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

Synaptic Recruitment Enhances Gap Termination Responses in Auditory Cortex

The ability to detect short gaps in noise is an important tool for assessing the temporal resolution in the auditory cortex. However, the mere existence of responses to temporal gaps bounded by two short broadband markers is surprising, because of the expected short-term suppression that is prevalent in auditory cortex. Here, we used in-vivo intracellular recordings in anesthetized rats to dissect the synaptic mechanisms that underlie gap-related responses. When a gap is bounded by two short markers, a gap termination response was evoked by the onset of the second marker with minimal contribution from the offset of the first marker. Importantly, we show that the gap termination response was driven by a different (potentially partially overlapping) synaptic population than that underlying the onset response to the first marker. This recruitment of additional synaptic resources is a novel mechanism contributing to the important perceptual task of gap detection.

A New Method for Assessing Masking and Residual Inhibition of Tinnitus

Tinnitus masking and residual inhibition (RI) are two well-known psychoacoustic measures of tinnitus. While it has long been suggested that they may provide diagnostic and prognostic information, these measures are still rarely performed in clinics, as they are too time consuming. Given this issue, the main goal of the present study was to validate a new method for assessing these measures. An acoustic sequence made of pulsed stimuli, which included a fixed stimulus duration and interstimulus interval, was applied to 68 tinnitus patients at two testing sites. First, the minimum masking level (MML) was measured by raising the stimulus intensity until the tinnitus was unheard during the stimulus presentation. Second, the level of the stimulus was further increased until the tinnitus was suppressed during the silence interval between the acoustic pulses. This level was called the minimum residual inhibition level (MRIL). The sequential measurement of MML and MRIL from the same stimulus condition offers several advantages such as time efficiency and the ability to compare results between the MRIL and MML. Our study confirms that, from this new approach, MML and MRIL can be easily and quickly obtained from a wide variety of patients displaying either normal hearing or different hearing loss configurations. Indeed, MML was obtained in all patients except one (98.5%), and some level of MRIL was found on 59 patients (86.7%). More so, this approach allows the categorization of tinnitus patients into different subgroups based on the properties of their MRIL.

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.

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.

Computational modelling suggests that temporal integration results from synaptic adaptation in auditory cortex

Incoming sounds are represented in the context of preceding events, and this requires a memory mechanism that integrates information over time. Here, it was demonstrated that response adaptation, the suppression of neural responses due to stimulus repetition, might reflect a computational solution that auditory cortex uses for temporal integration. Adaptation is observed in single-unit measurements as two-tone forward masking effects and as stimulus-specific adaptation (SSA). In non-invasive observations, the amplitude of the auditory N1m response adapts strongly with stimulus repetition, and it is followed by response recovery (the so-called mismatch response) to rare deviant events. The current computational simulations described the serial core-belt-parabelt structure of auditory cortex, and included synaptic adaptation, the short-term, activity-dependent depression of excitatory corticocortical connections. It was found that synaptic adaptation is sufficient for columns to respond selectively to tone pairs and complex tone sequences. These responses were defined as combination sensitive, thus reflecting temporal integration, when a strong response to a stimulus sequence was coupled with weaker responses both to the time-reversed sequence and to the isolated sequence elements. The temporal complexity of the stimulus seemed to be reflected in the proportion of combination-sensitive columns across the different regions of the model. Our results suggest that while synaptic adaptation produces facilitation and suppression effects, including SSA and the modulation of the N1m response, its functional significance may actually be in its contribution to temporal integration. This integration seems to benefit from the serial structure of auditory cortex.

Forward masking in the medial nucleus of the trapezoid body of the rat

Perception of acoustic stimuli is modulated by the temporal and spectral relationship between sound components. Forward masking experiments show that the perception threshold for a probe tone is significantly impaired by a preceding masker stimulus. Forward masking has been systematically studied at the level of the auditory nerve, cochlear nucleus, inferior colliculus and auditory cortex, but not yet in the superior olivary complex. The medial nucleus of the trapezoid body (MNTB), a principal cell group of the superior olive, plays an essential role in sound localization. The MNTB receives excitatory input from the contralateral cochlear nucleus via the calyces of Held and innervates the ipsilateral lateral and medial superior olives, as well as the superior paraolivary nucleus. Here, we performed single-unit extracellular recordings in the MNTB of rats. Using a forward masking paradigm previously employed in studies of the inferior colliculus and auditory nerve, we determined response thresholds for a 20-ms characteristic frequency pure tone (the probe), and then presented it in conjunction with another tone (the masker) that was varied in intensity, duration, and frequency; we also systematically varied the masker-to-probe delay. Probe response thresholds increased and response magnitudes decreased when a masker was presented. The forward suppression effects were greater when masker level and masker duration were increased, when the masker frequency approached the MNTB unit's characteristic frequency, and as the masker-to-probe delay was shortened. Probe threshold shifts showed an exponential decay as the masker-to-probe delay increased.

Synaptic plasticity in the auditory system: a review

Synaptic transmission via chemical synapses is dynamic, i.e., the strength of postsynaptic responses may change considerably in response to repeated synaptic activation. Synaptic strength is increased during facilitation, augmentation and potentiation, whereas a decrease in synaptic strength is characteristic for depression and attenuation. This review attempts to discuss the literature on short-term and long-term synaptic plasticity in the auditory brainstem of mammals and birds. One hallmark of the auditory system, particularly the inner ear and lower brainstem stations, is information transfer through neurons that fire action potentials at very high frequency, thereby activating synapses >500 times per second. Some auditory synapses display morphological specializations of the presynaptic terminals, e.g., calyceal extensions, whereas other auditory synapses do not. The review focuses on short-term depression and short-term facilitation, i.e., plastic changes with durations in the millisecond range. Other types of short-term synaptic plasticity, e.g., posttetanic potentiation and depolarization-induced suppression of excitation, will be discussed much more briefly. The same holds true for subtypes of long-term plasticity, like prolonged depolarizations and spike-time-dependent plasticity. We also address forms of plasticity in the auditory brainstem that do not comprise synaptic plasticity in a strict sense, namely short-term suppression, paired tone facilitation, short-term adaptation, synaptic adaptation and neural adaptation. Finally, we perform a meta-analysis of 61 studies in which short-term depression (STD) in the auditory system is opposed to short-term depression at non-auditory synapses in order to compare high-frequency neurons with those that fire action potentials at a lower rate. This meta-analysis reveals considerably less STD in most auditory synapses than in non-auditory ones, enabling reliable, failure-free synaptic transmission even at frequencies >100 Hz. Surprisingly, the calyx of Held, arguably the best-investigated synapse in the central nervous system, depresses most robustly. It will be exciting to reveal the molecular mechanisms that set high-fidelity synapses apart from other synapses that function much less reliably.

Response to best-frequency tone bursts in the ventral cochlear nucleus is governed by ordered inter-spike interval statistics

The spike trains generated by short constant-amplitude constant-frequency tone bursts in the ventral cochlear nucleus of the anaesthetised guinea pig are examined. Spikes are grouped according to the order in which they occur following the onset of the stimulus. It is found that successive inter-spike intervals have low statistical dependence according to information-theoretic measures. This is in contrast to previous observations with long-duration tone bursts in the cat dorsal and posteroventral cochlear nuclei and lateral superior olive, where it was found that long intervals tended to be followed by shorter ones and vice versa. The interval distributions can also be reasonably modelled by a shifted Gamma distribution parameterised by the dead-time and the mean and coefficient of variation of the dead-time corrected ISI distribution. Knowledge of those three parameters for each interval is sufficient to determine the peri-stimulus time histogram and the regularity measures used to classify these neurons.

Central auditory masking by an illusory tone

Many natural sounds fluctuate over time. The detectability of sounds in a sequence can be reduced by prior stimulation in a process known as forward masking. Forward masking is thought to reflect neural adaptation or neural persistence in the auditory nervous system, but it has been unclear where in the auditory pathway this processing occurs. To address this issue, the present study used a “Huggins pitch” stimulus, the perceptual effects of which depend on central auditory processing. Huggins pitch is an illusory tonal sensation produced when the same noise is presented to the two ears except for a narrow frequency band that is different (decorrelated) between the ears. The pitch sensation depends on the combination of the inputs to the two ears, a process that first occurs at the level of the superior olivary complex in the brainstem. Here it is shown that a Huggins pitch stimulus produces more forward masking in the frequency region of the decorrelation than a noise stimulus identical to the Huggins-pitch stimulus except with perfect correlation between the ears. This stimulus has a peripheral neural representation that is identical to that of the Huggins-pitch stimulus. The results show that processing in, or central to, the superior olivary complex can contribute to forward masking in human listeners.

Adaptation in the auditory midbrain of the barn owl (Tyto alba) induced by tonal double stimulation

During hunting, the barn owl typically listens to several successive sounds as generated, for example, by rustling mice. As auditory cells exhibit adaptive coding, the earlier stimuli may influence the detection of the later stimuli. This situation was mimicked with two double-stimulus paradigms, and adaptation was investigated in neurons of the barn owl's central nucleus of the inferior colliculus. Each double-stimulus paradigm consisted of a first or reference stimulus and a second stimulus (probe). In one paradigm (second level tuning), the probe level was varied, whereas in the other paradigm (inter-stimulus interval tuning), the stimulus interval between the first and second stimulus was changed systematically. Neurons were stimulated with monaural pure tones at the best frequency, while the response was recorded extracellularly. The responses to the probe were significantly reduced when the reference stimulus and probe had the same level and the inter-stimulus interval was short. This indicated response adaptation, which could be compensated for by an increase of the probe level of 5-7 dB over the reference level, if the latter was in the lower half of the dynamic range of a neuron's rate-level function. Recovery from adaptation could be best fitted with a double exponential showing a fast (1.25 ms) and a slow (800 ms) component. These results suggest that neurons in the auditory system show dynamic coding properties to tonal double stimulation that might be relevant for faithful upstream signal propagation. Furthermore, the overall stimulus level of the masker also seems to affect the recovery capabilities of auditory neurons.

An exact method of regularity analysis for auditory brainstem neurons (L)

The standard regularity analysis for spike trains in cochlear nucleus neurons evoked by tonebursts first proposed by Bourk is widely used, primarily as one of the criteria for classification of such neurons. It is shown that this procedure does not estimate quite what it is supposed to, and introduces unnecessary noise to its results due to its use of bins. Instead the desired quantities (mean and coefficient of variation of the lengths of all inter-spike intervals in progress as a function of time since stimulus onset) can all be exactly calculated directly from the spike train without the need for data binning. The implications for classification and other studies are discussed.

Forward suppression in the auditory cortex is frequency-specific

We investigated how physiologically observed forward suppression interacts with stimulus frequency in neuronal responses in the guinea pig auditory cortex. The temporal order and frequency proximity of sounds influence both their perception and neuronal responses. Psychophysically, preceding sounds (conditioners) can make successive sounds (probes) harder to hear. These effects are larger when the two sounds are spectrally similar. Physiological forward suppression is usually maximal for conditioner tones near to a unit's characteristic frequency (CF), the frequency to which a neuron is most sensitive. However, in most physiological studies, the frequency of the probe tone and CF are identical, so the role of unit CF and probe frequency cannot be distinguished. Here, we systemically varied the frequency of the probe tone, and found that the tuning of suppression was often more closely related to the frequency of the probe tone than to the unit's CF, i.e. suppressed tuning was specific to probe frequency. This relationship was maintained for all measured gaps between the conditioner and the probe tones. However, when the probe frequency and CF were similar, CF tended to determine suppressed tuning. In addition, the bandwidth of suppression was slightly wider for off-CF probes. Changes in tuning were also reflected in the firing rate in response to probe tones, which was maximally reduced when probe and conditioner tones were matched in frequency. These data are consistent with the idea that cortical neurons receive convergent inputs with a wide range of tuning properties that can adapt independently.

Contribution of the mouse calyx of Held synapse to tone adaptation

The calyx of Held synapse is a giant synapse in the medial nucleus of the trapezoid body (MNTB) of the ventral brainstem, which is involved in sound localization. Although it has many release sites, it can show transmission failures and display an increase in synaptic delay during high-frequency signalling. Its apparent lack of reliability and precision raises the question whether this synapse makes a sizeable contribution to tone adaptation, the decline in response to sustained or repetitive auditory stimuli. We observed evidence for the presence of both ipsilateral and contralateral inhibition, but these effects were already present in the inputs to the MNTB, suggesting that synaptic inhibition within the MNTB does not contribute to tone adaptation. During trains of brief tones at variable intervals, there were no clear changes in reliability or precision at tone intervals of 20 ms or longer. A progressive decrease in the number of spikes measured in the MNTB was observed at shorter tone intervals, but this decrease largely originated upstream from the MNTB. In addition, for tones with short intervals, during the train a progressive increase in first-spike latencies was observed, but much smaller changes were observed in the delay between excitatory postsynaptic potentials and postsynaptic action potentials within the MNTB. We conclude that despite the failures and variability in synaptic delay that are present at the calyx of Held synapse, their contribution to tone adaptation is relatively small compared with upstream factors.

Forward masking estimated by signal detection theory analysis of neuronal responses in primary auditory cortex

Psychophysical forward masking is an increase in threshold of detection of a sound (probe) when it is preceded by another sound (masker). This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation. Studies in the auditory nerve and cochlear nucleus using signal detection theory techniques to derive neuronal thresholds showed that in centrally projecting neurons, increases in masked thresholds were significantly smaller than the changes measured psychophysically. Larger threshold shifts have been reported in the inferior colliculus of awake marmoset. The present study investigated the magnitude of forward masking in primary auditory cortical neurons of anaesthetised guinea-pigs. Responses of cortical neurons to unmasked and forward masked tones were measured and probe detection thresholds estimated using signal detection theory methods. Threshold shifts were larger than in the auditory nerve, cochlear nucleus and inferior colliculus. The larger threshold shifts suggest that central, and probably cortical, processes contribute to forward masking. However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically. Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.

Efferent pathways modulate hyperactivity in inferior colliculus

Animal models have demonstrated that mild hearing loss caused by acoustic trauma results in spontaneous hyperactivity in the central auditory pathways. This hyperactivity has been hypothesized to be involved in the generation of tinnitus, a phantom auditory sensation. We have recently shown that such hyperactivity, recorded in the inferior colliculus, is still dependent on cochlear neural output for some time after recovery (up to 6 weeks). We have now studied the capacity of an intrinsic efferent system, i.e., the olivocochlear system, to alter hyperactivity. This system is known to modulate cochlear neural output. Anesthetized guinea pigs were exposed to a loud sound and after 2 or 3 weeks of recovery, single-neuron recordings in inferior colliculus were made to confirm hyperactivity. Olivocochlear axons were electrically stimulated and effects on cochlear neural output and on highly spontaneous neurons in inferior colliculus were assessed. Olivocochlear stimulation suppressed spontaneous hyperactivity in the inferior colliculus. This result is in agreement with our earlier finding that hyperactivity can be modulated by altering cochlear neural output. Interestingly, the central suppression was generally much larger and longer lasting than reported previously for primary afferents. Blockade of the intracochlear effects of olivocochlear system activation eliminated some but not all of the effects observed on spontaneous activity, suggesting also a central component to the effects of stimulation. More research is needed to investigate whether these central effects of olivocochlear efferent stimulation are due to central intrinsic circuitry or to coactivation of central efferent collaterals to the cochlear nucleus.

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).

Wide-dynamic-range forward suppression in marmoset inferior colliculus neurons is generated centrally and accounts for perceptual masking

An organism's ability to detect and discriminate sensory inputs depends on the recent stimulus history. For example, perceptual detection thresholds for a brief tone can be elevated by as much as 50 dB when following a masking stimulus. Previous work suggests that such forward masking is not a direct result of peripheral neural adaptation; the central pathway apparently modifies the representation in a way that further attenuates the input's response to short probe signals. Here, we show that much of this transformation is complete by the level of the inferior colliculus (IC). Single-neuron extracellular responses were recorded in the central nucleus of the awake marmoset IC. The threshold for a 20 ms probe tone presented at best frequency was determined for various masker-probe delays, over a range of masker sound pressure levels (SPLs) and frequencies. The most striking aspect of the data was the increased potency of forward maskers as their SPL was increased, despite the fact that the excitatory response to the masker was often saturating or nonmonotonic over the same range of levels. This led to probe thresholds at high masker levels that were almost always higher than those observed in the auditory nerve. Probe threshold shifts were not usually caused by a persistent excitatory response to the masker; instead we propose a wide-dynamic-range inhibitory mechanism locked to sound offset as an explanation for several key aspects of the data. These findings further delineate the role of subcortical auditory processing in the generation of a context-dependent representation of ongoing acoustic scenes.

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.

Rapid neural adaptation to sound level statistics

Auditory neurons must represent accurately a wide range of sound levels using firing rates that vary over a far narrower range of levels. Recently, we demonstrated that this "dynamic range problem" is lessened by neural adaptation, whereby neurons adjust their input-output functions for sound level according to the prevailing distribution of levels. These adjustments in input-output functions increase the accuracy with which levels around those occurring most commonly are coded by the neural population. Here, we examine how quickly this adaptation occurs. We recorded from single neurons in the auditory midbrain during a stimulus that switched repeatedly between two distributions of sound levels differing in mean level. The high-resolution analysis afforded by this stimulus showed that a prominent component of the adaptation occurs rapidly, with an average time constant across neurons of 160 ms after an increase in mean level, much faster than our previous experiments were able to assess. This time course appears to be independent of both the timescale over which sound levels varied and that over which sound level distributions varied, but is related to neural characteristic frequency. We find that adaptation to an increase in mean level occurs more rapidly than to a decrease. Finally, we observe an additional, slow adaptation in some neurons, which occurs over a timescale of tens of seconds. Our findings provide constraints in the search for mechanisms underlying adaptation to sound level. They also have functional implications for the role of adaptation in the representation of natural sounds.

Rebound depolarization in single units of the ventral cochlear nucleus: a contribution to grouping by common onset?

Simultaneous grouping by common onset time is believed to be a powerful cue in auditory perception; components that start or stop roughly at the same time are judged as far more likely to have originated from the same source. Here we report a simple experiment designed to simulate a complex psychophysical paradigm first described by Darwin and Sutherland [(1984) Grouping frequency components of vowels. When is a harmonic not a harmonic? Quarterly J of Experimental Psychology: Hum Exp Psychol 36(A):193-208]. It is possible to change the perception of the vowel /I/ to /epsilon/ by manipulating the harmonics around the first formant (F1). Increasing the amplitude of one harmonic around F1 caused the perception of the vowel to change from /I/ to /epsilon/. Extending the increased component before the vowel could, however, greatly reduce this change. The role of neural adaptation in this effect was questioned by repeating the experiment but this time using a 'captor' tone which was switched on with the asynchronous harmonic and off when the vowel started. This time the vowel percept did change in a fashion analogous to the effect of an increase in the amplitude of the fourth harmonic (which is close to F1). This effect was explained by assuming that the captor had grouped with the leading portion of the asynchronous component enabling the remainder of the asynchronous component to be grouped with the remainder of the components. We propose a relatively low-level neuronal explanation for this grouping effect: the captor reduces the neural response to the leading segment of the asynchronous component by activating across-frequency suppression, either from the cochlea, or acting via a wideband inhibitor in the ventral cochlear nucleus. The reduction in neural response results in a release from adaptation with the offset of the captor terminating the inhibition, such that the response to the continuation of that component is now enhanced. Using a simplified paradigm we show that both primary-like and chopper units in the ventral cochlear nucleus of the anesthetized guinea pig may show a rebound in excitation when a captor is positioned so as to stimulate the suppressive sidebands in its receptive field. The strength of the rebound was positively correlated with the strength of the suppression. These and other results are consistent with the view that low-level mechanisms underlie the psychophysical captor effect.