Recovery of forward-masked responses in ventral cochlear nucleus neurons

Neuronal population model of globular bushy cells covering unit-to-unit variability

Computations of acoustic information along the central auditory pathways start in the cochlear nucleus. Bushy cells in the anteroventral cochlear nucleus, which innervate monaural and binaural stations in the superior olivary complex, process and transfer temporal cues relevant for sound localization. These cells are categorized into two groups: spherical and globular bushy cells (SBCs/GBCs). Spontaneous rates of GBCs innervated by multiple auditory nerve (AN) fibers are generally lower than those of SBCs that receive a small number of large AN synapses. In response to low-frequency tonal stimulation, both types of bushy cells show improved phase-locking and entrainment compared to AN fibers. When driven by high-frequency tones, GBCs show primary-like-with-notch or onset-L peristimulus time histograms and relatively irregular spiking. However, previous in vivo physiological studies of bushy cells also found considerable unit-to-unit variability in these response patterns. Here we present a population of models that can simulate the observed variation in GBCs. We used a simple coincidence detection model with an adaptive threshold and systematically varied its six parameters. Out of 567000 parameter combinations tested, 7520 primary-like-with-notch models and 4094 onset-L models were selected that satisfied a set of physiological criteria for a GBC unit. Analyses of the model parameters and output measures revealed that the parameters of the accepted model population are weakly correlated with each other to retain major GBC properties, and that the output spiking patterns of the model are affected by a combination of multiple parameters. Simulations of frequency-dependent temporal properties of the model GBCs showed a reasonable fit to empirical data, supporting the validity of our population modeling. The computational simplicity and efficiency of the model structure makes our approach suitable for future large-scale simulations of binaural information processing that may involve thousands of GBC units.

In the auditory system, specialized neuronal circuits process various types of acoustic information. A group of neurons, called globular bushy cells (GBCs), faithfully transfer timing information of acoustic signals to their downstream neurons responsible for the perception of sound location. Previous physiological studies found representative activity patterns of GBCs, but with substantial individual variations among them. In this study, we present a population of models, instead of creating one best model, to account for the observed variations of GBCs. We varied all six parameters of a simple auditory neuron model and selected the combinations of parameters that led to acceptable activity patterns of GBCs. In total, we tested more than half a million combinations and accepted ~11600 GBC models. Temporal spiking patterns of real GBCs depend on the sound frequency, and our model population was able to replicate this trend. The model used here is computationally efficient and can thus serve as a building block for future large-scale simulations of auditory information processing.

Diverse effects of stimulus history in waking mouse auditory cortex

Responses to auditory stimuli are often strongly influenced by recent stimulus history. For example, in a paradigm called forward suppression, brief sounds can suppress the perception of, and the neural responses to, a subsequent sound, with the magnitude of this suppression depending on both the spectral and temporal distances between the sounds. As a step towards understanding the mechanisms that generate these adaptive representations in awake animals, we quantitatively characterize responses to two-tone sequences in the auditory cortex of waking mice. We find that cortical responses in a forward suppression paradigm are more diverse in waking mice than previously appreciated, that these responses vary between cells with different firing characteristics and waveform shapes, but that the variability in these responses is not substantially related to cortical depth or columnar location. Moreover, responses to the first tone in the sequence are often not linearly related to the suppression of the second tone response, suggesting that spike-frequency adaptation of cortical cells is not a large contributor to forward suppression or its variability. Instead, we use a simple multilayered model to show that cell-to-cell differences in the balance of intracortical inhibition and excitation will naturally produce such a diversity of forward interactions. We propose that diverse inhibitory connectivity allows the cortex to encode spectro-temporally fluctuating stimuli in multiple parallel ways.NEW & NOTEWORTHY Behavioral and neural responses to auditory stimuli are profoundly influenced by recent sounds, yet how this occurs is not known. Here, the authors show in the auditory cortex of awake mice that the quality of history-dependent effects is diverse and related to cell type, response latency, firing rates, and receptive field bandwidth. In a cortical model, differences in excitatory-inhibitory balance can produce this diversity, providing the cortex with multiple ways of representing temporally complex information.

The time scale of adaptation in tonal sequence processing by the mouse auditory midbrain neurons

The time course of poststimulatory adaptation of the inferior colliculus central nucleus (ICC) of CBB6F₁ hybrid mice to sound sequences, specifically, series of four tonal stimuli presented at intervals of 0, 2, 4, 10, 20, 50, 100, 200, 500, 700, 1000, and 1500 ms were studied. Assessment of the adaptation of the entire neuronal population have shown that, at an interstimulus interval of 0-200 ms, the response to the first tone in a series is significantly stronger than those to the second to fourth tones, the strengths of the latter three responses not differing significantly from one another. If the interstimulus interval is 500 ms or longer, the response to none of the tones in a series differs significantly in strength from the others. The role of adaptation of midbrain neurons to the grouping of components of bioacoustic stimuli is discussed.

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.

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.

Why do forward maskers affect auditory intensity discrimination? Evidence from "molecular psychophysics"

Nonsimultaneous maskers can strongly impair performance in an auditory intensity discrimination task. Using methods of molecular psychophysics, we quantified the extent to which (1) a masker-induced impairment of the representation of target intensity (i.e., increase in internal noise) and (2) a systematic influence of the masker intensities on the decision variable contribute to these effects. In a two-interval intensity discrimination procedure, targets were presented in quiet, and combined with forward maskers. The lateralization of the maskers relative to the targets was varied via the interaural time difference. Intensity difference limens (DLs) were strongly elevated under forward masking but less with contralateral than with ipsilateral maskers. For most listeners and conditions, perceptual weights measuring the relation between the target and masker levels and the response in the intensity discrimination task were positive and significant. Higher perceptual weights assigned to the maskers corresponded to stronger elevations of the intensity DL. The maskers caused only a weak increase in internal noise, unrelated to target level and masker lateralization. The results indicate that the effects of forward masking on intensity discrimination are determined by an inclusion of the masker intensities in the decision variable, compatible with the hypothesis that the impairment in performance is to a large part caused by difficulties in directing selective attention to the targets. The effects of masker lateralization are evidence for top-down influences, and the observed positive signs of the masker weights suggest that the relevant mechanisms are located at higher processing stages rather than in the auditory periphery.

Neural adaptation to silence in the human auditory cortex: a magnetoencephalographic study

INTRODUCTION

Previous studies demonstrated that a decrement in the N1m response, a major deflection in the auditory evoked response, with sound repetition was mainly caused by bottom-up driven neural refractory periods following brain activation due to sound stimulations. However, it currently remains unknown whether this decrement occurs with a repetition of silences, which do not induce refractoriness.

METHODS

In the present study, we investigated decrements in N1m responses elicited by five repetitive silences in a continuous pure tone and by five repetitive pure tones in silence using magnetoencephalography.

RESULTS

Repetitive sound stimulation differentially affected the N1m decrement in a sound type-dependent manner; while the N1m amplitude decreased from the 1st to the 2nd pure tone and remained constant from the 2nd to the 5th pure tone in silence, a gradual decrement was observed in the N1m amplitude from the 1st to the 5th silence embedded in a continuous pure tone.

CONCLUSIONS

Our results suggest that neural refractoriness may mainly cause decrements in N1m responses elicited by trains of pure tones in silence, while habituation, which is a form of the implicit learning process, may play an important role in the N1m source strength decrements elicited by successive silences in a continuous pure tone.

Modulatory effects of spectral energy contrasts on lateral inhibition in the human auditory cortex: an MEG study

We investigated the modulation of lateral inhibition in the human auditory cortex by means of magnetoencephalography (MEG). In the first experiment, five acoustic masking stimuli (MS), consisting of noise passing through a digital notch filter which was centered at 1 kHz, were presented. The spectral energy contrasts of four MS were modified systematically by either amplifying or attenuating the edge-frequency bands around the notch (EFB) by 30 dB. Additionally, the width of EFB amplification/attenuation was varied (3/8 or 7/8 octave on each side of the notch). N1m and auditory steady state responses (ASSR), evoked by a test stimulus with a carrier frequency of 1 kHz, were evaluated. A consistent dependence of N1m responses upon the preceding MS was observed. The minimal N1m source strength was found in the narrowest amplified EFB condition, representing pronounced lateral inhibition of neurons with characteristic frequencies corresponding to the center frequency of the notch (NOTCH CF) in secondary auditory cortical areas. We tested in a second experiment whether an even narrower bandwidth of EFB amplification would result in further enhanced lateral inhibition of the NOTCH CF. Here three MS were presented, two of which were modified by amplifying 1/8 or 1/24 octave EFB width around the notch. We found that N1m responses were again significantly smaller in both amplified EFB conditions as compared to the NFN condition. To our knowledge, this is the first study demonstrating that the energy and width of the EFB around the notch modulate lateral inhibition in human secondary auditory cortical areas. Because it is assumed that chronic tinnitus is caused by a lack of lateral inhibition, these new insights could be used as a tool for further improvement of tinnitus treatments focusing on the lateral inhibition of neurons corresponding to the tinnitus frequency, such as the tailor-made notched music training.

Age differences in the neuroelectric adaptation to meaningful sounds

Much of what we know regarding the effect of stimulus repetition on neuroelectric adaptation comes from studies using artificially produced pure tones or harmonic complex sounds. Little is known about the neural processes associated with the representation of everyday sounds and how these may be affected by aging. In this study, we used real life, meaningful sounds presented at various azimuth positions and found that auditory evoked responses peaking at about 100 and 180 ms after sound onset decreased in amplitude with stimulus repetition. This neural adaptation was greater in young than in older adults and was more pronounced when the same sound was repeated at the same location. Moreover, the P2 waves showed differential patterns of domain-specific adaptation when location and identity was repeated among young adults. Background noise decreased ERP amplitudes and modulated the magnitude of repetition effects on both the N1 and P2 amplitude, and the effects were comparable in young and older adults. These findings reveal an age-related difference in the neural processes associated with adaptation to meaningful sounds, which may relate to older adults' difficulty in ignoring task-irrelevant stimuli.

A fast cholinergic modulation of the primary acoustic startle circuit in rats

Cochlear root neurons (CRNs) are the first brainstem neurons which initiate and participate in the full expression of the acoustic startle reflex. Although it has been suggested that a cholinergic pathway from the ventral nucleus of the trapezoid body (VNTB) conveys auditory prepulses to the CRNs, the neuronal origin of the VNTB-CRNs projection and the role it may play in the cochlear root nucleus remain uncertain. To determine the VNTB neuronal type which projects to CRNs, we performed tract-tracing experiments combined with mechanical lesions, and morphometric analyses. Our results indicate that a subpopulation of non-olivocochlear neurons projects directly and bilaterally to CRNs via the trapezoid body. We also performed a gene expression analysis of muscarinic and nicotinic receptors which indicates that CRNs contain a cholinergic receptor profile sufficient to mediate the modulation of CRN responses. Consequently, we investigated the effects of auditory prepulses on the neuronal activity of CRNs using extracellular recordings in vivo. Our results show that CRN responses are strongly inhibited by auditory prepulses. Unlike other neurons of the cochlear nucleus, the CRNs exhibited inhibition that depended on parameters of the auditory prepulse such as intensity and interstimulus interval, showing their strongest inhibition at short interstimulus intervals. In sum, our study supports the idea that CRNs are involved in the auditory prepulse inhibition of the acoustic startle reflex, and confirms the existence of multiple cholinergic pathways that modulate the primary acoustic startle circuit.

The adaptive pattern of the auditory N1 peak revealed by standardized low-resolution brain electromagnetic tomography

The N1 peak in the late auditory evoked potential (LAEP) decreases in amplitude following stimulus repetition, displaying an adaptive pattern. The present study explored the functional neural substrates that may underlie the N1 adaptive pattern using standardized Low Resolution Electromagnetic Tomography (sLORETA). Fourteen young normal hearing (NH) listeners participated in the study. Tone bursts (80 dB SPL) were binaurally presented via insert earphones in trains of 10; the inter-stimulus interval was 0.7s and the inter-train interval was 15s. Current source density analysis was performed for the N1 evoked by the 1st, 2nd and 10th stimuli (S(1), S(2) and S(10)) at 3 different timeframes that corresponded to the latency ranges of the N1 waveform subcomponents (70-100, 100-130 and 130-160 ms). The data showed that S(1) activated broad regions in different cortical lobes and the activation was much smaller for S(2) and S(10). Response differences in the LAEP waveform and sLORETA were observed between S(1) and S(2), but not between the S(2) and S(10). The sLORETA comparison map between S(1) and S(2) responses showed that the activation was located in the parietal lobe for the 70-100 ms timeframe, the frontal and limbic lobes for the 100-130 ms timeframe, and the frontal lobe for the 130-160 ms timeframe. These sLORETA comparison results suggest a parieto-frontal network that might help to sensitize the brain to novel stimuli by filtering out repetitive and irrelevant stimuli. This study demonstrates that sLORETA may be useful for identifying generators of scalp-recorded event related potentials and for examining the physiological features of these generators. This technique could be especially useful for cortical source localization in individuals who cannot be examined with functional magnetic resonance imaging or magnetoencephalography (e.g., cochlear implant users).

Mismatch negativity and adaptation measures of the late auditory evoked potential in cochlear implant users

A better understanding of the neural correlates of large variability in cochlear implant (CI) patients' speech performance may allow us to find solutions to further improve CI benefits. The present study examined the mismatch negativity (MMN) and the adaptation of the late auditory evoked potential (LAEP) in 10 CI users. The speech syllable /da/ and 1-kHz tone burst were used to examine the LAEP adaptation. The amount of LAEP adaptation was calculated according to the averaged N1-P2 amplitude for the LAEPs evoked by the last 3 stimuli and the amplitude evoked by the first stimulus. For the MMN recordings, the standard stimulus (1-kHz tone) and the deviant stimulus (2-kHz tone) were presented in an oddball condition. Additionally, the deviants alone were presented in a control condition. The MMN was derived by subtracting the response to the deviants in the control condition from the oddball condition. Results showed that good CI performers displayed a more prominent LAEP adaptation than moderate-to-poor performers. Speech performance was significantly correlated to the amount of LAEP adaptation for the 1-kHz tone bursts. Good performers displayed large MMNs and moderate-to-poor performers had small or absent MMNs. The abnormal electrophysiological findings in moderate-to-poor performers suggest that long-term deafness may cause damage not only at the auditory cortical level, but also at the cognitive level.

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.

Responses of neurons in the cat primary auditory cortex to sequential sounds

In the natural acoustic environment sounds frequently arrive at the two ears in quick succession. The responses of a cortical neuron to acoustic stimuli can be dramatically altered, usually suppressed, by a preceding sound. The purpose of this study was to determine if the binaural interaction evoked by a preceding sound is involved in subsequent suppressive interactions observed in auditory cortex neurons. Responses of neurons in the primary auditory cortex (AI) exhibiting binaural suppressive interactions (EO/I) were studied in barbiturate-anesthetized cats. For the majority (72.5%) of EO/I neurons studied, the response to a monaural contralateral stimulus was suppressed by a preceding monaural contralateral stimulus, but was not changed by a preceding monaural ipsilateral stimulus. For this subset of EO/I neurons, when a monaural contralateral stimulus was preceded by a binaural stimulus, the level of both the ipsilateral and the contralateral component of the binaural stimulus influenced the response to the subsequent monaural contralateral stimulus. When the contralateral level of the binaural stimulus was constant, increasing its ipsilateral level decreased the suppression of the response to the subsequent monaural contralateral stimulus. When the ipsilateral level of the binaural stimulus was constant, increasing its contralateral level increased the suppression of the response to the subsequent monaural contralateral stimulus. These results demonstrate that the sequential inhibition of responses of AI neurons is a function of the product of a preceding binaural interaction. The magnitude of the response to the contralateral stimulus is related to, but not determined by the magnitude of the response to the preceding binaural stimulus. Possible mechanisms of this sequential interaction are discussed.

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.

Spectro-temporal sound density-dependent long-term adaptation in cat primary auditory cortex

Sensory systems use adaptive strategies to code for the changing environment on different time scales. Short-term adaptation (up to 100 ms) reflects mostly synaptic suppression mechanisms after response to a stimulus. Long-term adaptation (up to a few seconds) is reflected in the habituation of neuronal responses to constant stimuli. Very long-term adaptation (several weeks) can lead to plastic changes in the cortex, most often facilitated during early development, by stimulus relevance or by behavioral states such as attention. In this study, we show that long-term adaptation with a time course of tens of minutes is detectable in anesthetized adult cat auditory cortex after a few minutes of listening to random-frequency tone pips. After the initial post-onset suppression, a slow recovery of the neuronal response strength to tones at or near their best frequency was observed for low-rate random sounds (four pips per octave per second) during stimulation. The firing rate at the end of stimulation (15 min) reached levels close to that observed during the initial onset response. The effect, visible for both spikes and, to a smaller extent, local field potentials, decreased with increasing spectro-temporal density of the sound. The spectro-temporal density of sound may therefore be of particular relevance in cortical processing. Our findings suggest that low stimulus rates may produce a specific acoustic environment that shapes the primary auditory cortex through very different processing than for spectro-temporally more dense and complex sounds.

Modeling the cochlear nucleus: a site for monaural echo suppression?

Echo suppression plays an important role in identifying and localizing auditory objects. One can distinguish between binaural and monaural echo suppression, although the former is the one commonly referred to. Based on biological findings we introduce and analyze a mathematical model for a neural implementation of monaural echo suppression in the cochlear nucleus. The model's behavior has been verified by analytical calculations as well as by numerical simulations for several types of input signal. It shows that in the perception of a pair of clicks the leading click suppresses the lagging one and that suppression is maximal for an interclick interval of 2-3 ms. Similarly, ongoing stimuli will be affected by the suppression mechanism primarily a couple of milliseconds after onset, resulting in a reduced perception of a sound shortly after its start. Both effects match experimental data.

Asymmetric lateral inhibitory neural activity in the auditory system: a magnetoencephalographic study

BACKGROUND

Decrements of auditory evoked responses elicited by repeatedly presented sounds with similar frequencies have been well investigated by means of electroencephalography and magnetoencephalography (MEG). However the possible inhibitory interactions between different neuronal populations remains poorly understood. In the present study, we investigated the effect of proceeding notch-filtered noises (NFNs) with different frequency spectra on a following test tone using MEG.

RESULTS

Three-second exposure to the NFNs resulted in significantly different N1m responses to a 1000 Hz test tone presented 500 ms after the offset of the NFNs. The NFN with a lower spectral edge closest to the test tone mostly decreased the N1m amplitude.

CONCLUSION

The decrement of the N1m component after exposure to the NFNs could be explained partly in terms of lateral inhibition. The results demonstrated that the amplitude of the N1m was more effectively influenced by inhibitory lateral connections originating from neurons corresponding to lower rather than higher frequencies. We interpret this effect of asymmetric lateral inhibition in the auditory system as an important contribution to reduce the asymmetric neural activity profiles originating from the cochlea.

Masking by inaudible sounds and the linearity of temporal summation

Many natural sounds, including speech and animal vocalizations, involve rapid sequences that vary in spectrum and amplitude. Each sound within a sequence has the potential to affect the audibility of subsequent sounds in a process known as forward masking. Little is known about the neural mechanisms underlying forward masking, particularly in more realistic situations in which multiple sounds follow each other in rapid succession. A parsimonious hypothesis is that the effects of consecutive sounds combine linearly, so that the total masking effect is a simple sum of the contributions from the individual maskers. The experiment reported here tests a counterintuitive prediction of this linear-summation hypothesis, namely that a sound that itself is inaudible should, under certain circumstances, affect the audibility of subsequent sounds. The results show that, when two forward maskers are combined, the second of the two maskers can continue to produce substantial masking, even when it is completely masked by the first masker. Thus, inaudible sounds can affect the perception of subsequent sounds. A model incorporating instantaneous compression (reflecting the nonlinear response of the basilar membrane in the cochlea), followed by linear summation of the effects of the maskers, provides a good account of the data. Despite the presence of multiple sources of nonlinearity in the auditory system, masking effects by sequential sounds combine in a manner that is well captured by a time-invariant linear system.

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.

Modulation of Level Response Areas and Stimulus Selectivity of Neurons in Cat Primary Auditory Cortex

Sounds commonly occur in sequences, such as in speech. It is therefore important to understand how the occurrence of one sound affects the response to a subsequent sound. We approached this question by determining how a conditioning stimulus alters the response areas of single neurons in the primary auditory cortex (AI) of barbiturate-anesthetized cats. The response areas consisted of responses to stimuli that varied in level at the two ears and delivered at the characteristic frequency of each cell. A binaural conditioning stimulus was then presented ≥50 ms before each of the stimuli comprising the level response area. An effective preceding stimulus alters the shape and severely reduces the size and response magnitude of the level response area. This ability of the preceding stimulus depends on its proximity in the level domain to the level response area, not on its absolute level or on the size of the response it evokes. Preceding stimuli evoke a nonlinear inhibition across the level response area that results in an increased selectivity of a cortical neuron for its preferred binaural stimuli. The selectivity of AI neurons during the processing of a stream of acoustic stimuli is likely to be restricted to a portion of their level response areas apparent in the tone-alone condition. Thus rather than being static, level response areas are fluid; they can vary greatly in extent, shape and response magnitude. The dynamic modulation of the level response area and level selectivity of AI neurons might be related to several tasks confronting the central auditory system.

Auditory stream segregation in monkey auditory cortex: effects of frequency separation, presentation rate, and tone duration

Auditory stream segregation refers to the organization of sequential sounds into "perceptual streams" reflecting individual environmental sound sources. In the present study, sequences of alternating high and low tones, "...ABAB...," similar to those used in psychoacoustic experiments on stream segregation, were presented to awake monkeys while neural activity was recorded in primary auditory cortex (A1). Tone frequency separation (AF), tone presentation rate (PR), and tone duration (TD) were systematically varied to examine whether neural responses correlate with effects of these variables on perceptual stream segregation. "A" tones were fixed at the best frequency of the recording site, while "B" tones were displaced in frequency from "A" tones by an amount = delta F. As PR increased, "B" tone responses decreased in amplitude to a greater extent than "A" tone responses, yielding neural response patterns dominated by "A" tone responses occurring at half the alternation rate. Increasing TD facilitated the differential attenuation of "B" tone responses. These findings parallel psychoacoustic data and suggest a physiological model of stream segregation whereby increasing delta F, PR, or TD enhances spatial differentiation of "A" tone and "B" tone responses along the tonotopic map in A1.

Effects of intensity of repetitive acoustic stimuli on neural adaptation in the ventral cochlear nucleus of the rat

To study neural adaptation as a function of stimulus intensity, auditory near-field evoked potentials were recorded from the ventral cochlear nucleus in awake Long Evans rats. Responses to 250-ms trains of repetitive clicks (pulse rates ranging from 100 to 1000 pulses per second) were collected at stimulus intensities of 5, 10, 30, 50 and 70 dB SPL. The amplitude of the first negative (N1) component of the average evoked potentials to individual pulses in the train was measured by using a subtraction method. The N1 responses were normalized with respect to the highest cochlear nucleus potential observed in the train, and then plotted as a function of click position in the train. As expected, the general trend of the curves was an exponential decay reaching a plateau more or less rapidly as a function of both intensity and rate of stimulation. Fitting these curves with exponential decay equations revealed that the rapid time constant decreased for increasing stimulus intensities whereas the short-term time constant is relatively independent of intensity. The amount of adaptation (expressed as the ratio of the plateau to the first peak amplitude) was substantially less prominent at low intensities (5-10 dB SPL) and low rates (100-200 pulses per second) than at higher intensities and high rates. These results indicate that adaptation patterns obtained in the ventral cochlear nucleus by using near-field evoked potentials exhibit properties comparable to those already present at the level of the auditory nerve.

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.

Time course of loudness recalibration: implications for loudness enhancement

Loudness recalibration, the effect of a relatively loud 2500-Hz recalibrating tone on the loudness of a relatively soft 2500-Hz target tone, was measured as a function of the interstimulus interval (ISI) between them. The loudness of the target tone, assessed by a 500-Hz comparison tone, declined when the ISI equaled or exceeded about 200 ms and leveled off at an ISI of about 700 ms. Notably, the target tone's loudness did not change significantly at very short ISIs (< 150 ms). The latter result is incompatible with the literature reporting loudness enhancement in this time window, but is compatible with the suggestion made by Scharf, Buus, and Nieder [J. Acoust. Soc. Am. 112, 807-810 (2002)] that early measurements of enhancement were contaminated by the influence of the recalibrating tone on the comparison tone when the two shared the same frequency. In a second experiment the frequency of the comparison tone was changed to 2500 Hz and the results of a loudness enhancement paradigm was successfully predicted from the time course of recalibration obtained in experiment 1.

Response adaptation of medial olivocochlear neurons is minimal

Response adaptation is a general characteristic of neurons. A number of studies have investigated the adaptation characteristics of auditory-nerve fibers, which send information to the brain about sound stimuli. However, there have been no previous adaptation studies of olivocochlear neurons, which provide efferent fibers to hair cells and auditory nerve dendrites in the auditory periphery. To study adaptation in efferent fibers, responses of single olivocochlear neurons were recorded to characteristic-frequency tones and noise, using anesthetized guinea pigs. To measure short-term adaptation, stimuli of 500 ms duration were presented, and the responses were displayed as peristimulus time histograms. These histograms showed regular peaks, indicating a "chopping" pattern of response. The rate during each chopping period as well as the general trend of the histogram could be well fit by an equation that expresses the firing rate as a sum of 1) a short-term adaptive rate that decays exponentially with time and 2) a constant steady-state rate. For the adaptation in medial olivocochlear (MOC) neurons, the average exponential time constant was 47 ms, which is roughly similar to that for short-term adaptation in auditory-nerve fibers. The amount of adaptation (expressed as a percentage decrease of onset firing rate), however, was substantially less in MOC neurons (average 31%) than in auditory-nerve fibers (average 63%). To test for adaptation over longer periods, we used noise and tones of 10 s duration. After the short-term adaptation, the responses of MOC neurons were almost completely sustained (average long-term adaptation 3%). However, in the same preparations, significant long-term adaptation was present in auditory-nerve fibers. These results indicate that the MOC response adaptation is minimal compared with that of auditory-nerve fibers. Such sustained responses may enable the MOC system to produce sustained effects in the periphery, supporting a role for this efferent system during ongoing stimuli of long duration.

Effects of auditory stimulus context on the representation of frequency in the gerbil inferior colliculus

Prior studies of dynamic conditioning have focused on modulation of binaural localization cues, revealing that the responses of inferior colliculus (IC) neurons to particular values of interaural phase and level disparities depend critically on the context in which they occur. Here we show that monaural frequency transitions, which do not simulate azimuthal motion, also condition the responses of IC neurons. We characterized single-unit responses to two frequency transition stimuli: a glide stimulus comprising two tones linked by a linear frequency sweep (origin-sweep-target) and a step stimulus consisting of one tone followed immediately by another (origin-target). Using sets of glide and step stimuli converging on a common target, we constructed conditioned response functions (RFs) depicting the variability in the response to an identical stimulus as a function of the preceding origin frequency. For nearly all cells, the response to the target depended on the origin frequency, even for origins outside the excitatory frequency response area of the cell. Results from conditioned RFs based on long (2-4 s) and short (200 ms) duration step stimuli indicate that conditioning effects can be induced in the absence of the dynamic sweep, and by stimuli of relatively short duration. Because IC neurons are tuned to frequency, changes in the origin frequency often change the "effective" stimulus duty cycle. In many cases, the enhancement of the target response appeared related to the decrease in the "effective" stimulus duty cycle rather than to the prior presentation of a particular origin frequency. Although this implies that nonselective adaptive mechanisms are responsible for conditioned responses, slightly more than half of IC neurons in each paradigm responded significantly differently to targets following origins that elicited statistically indistinguishable responses. The prevailing influence of stimulus context when discharge history is controlled demonstrates that not all the mechanisms governing conditioning depend on the discharge history of the recorded neuron. Selective adaptation among the neuron's variously tuned afferents may help engender stimulus-specific conditioning. The demonstration that conditioning effects reflect sensitivity to spectral as well as spatial stimulus contrast has broad implications for the processing of a wide range of dynamic acoustic signals and sound sequences.

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.

The precedence effect

In a reverberant environment, sounds reach the ears through several paths. Although the direct sound is followed by multiple reflections, which would be audible in isolation, the first-arriving wavefront dominates many aspects of perception. The "precedence effect" refers to a group of phenomena that are thought to be involved in resolving competition for perception and localization between a direct sound and a reflection. This article is divided into five major sections. First, it begins with a review of recent work on psychoacoustics, which divides the phenomena into measurements of fusion, localization dominance, and discrimination suppression. Second, buildup of precedence and breakdown of precedence are discussed. Third measurements in several animal species, developmental changes in humans, and animal studies are described. Fourth, recent physiological measurements that might be helpful in providing a fuller understanding of precedence effects are reviewed. Fifth, a number of psychophysical models are described which illustrate fundamentally different approaches and have distinct advantages and disadvantages. The purpose of this review is to provide a framework within which to describe the effects of precedence and to help in the integration of data from both psychophysical and physiological experiments. It is probably only through the combined efforts of these fields that a full theory of precedence will evolve and useful models will be developed.

Post-stimulatory suppression, facilitation and tuning for delays shape responses of inferior colliculus neurons to sequential pure tones

Temporal changes in the excitability of inferior colliculus (IC) neurons will shape their responses to complex stimuli. Single-unit responses of rat IC neurons to the second (probe) of a pair of tones exhibited suppression, facilitation and delay tuned effects. Responses to probe tones were markedly suppressed (by 76% for contralateral stimulation with equal intensity tone pairs) during contralateral and binaural stimulation in 60% of IC neurons. Suppression developed rapidly as a function of the duration of the initial tone, and approached maximum for tones of less than 200 ms. Suppression decreased as the interval between tones increased, and this recovery of responsiveness was often exponential (time constants: mean: 271.4 ms; median: 72.8 ms; n = 47), and independent of the duration and intensity of preceding stimulation. Facilitation of responses to probe tones was observed chiefly in neurons with 'pauser/buildup' response patterns, and decreased as the intertone interval increased. The greatest suppression of responses to probe tones occurred only after intertone intervals of 32 ms (delayed minimum; n = 8) in 11% of IC neurons. Other IC neurons exhibited an increased excitability to probe tones presented 128 ms after stimulation (delayed maximum; n = 7). The latencies of the later neurons' responses were longer (mean: 29.5 ms) than other IC neurons. The role of suppression in sound localization and echo suppression, and the relationship between 'delay tuning' effects and encoding of complex stimuli are discussed.

Responses of anteroventral cochlear nucleus neurons of the unanesthetized decerebrate cat to click pairs as simulated echoes

To elucidate the contribution of the anteroventral cochlear nucleus (AVCN) to 'echo' processing, this study documents the responses of AVCN neurons to simulated echoes and compares them to those of auditory nerve (AN) fibers. Single unit discharges were recorded from 121 units in the AVCN of 21 unanesthetized decerebrate cats in response to click pairs with inter-click intervals ranging from 1 to 32 ms between 45 and 105 dB SPL re 20 microPa. Units were classified according to the post-stimulus time histogram (PSTH) and excitatory-inhibitory response area (EI-area) schemes. Based on their spontaneous rates (SR), units were subdivided into low- ( < 20 spikes/s) and high- ( > 20 spikes/s) SR groups. A majority of the units exhibited second-click responses whose recovery time courses were similar to those of AN fibers. These units included primary-like, chopper and onset units in the PSTH scheme and Types I, I/III and III units in the EI-area scheme. A minority of the units exhibited responses that were distinct from those of AN fibers, in that they had second-click response recovery times that were either markedly reduced or prolonged. This group of units included those with primary-like, chopper and onset PSTHs and Type I/III and III EI-areas. No significant difference was found in the second-click response among various PSTH or EI-area types. High-SR AVCN units exhibited a decrease in the second-click response with increasing level. In contrast, low-SR AVCN units showed little level-dependent change in the second-click responses. This SR-based difference was similar to that previously found among AN fibers. The present results suggest that, although a majority of AVCN units exhibit similar time courses of second-click response recovery to those of AN fibers, there do exist mechanisms in the cochlear nucleus that can substantially alter this representation. Furthermore, the difference between the second-click response recovery functions of low- and high-SR AVCN units and the consistency of this finding between AVCN and AN suggest that SR represents an important dimension for signal representation in the AVCN neurons.

Influence of centrifugal pathways on forward masking of ventral cochlear nucleus neurons

When responses to one part of a sequence of auditory signals reduce the responses to a subsequent portion of the signal, "forward masking" results. Although forward masking occurs in the auditory nerve, that observed in the ventral cochlear nucleus (VCN) more closely resembles psychophysical forward masking. In contrast to the auditory nerve in which the amount of forward masking is proportional to the amount of excitation produced by the masker, most VCN neurons show a poor correlation between forward masking and excitation produced by the masker, indicating a more complex interaction between responses to adjacent signals. This study tested the hypothesis that one component of forward masking is produced by inputs from centrifugal neural connections to the VCN. The centrifugal pathways were interrupted with knife-cut lesions medial to the CN. Responses of single units obtained 60 minutes after the lesions were compared to those obtained before the lesions. In primarylike, sustained chopper and on units the lesions resulted in a reduction in forward masking and enhanced recovery. In contrast, lesions resulted in increased masking in primarylike-notch and low-intensity chopper units. The relationship between masker-elicited excitation and forward masking became more monotonic for transient choppers and on units, approaching that observed for auditory nerve fibers. These effects are probably the result of removal of both inhibitory and excitatory inputs, ultimately reflecting a balance of excitation and inhibition to each neural population in the VCN.

Responses of chopper units in the ventral cochlear nucleus of the anaesthetised guinea pig to clicks-in-noise and click trains

Auditory brainstem responses (ABRs) have been measured with clicks, clicks masked by noise, click trains and pseudorandom maximum length sequences (MLS) of clicks. To investigate the neuronal populations contributing to the ABR under these stimulation conditions, we measured the extracellular responses of ventral cochlear nucleus (VCN) units in the urethane-anaesthetised guinea pig. We studied 23 chopper, 7 primary-like and 7 onset units. This report focuses on the responses from chopper units. The probability of discharge for chopper units increased with increasing click level reaching nearly 100% in many units, over a range of about 20-30 dB. Following each response to a click there was a 5-10 ms suppression of the spontaneous or noise evoked activity. As the level of the noise was increased over a range of 20-30 dB, the response to the clicks gradually decreased leading to a complete abolition of the click response at high noise levels. In a few units, low level noise produced a facilitation of the response to single clicks. In response to constant level equally spaced click trains, discharge probability increased with increasing minimum pulse interval (MPI), approaching 100% for MPIs of 4-8 ms in some units. The recovery afforded by the gaps in the MLS train often resulted in higher discharge probability for MLS than click trains with the same MPI, while response probabilities for MLS and click trains were similar when compared at equivalent average click rates. At short MPIs (0.5 and 1.0 ms), peri stimulus time histograms in response to click trains resembled those to best frequency (BF) tones and noisebursts, with chopping peaks unrelated to unit BF. VCN units show highly synchronised and reliable responses to click trains, MLS trains and clicks masked by noise. The decrease in discharge rate and increase in latency of chopper units with decreasing click level, increasing click rate and increasing masker level parallel the peak amplitude and latency changes observed in the auditory brainstem response.

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.

Forward-masked intensity discrimination: duration effects and spectral effects

Three experiments were completed to examine the effect of masker duration and spectrum on forward-masked intensity discrimination. Four listeners participated in each experiment. Intensity discrimination was measured in quiet and in the presence of forward maskers using adaptive forced-choice procedures. The standard duration was either short (10 ms) or long (250 ms) in experiment 1 and short (10 ms) in experiment 2. The standard always occurred 100 ms after the offset of the masker. In the first experiment employing 1.0-kHz maskers and standards, a short duration masker (10 ms) produced more masking than a long duration masker (250 ms). A mid-level elevation of the Weber fraction was observed for all conditions. To ensure that the results of experiment 1 were not influenced by off-frequency listening, the second experiment employed a broadband noise masker. As before, a short duration (10 ms) masker produced more masking than a long duration masker (100 ms) and a mid-level elevation of Weber fractions was observed. This outcome is inconsistent with a peripheral sensory effect for which an increase in masker duration should result in a greater amount of adaptation, and, as a consequence more masking. A third experiment employing a broadband noise masker and standard showed the greatest amount of masking for low-level standards, but only when the duration of the masker and standard was short. This result is similar to one seen for a single listener in the first experiment for short duration tonal maskers and standards. For this listener, a second tone presented at 4.133 kHz presented simultaneously with the 1.0 kHz masker reduced significantly the amount of masking for low-level standards, but the mid-level elevation of the Weber fraction remained. Taken together, these results suggest that perceptual similarity plays a role in forward-masked intensity discrimination but does not account entirely for the mid-level elevation of the Weber fraction.

GABA- and glycine-immunoreactive projections from the superior olivary complex to the cochlear nucleus in guinea pig

Retrograde transport of horseradish peroxidase was combined with immunocytochemistry to identify the origins of potential gamma-aminobutyric acid (GABA) -ergic and glycinergic inputs to different subdivisions of the cochlear nucleus. Projection neurons in the inferior colliculus, superior olivary complex, and contralateral cochlear nucleus were examined, but only those from the superior olivary complex contained significant numbers of GABA- or glycine-immunoreactive neurons. The majority of these were in periolivary nuclei ipsilaterally, with a sizeable contribution from the contralateral ventral nucleus of the trapezoid body. Overall, 80% of olivary neurons projecting to the cochlear nucleus were immunoreactive for GABA, glycine, or both. Most glycine-immunoreactive projection neurons were located ipsilaterally, in the lateral and ventral nuclei of the trapezoid body and the dorsal periolivary nucleus. This suggests that glycine is the predominant neurotransmitter used by ipsilateral olivary projections. Most GABA-immunoreactive cells were located bilaterally in the ventral nuclei of the trapezoid body. The contralateral olivary projection was primarily GABA-immunoreactive and provided almost half the GABA-immunoreactive projections to the cochlear nucleus. This suggests that GABA is the predominant neurotransmitter used by contralateral olivary projections. The present results suggest that the superior olivary complex is the most important extrinsic source of inhibitory inputs to the cochlear nucleus. Individual periolivary nuclei differ in the strength and the transmitter content of their projections to the cochlear nucleus and may perform different roles in acoustic processing in the cochlear nucleus.

Short-term adaptation of excitation and inhibition shapes binaural processing

The temporal dependence of neuronal responses in the superior olivary complex (SOC) and central nucleus of the inferior colliculus (ICC) were examined using modified forward masking paradigms. Masking and probe tones were at the unit's best frequency and at the same intensity (20-30 dB above threshold). Short-term adaptation was observed in 85 of 113 SOC and in 32 of 50 ICC neurons, and resulted in an average decrease of probe responses (1 or 2 ms after the masker) of 56.3% in SOC neurons and 83.1% in ICC neurons. Recovery from adaptation followed exponential trends, with mean time constants of 106.1 ms and 226.9 ms for SOC and ICC neurons, respectively. Adaptation of inhibition was observed in the lateral superior olive, and may also affect many of the neurons studied. Other ICC neurons (n = 7) exhibited facilitation of probe tone responses, while 6 ICC neurons exhibited more complex temporal changes in responsiveness following a masker.

Time course of forward masking tuning curves in cat primary auditory cortex

Nonsimultaneous two-tone interactions were studied in the primary auditory cortex of anesthetized cats. Poststimulatory effects of pure tone bursts (masker) on the evoked activity of a fixed tone burst (probe) were investigated. The temporal interval from masker onset to probe onset (stimulus onset asynchrony), masker frequency, and intensity were parametrically varied. For all of the 53 single units and 58 multiple-unit clusters, the neural activity of the probe signal was either inhibited, facilitated, and/or delayed by a limited set of masker stimuli. The stimulus range from which forward inhibition of the probe was induced typically was centered at and had approximately the size of the neuron's excitatory receptive field. This "masking tuning curve" was usually V shaped, i.e., the frequency range of inhibiting masker stimuli increased with the masker intensity. Forward inhibition was induced at the shortest stimulus onset asynchrony between masker and probe. With longer stimulus onset asynchronies, the frequency range of inhibiting maskers gradually became smaller. Recovery from forward inhibition occurred first at the lower- and higher-frequency borders of the masking tuning curve and lasted the longest for frequencies close to the neuron's characteristic frequency. The maximal duration of forward inhibition was measured as the longest period over which reduction of probe responses was observed. It was in the range of 53-430 ms, with an average of 143 +/- 71 (SD) ms. Amount, duration and type of forward inhibition were weakly but significantly correlated with "static" neural receptive field properties like characteristic frequency, bandwidth, and latency. For the majority of neurons, the minimal inhibitory masker intensity increased when the stimulus onset asynchrony became longer. In most cases the highest masker intensities induced the longest forward inhibition. A significant number of neurons, however, exhibited longest periods of inhibition after maskers of intermediate intensity. The results show that the ability of cortical cells to respond with an excitatory activity depends on the temporal stimulus context. Neurons can follow higher repetition rates of stimulus sequences when successive stimuli differ in their spectral content. The differential sensitivity to temporal sound sequences within the receptive field of cortical cells as well as across different cells could contribute to the neural processing of temporally structured stimuli like speech and animal vocalizations.

Excitatory and inhibitory response adaptation in the superior olive complex affects binaural acoustic processing

Short-term adaptation was examined in single unit recordings from 113 superior olive neurons of anaesthetized 3- to 6-month-old Long-Evans rats. Responses to an equal intensity BF probe tone presented 1 ms after an 'adapting' BF tone were adapted by 56.3 +/- 2.6% (mean +/- S.E.) compared to responses at a 512 ms delay. The rapid decrease in discharge rate during adapting tones often approximated exponential time courses with time constants of less than 20 ms. The recovery from adaptation was exponential with time constants of 106 +/- 20.0 ms. The magnitude of adaptation and time course of recovery following monaural stimulation of binaurally excited (EE) neurons were not significantly different in both input pathways. Additionally, in 60% of EE neurons, an 'adapting' tone presented to one ear reduced subsequent responses to probe tones presented to the opposite ear. Binaural stimulation resulted in equal or greater adaptation of responses than monaural stimulation of either ear. The recovery of binaural excitatory responses generally followed a time course between recovery functions for ipsilateral and contralateral monaural stimuli. Lateral Superior Olive (LSO) neurons encode sound source location through the interaction of ipsilateral excitation and contralateral inhibition (IE). Ipsilaterally driven excitatory responses in LSO neurons exhibited the greatest magnitude of adaptation (68.5 +/- 21.1%). Adaptation of inhibition was observed in over half of IE neurons. Responses of LSO neurons to binaural BF probe stimuli were greatest immediately after a 200 ms BF 'inhibitory adapting' stimulus to the contralateral ear, and decreased with greater interstimulus delays. Responses to binaural stimulation were constant after prior binaural adaptation, when the magnitude and recovery of adaptation to monaural stimuli were similar for excitation and inhibition (8/25 IE cells). The functional significance and possible sites of adaptation processes are discussed.

Auditory evoked potentials in aged gerbils: responses elicited by noises separated by a silent gap

The compound action potential (CAP) and the auditory brainstem response (ABR; waves ii and iv) were recorded in young (4-8 month) and aged (33-37 month) gerbils using a paradigm similar to that used in some psychophysical studies of gap detection (a pair of identical low-pass noises separated by a silent gap). Response amplitudes were analyzed in terms of absolute amplitudes and the 'amplitude ratio' (the amplitude of the response to the second noise of a pair divided by that to the first). Response latencies were analyzed in terms of the absolute latencies as well as the 'latency shift' (the latency of the response to the second noise minus that to the first). Response amplitudes were much smaller in the aged subjects for both the first and second stimuli of a pair. There were minimal changes in amplitude ratios across age for both the CAP and ABR. Absolute latencies were similar between groups for the first stimulus of a pair, but latencies to wave iv were much longer for the aged subjects when the gap was short. Thus, the latency shift for the aged group was much longer for wave iv in the aged compared to the young group, but were similar between groups for the CAP or wave ii of the ABR. The results suggest that there may be changes in coding of temporal information in the auditory brainstem of aged gerbils which are not a direct result of abnormal temporal processing in the auditory periphery.