excitatory response map (+/- 0.33 octaves) often extending to low stimulus levels. In 25 neurones, higher threshold contralateral inhibitory responses were found, mostly centred at frequencies greater than 0.33 octaves below the ipsilateral excitation. A few neurones (n = 8) exhibited responses consistent with excitatory input from the contralateral ear, which was closely aligned with the ipsilateral excitation, and were found exclusively in the dorsal cochlear nucleus. The latency of the contralateral interaction was, on average, longer than the ipsilateral latency. Interaural level difference curves are similar to other reports from the cochlear nucleus. Our results are consistent with the idea that contralateral interactions arise from a variety of direct and indirect neuronal projections.

The temporal representation of the delay of iterated rippled noise with positive or negative gain by chopper units in the cochlear nucleus.

The role of chopper units in representing the pitch of complex sounds is unresolved. Traditionally chopper units have been regarded as primarily responding to the stimulus envelope of complex stimuli. This has been supported by the response of chopper units to iterated rippled noise (IRN) as they can provide a robust representation of the delay of IRN with positive gain (+) in their first-order interspike intervals and for some chopper units this representation is relatively level independent. The envelope modulation of IRN(+), and pitch, is at the reciprocal of the delay, the pitch of IRN with negative gain (IRN(-)) is often at twice the delay. This distinction between IRN(+) and IRN(-) can be used to help determine whether a unit is simply responding to modulation or to stimulus fine structure. Chopper units with relatively high best frequencies (BF) are unable to represent the distinction between IRN(+) and IRN(-). However, in this study it is shown that at least some chopper units, with low BFs (<1.25 kHz), can represent the pitch of the IRN(-) as perceived perceptually.

The time course of recovery from suppression and facilitation from single units in the mammalian cochlear nucleus.

The responses to two identical, consecutive pure tone stimuli with varying inter-stimulus intervals (delta ts) were measured for 89 neurons in the cochlear nucleus of the anaesthetised guinea pig. We observed two main effects; either a decrease (suppression) or an increase (facilitation) in response to the second tone followed by an exponential recovery. Response behaviour correlated with the unit type; primary-like, primary-like with notch and transient-chopper units showed a recovery from suppression that was very similar to that already reported in the auditory nerve. For chopper units the strength of the adaptation was correlated with the units regularity of spike discharge; sustained chopper (CS) units showed less suppression than transient choppers. Onset units showed complete suppression at short delta ts. Pause/Build (PB) units responded with increased activity to the second tone. In contrast to previous studies in the cochlear nucleus the recovery from suppression or facilitation was well described by a single exponential function, enabling us to define a recovery time constant and a maximum suppression/facilitation. There appeared to be a hierarchy in the time constant of recovery with PB and CS units showing the longest recovery times and onset units showing the shortest.

Temporal representation of the delay of iterated rippled noise in the dorsal cochlear nucleus.

It has been suggested that the dorsal cochlear nucleus (DCN) is involved in the temporal representation of both envelope periodicity and pitch. This hypothesis is tested using iterated rippled noise (IRN), which is generated by a cascade of delay and add [IRN(+)] or delay and subtract [IRN(-)] operations. The autocorrelation functions (ACFs) of the waveform and the envelope of IRN(+) have a first peak at the delay, which corresponds to the perceived pitch of the IRN. With the same delay, the pitch of IRN(-) is generally an octave lower than for IRN(+). This is reflected in a first peak at twice the delay in the ACF of the waveform for IRN(-). In contrast, for identical delays, the ACF of the envelope for both IRN(-) and IRN(+) is the same. Thus the use of IRN allows the distinction between envelope- or fine-structure sensitivity. Recordings were made from 135 single units (BFs <5 kHz) in the DCN of the anesthetized guinea pig using IRN with delays ranging from 1 to 32 ms. In our sample 42% were sensitive to the periodicity of IRN(+) and were tuned to a particular delay in their first-order interspike interval histograms (ISIHs). This tuning was highly correlated with their response to white noise. Most units with best frequencies (BFs) <500 Hz show a different all-order ISIH for IRN(+) and IRN(-), which corresponds to the perceived pitch difference, whereas units with higher BFs show a similar response to IRN(+) and IRN(-). The results indicate that low-frequency units (BF <500 Hz) in the DCN may be involved in the representation of the waveform fine structure, although units with BFs >500 Hz are able to encode only the envelope periodicity of broadband IRN in their temporal discharge characteristics.

Responses of dorsal cochlear nucleus neurons to signals in the presence of modulated maskers.

The detection of a signal in noise is enhanced when the masking noise is coherently modulated over a wide range of frequencies. This phenomenon, known as comodulation masking release (CMR), has been attributed to across-channel processing; however, the relative contribution of different stages in the auditory system to such across-channel processing is unknown. It has been hypothesized that wideband or lateral inhibition may underlie a physiological correlate of CMR. To further test this hypothesis, we have measured the responses of single units from the dorsal cochlear nucleus in which wideband inhibition is particularly pronounced. Using a sinusoidally amplitude-modulated tone at the best frequency of each unit as a masker, a pure-tone signal was added in the dips of the masker modulation. Flanking bands (FBs, also amplitude-modulated pure tones) were positioned to fall within the inhibitory sidebands of the receptive field of the unit. The FBs were either in phase (comodulated) or out of phase (codeviant) with the on-frequency masker. For the majority of units, the addition of the comodulated FBs produced a strong reduction in the response to the masker modulation, making the signal more salient in the post stimulus time histograms. The change in spike rate in response to the signal between the masker and signal-plus-masker conditions was greatest for the comodulated condition for 29 of 45 units. These results are consistent with the hypothesis that wideband inhibition may play a role in across-channel processing at an early stage in the auditory pathway.

The psychophysics and physiology of comodulation masking release.

The ability to detect auditory signals from background noise may be enhanced by the addition of energy in frequency regions well removed from the frequency of the signal. However, it is important that this energy is amplitude-modulated in a coherent way across frequencies, i.e. comodulated. This enhancement of signal detectability is known as comodulation masking release (CMR), and in this review we show that CMR is largest if: (1) the total masker's bandwidth is large, (2) the modulation frequency is low, (3) the modulation depth is high, (4) the envelope is regular and, (5) the masker's spectrum level is high. Possible physiological correlates of CMR have been found at different levels of the auditory pathway. Current hypotheses for the underlying physiological mechanisms, including wide-band inhibition or the disruption of masker modulation envelope response, are discussed.