Transmission delay of phase-locked cells in the medial geniculate body

Time as a supervisor: temporal regularity and auditory object learning

Sensory systems appear to learn to transform incoming sensory information into perceptual representations, or "objects," that can inform and guide behavior with minimal explicit supervision. Here, we propose that the auditory system can achieve this goal by using time as a supervisor, i.e., by learning features of a stimulus that are temporally regular. We will show that this procedure generates a feature space sufficient to support fundamental computations of auditory perception. In detail, we consider the problem of discriminating between instances of a prototypical class of natural auditory objects, i.e., rhesus macaque vocalizations. We test discrimination in two ethologically relevant tasks: discrimination in a cluttered acoustic background and generalization to discriminate between novel exemplars. We show that an algorithm that learns these temporally regular features affords better or equivalent discrimination and generalization than conventional feature-selection algorithms, i.e., principal component analysis and independent component analysis. Our findings suggest that the slow temporal features of auditory stimuli may be sufficient for parsing auditory scenes and that the auditory brain could utilize these slowly changing temporal features.

Corticofugal Modulation of Temporal and Rate Representations in the Inferior Colliculus of the Awake Marmoset

Temporal processing is crucial for auditory perception and cognition, especially for communication sounds. Previous studies have shown that the auditory cortex and the thalamus use temporal and rate representations to encode slowly and rapidly changing time-varying sounds. However, how the primate inferior colliculus (IC) encodes time-varying sounds at the millisecond scale remains unclear. In this study, we investigated the temporal processing by IC neurons in awake marmosets to Gaussian click trains with varying interclick intervals (2-100 ms). Strikingly, we found that 28% of IC neurons exhibited rate representation with nonsynchronized responses, which is in sharp contrast to the current view that the IC only uses a temporal representation to encode time-varying signals. Moreover, IC neurons with rate representation exhibited response properties distinct from those with temporal representation. We further demonstrated that reversible inactivation of the primary auditory cortex modulated 17% of the stimulus-synchronized responses and 21% of the nonsynchronized responses of IC neurons, revealing that cortico-colliculus projections play a role, but not a crucial one, in temporal processing in the IC. This study has significantly advanced our understanding of temporal processing in the IC of awake animals and provides new insights into temporal processing from the midbrain to the cortex.

Cortical Coding of Auditory Features

How the cerebral cortex encodes auditory features of biologically important sounds, including speech and music, is one of the most important questions in auditory neuroscience. The pursuit to understand related neural coding mechanisms in the mammalian auditory cortex can be traced back several decades to the early exploration of the cerebral cortex. Significant progress in this field has been made in the past two decades with new technical and conceptual advances. This article reviews the progress and challenges in this area of research.

On Zwicker tones and musical pitch in the likely absence of phase locking corresponding to the pitch

It was assessed whether Zwicker tones (ZTs) (an auditory afterimage produced by a band-stop noise) have a musical pitch. First (stage I), musically trained subjects adjusted the frequency, level, and decay time of an exponentially decaying diotic sinusoid to sound similar to the ZT they perceived following the presentation of diotic broadband noise, for various band-stop positions. Next (stage II), subjects adjusted a sinusoid in frequency and level so that its pitch was a specified musical interval below that of either a preceding ZT or a preceding sinusoid, and so that it was equally loud. For each subject the reference sinusoid corresponded to their adjusted sinusoid from stage I. Subjects selected appropriate frequency ratios for ZTs, although the standard deviations of the adjustments were larger for the ZTs than for the equally salient sinusoids by a factor of 1.0-2.2. Experiments with monaural stimuli led to similar results, although the pitch of the ZTs could differ for monaural and diotic presentation of the ZT-exciting noise. The results suggest that a weak musical pitch may exist in the absence of phase locking in the auditory nerve to the frequency corresponding to the pitch (or harmonics thereof) at the time of the percept.

Hierarchical effects of task engagement on amplitude modulation encoding in auditory cortex

We recorded from middle lateral belt (ML) and primary (A1) auditory cortical neurons while animals discriminated amplitude-modulated (AM) sounds and also while they sat passively. Engagement in AM discrimination improved ML and A1 neurons' ability to discriminate AM with both firing rate and phase-locking; however, task engagement affected neural AM discrimination differently in the two fields. The results suggest that these two areas utilize different AM coding schemes: a "single mode" in A1 that relies on increased activity for AM relative to unmodulated sounds and a "dual-polar mode" in ML that uses both increases and decreases in neural activity to encode modulation. In the dual-polar ML code, nonsynchronized responses might play a special role. The results are consistent with findings in the primary and secondary somatosensory cortices during discrimination of vibrotactile modulation frequency, implicating a common scheme in the hierarchical processing of temporal information among different modalities. The time course of activity differences between behaving and passive conditions was also distinct in A1 and ML and may have implications for auditory attention. At modulation depths ≥ 16% (approximately behavioral threshold), A1 neurons' improvement in distinguishing AM from unmodulated noise is relatively constant or improves slightly with increasing modulation depth. In ML, improvement during engagement is most pronounced near threshold and disappears at highly suprathreshold depths. This ML effect is evident later in the stimulus, and mainly in nonsynchronized responses. This suggests that attention-related increases in activity are stronger or longer-lasting for more difficult stimuli in ML.

Neural coding of temporal information in auditory thalamus and cortex

How the brain processes temporal information embedded in sounds is a core question in auditory research. This article synthesizes recent studies from our laboratory regarding neural representations of time-varying signals in auditory cortex and thalamus in awake marmoset monkeys. Findings from these studies show that 1) the primary auditory cortex (A1) uses a temporal representation to encode slowly varying acoustic signals and a firing rate-based representation to encode rapidly changing acoustic signals, 2) the dual temporal-rate representation in A1 represent a progressive transformation from the auditory thalamus, 3) firing rate-based representations in the form of a monotonic rate-code are also found to encode slow temporal repetitions in the range of acoustic flutter in A1 and more prevalently in the cortical fields rostral to A1 in the core region of the marmoset auditory cortex, suggesting further temporal-to-rate transformations in higher cortical areas. These findings indicate that the auditory cortex forms internal representations of temporal characteristic structures. We suggest that such transformations are necessary for the auditory cortex to perform a wide range of functions including sound segmentation, object processing and multi-sensory integration.

Neural coding of temporal information in auditory thalamus and cortex

How the brain processes temporal information embedded in sounds is a core question in auditory research. This article synthesizes recent studies from our laboratory regarding neural representations of time-varying signals in auditory cortex and thalamus in awake marmoset monkeys. Findings from these studies show that 1) the primary auditory cortex (A1) uses a temporal representation to encode slowly varying acoustic signals and a firing rate-based representation to encode rapidly changing acoustic signals, 2) the dual temporal-rate representations in A1 represent a progressive transformation from the auditory thalamus, 3) firing rate-based representations in the form of monotonic rate-code are also found to encode slow temporal repetitions in the range of acoustic flutter in A1 and more prevalently in the cortical fields rostral to A1 in the core region of marmoset auditory cortex, suggesting further temporal-to-rate transformations in higher cortical areas. These findings indicate that the auditory cortex forms internal representations of temporal characteristics of sounds that are no longer faithful replicas of their acoustic structures. We suggest that such transformations are necessary for the auditory cortex to perform a wide range of functions including sound segmentation, object processing and multi-sensory integration.

Phase-locked responses to pure tones in the auditory thalamus

Accurate temporal coding of low-frequency tones by spikes that are locked to a particular phase of the sine wave (phase-locking), occurs among certain groups of neurons at various processing levels in the brain. Phase-locked responses have previously been studied in the inferior colliculus and neocortex of the guinea pig and we now describe the responses in the auditory thalamus. Recordings were made from 241 single units, 32 (13%) of which showed phase-locked responses. Units with phase-locked responses were mainly (82%) located in the ventral division of the medial geniculate body (MGB), and also the medial division (18%), but were not found in the dorsal or shell divisions. The upper limiting frequency of phase-locking varied greatly between units (60-1,100 Hz) and between anatomical divisions. The upper limit in the ventral division was 520 Hz and in the medial was 1,100 Hz. The range of steady-state delays calculated from phase plots also varied: ventral division, 8.6-14 ms (mean 11.1 ms; SD 1.56); medial division, 7.5-11 ms (mean 9.3 ms; SD 1.5). Taken together, these measurements are consistent with the medial division receiving a phase-locked input directly from the brain stem, without an obligatory relay in the inferior colliculus. Cells in both the ventral and medial divisions of the MGB showed a response that phase-locked to the fundamental frequency of a guinea pig purr and may be involved in analyzing communication calls.

Neural coding strategies in auditory cortex

In contrast to the visual system, the auditory system has longer subcortical pathways and more spiking synapses between the peripheral receptors and the cortex. This unique organization reflects the needs of the auditory system to extract behaviorally relevant information from a complex acoustic environment using strategies different from those used by other sensory systems. The neural representations of acoustic information in auditory cortex can be characterized by three types: (1) isomorphic (faithful) representations of acoustic structures; (2) non-isomorphic transformations of acoustic features and (3) transformations from acoustical to perceptual dimensions. The challenge facing auditory neurophysiologists is to understand the nature of the latter two transformations. In this article, I will review recent studies from our laboratory regarding temporal discharge patterns in auditory cortex of awake marmosets and cortical representations of time-varying signals. Findings from these studies show that (1) firing patterns of neurons in auditory cortex are dependent on stimulus optimality and context and (2) the auditory cortex forms internal representations of sounds that are no longer faithful replicas of their acoustic structures.

Phase-locked responses to pure tones in the inferior colliculus

In the auditory system, some ascending pathways preserve the precise timing information present in a temporal code of frequency. This can be measured by studying responses that are phase-locked to the stimulus waveform. At each stage along a pathway, there is a reduction in the upper frequency limit of the phase-locking and an increase in the steady-state latency. In the guinea pig, phase-locked responses to pure tones have been described at various levels from auditory nerve to neocortex but not in the inferior colliculus (IC). Therefore we made recordings from 161 single units in guinea pig IC. Of these single units, 68% (110/161) showed phase-locked responses. Cells that phase-locked were mainly located in the central nucleus but also occurred in the dorsal cortex and external nucleus. The upper limiting frequency of phase-locking varied greatly between units (80-1,034 Hz) and between anatomical divisions. The upper limits in the three divisions were central nucleus, >1,000 Hz; dorsal cortex, 700 Hz; external nucleus, 320 Hz. The mean latencies also varied and were central nucleus, 8.2 +/- 2.8 (SD) ms; dorsal cortex, 17.2 ms; external nucleus, 13.3 ms. We conclude that many cells in the central nucleus receive direct inputs from the brain stem, whereas cells in the external and dorsal divisions receive input from other structures that may include the forebrain.

Phase-locked responses to pure tones in the primary auditory cortex

At the level of the brainstem, precise temporal information is essential for some aspects of binaural processing, while at the level of the cortex, rate and place mechanisms for neural coding seem to predominate. However, we now show that precise timing of steady-state responses to pure tones occurs in the primary auditory cortex (AI). Recordings were made from 163 multi-units in guinea pig AI. All units increased their firing rate in response to pure tones at 100 Hz and 46 (28%) gave sustained responses which were synchronised with the stimulus waveform (phase-locking). The phase-locking units were clustered together in columns. Phase-locking was generally strongest in layers III and IV but was also recorded in layers I, II and V. Good phase-locking was observed over a range of 60-250 Hz: some units (30%) were narrow band while others (37%) were low-pass (33% were not determined). Phase-locking strength was also influenced by sound level: some units showed monotonic increases in strength with level and others were non-monotonic. Ten of the units provided a good temporal representation of the fundamental frequency (270 Hz) of a guinea pig vocalisation (rumble) and may be involved in analysing communication calls.

Neural representations of sinusoidal amplitude and frequency modulations in the primary auditory cortex of awake primates

We investigated neural coding of sinusoidally modulated tones (sAM and sFM) in the primary auditory cortex (A1) of awake marmoset monkeys, demonstrating that there are systematic cortical representations of embedded temporal features that are based on both average discharge rate and stimulus-synchronized discharge patterns. The rate-representation appears to be coded alongside the stimulus-synchronized discharges, such that the auditory cortex has access to both rate and temporal representations of the stimulus at high and low frequencies, respectively. Furthermore, we showed that individual auditory cortical neurons, as well as populations of neurons, have common features in their responses to both sAM and sFM stimuli. These results may explain the similarities in the perception of sAM and sFM stimuli as well as the different perceptual qualities effected by different modulation frequencies. The main findings include the following. 1) Responses of cortical neurons to sAM and sFM stimuli in awake marmosets were generally much stronger than responses to unmodulated tones. Some neurons responded to sAM or sFM stimuli but not to pure tones. 2) The discharge rate-based modulation transfer function typically had a band-pass shape and was centered at a preferred modulation frequency (rBMF). Population-averaged mean firing rate peaked at 16- to 32-Hz modulation frequency, indicating that the A1 was maximally excited by this frequency range of temporal modulations. 3) Only approximately 60% of recorded units showed statistically significant discharge synchrony to the modulation waveform of sAM or sFM stimuli. The discharge synchrony-based best modulation frequency (tBMF) was typically lower than the rBMF measured from the same neuron. The distribution of rBMF over the population of neurons was approximately one octave higher than the distribution of tBMF. 4) There was a high degree of similarity between cortical responses to sAM and sFM stimuli that was reflected in both discharge rate- or synchrony-based response measures. 5) Inhibition appeared to be a contributing factor in limiting responses at modulation frequencies above the rBMF of a neuron. And 6) neurons with shorter response latencies tended to have higher tBMF and maximum discharge synchrony frequency than those with longer response latencies. rBMF was not significantly correlated with the minimum response latency.

Temporal properties of chronic cochlear electrical stimulation determine temporal resolution of neurons in cat inferior colliculus

As cochlear implants have become increasingly successful in the rehabilitation of adults with profound hearing impairment, the number of pediatric implant subjects has increased. We have developed an animal model of congenital deafness and investigated the effect of electrical stimulus frequency on the temporal resolution of central neurons in the developing auditory system of deaf cats. Maximum following frequencies (Fmax) and response latencies of isolated single neurons to intracochlear electrical pulse trains (charge balanced, constant current biphasic pulses) were recorded in the contralateral inferior colliculus (IC) of two groups of neonatally deafened, barbiturate-anesthetized cats: animals chronically stimulated with low-frequency signals (< or = 80 Hz) and animals receiving chronic high-frequency stimulation (> or = 300 pps). The results were compared with data from unstimulated, acutely deafened and implanted adult cats with previously normal hearing (controls). Characteristic differences were seen between the temporal response properties of neurons in the external nucleus (ICX; approximately 16% of the recordings) and neurons in the central nucleus (ICC; approximately 81% of all recordings) of the IC: 1) in all three experimental groups, neurons in the ICX had significantly lower Fmax and longer response latencies than those in the ICC. 2) Chronic electrical stimulation in neonatally deafened cats altered the temporal resolution of neurons exclusively in the ICC but not in the ICX. The magnitude of this effect was dependent on the frequency of the chronic stimulation. Specifically, low-frequency signals (30 pps, 80 pps) maintained the temporal resolution of ICC neurons, whereas higher-frequency stimuli significantly improved temporal resolution of ICC neurons (i.e., higher Fmax and shorter response latencies) compared with neurons in control cats. Furthermore, Fmax and latencies to electrical stimuli were not correlated with the tonotopic gradient of the ICC, and changes in temporal resolution following chronic electrical stimulation occurred uniformly throughout the entire ICC. In all three experimental groups, increasing Fmax was correlated with shorter response latencies. The results indicate that the temporal features of the chronically applied electrical signals critically influence temporal processing of neurons in the cochleotopically organized ICC. We suggest that such plastic changes in temporal processing of central auditory neurons may contribute to the intersubject variability and gradual improvements in speech recognition performance observed in clinical studies of deaf children using cochlear implants.

Lemniscal and non-lemniscal synaptic transmission in rat auditory thalamus

1. The central auditory pathway linking the inferior colliculus (IC) and the medial geniculate body (MGB) of the thalamus consists of a segregated ventral lemniscal and dorsal non-lemniscal projection whose synaptic transmission mechanisms remain unknown. Extracellular and intracellular recordings combined with axonal tract tracing and cell staining were made from lemniscal and non-lemniscal divisions of adult rat MGB maintained acutely in in vitro explants containing parallel tectothalamic projections. 2. Biocytin deposition within the brachium of the IC revealed dense axonal fibres projecting to the MGB. Thin axonal terminals were found throughout the ventral (MGv) and dorsal (MGd) divisions of the MGB. Bushy cells with tufted or bitufted dendritic branches were primarily found in the MGv. In the MGd, cells were mainly seen as stellate neurones having a radiate dendritic arbor. 3. Electrical stimulation of the brachium of IC invariably elicits fast, excitatory synaptic potentials in both MGv and MGd cells. The evoked responses occurred monosynaptically and were exclusively mediated by glutamate acting on both N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Non-lemniscal MGd neurones recorded extracellularly exhibited a strong tendency to discharge in bursts in response to brachium stimulation. In contrast, a large proportion of ventral lemniscal cells tended to discharge in single or dual spikes. Intracellularly, MGd cells, but not MGv cells, showed a predominant, slow synaptic potential mediated by NMDA receptors. 4. It is concluded that the central auditory circuitry linking the tectum and the thalamus is connected monosynaptically via glutamatergic synapses. Lemniscal and non-lemniscal thalamic neurones possess distinct response properties which cannot be accounted for by a differential transmitter system or polysynaptic delays as postulated previously.

Selectivity for temporal characteristics of sound and interaural time difference of auditory midbrain neurons in the grassfrog: a system theoretical approach

The selectivity for temporal characteristics of sound and interaural time difference (ITD) was investigated in the torus semicircularis (TS) of the grassfrog. Stimuli were delivered by means of a closed sound system and consisted of binaurally presented Poisson distributed condensation clicks, and pseudo-random (RAN) or equidistant (EQU) click trains of which ITD was varied. With RAN and EQU trains, 86% of the TS units demonstrated a clear selectivity for ITD. Most commonly, these units had monotonically increasing ITD-rate functions. In general, units responding to Poisson clicks, responded also to RAN and EQU trains. One category of units which showed strong time-locking had comparable selectivities for ITD with both stimulus ensembles. A second category of units showed a combined selectivity for temporal structure and ITD. These units responded exclusively to EQU trains in a nonsynchronized way. From the responses obtained with the Poisson click ensemble so-called Poisson system kernels were determined, in analogy to the Wiener-Volterra functional expansion for nonlinear systems. The kernel analysis was performed up to second order. Contralateral (CL) first order kernels usually had positive or combinations of positive and negative regions, indicating that the contralateral ear exerted an excitatory or combined excitatory-inhibitory influence upon the neural response. Ipsilateral (IL), units were characterized by first order kernels which were not significantly different from zero, or kernels in which a single negative region was present. A large variety of CL second order kernels has been observed whereas rarely IL second order kernels were encountered. About 35% of the units possessed nonzero second order cross kernels, which indicates that CL and IL neural processes are interacting in a nonlinear way. Units demonstrating a pronounced selectivity for ITD, were generally characterized by positive CL combined with negative IL first order kernels. Findings suggested that, in the grassfrog, neural selectivity for ITD mainly is established by linear interaction of excitatory and inhibitory processes originating from the CL and IL ear, respectively. Units exhibiting strong time-locking to Poisson clicks and RAN and EQU trains had significantly shorter response latencies than moderately time-locking units. In the first category of units, a substantial higher number of nonzero first and second order kernels was observed. It was concluded that nonlinear response properties, as observed in TS units, most likely have to be ascribed to nonlinear characteristics of neural components located in the auditory nervous system peripheral to the torus semicircularis.

Frequency selectivity is related to temporal processing in parallel thalamocortical auditory pathways

Lemniscal and non-lemniscal parallel thalamocortical auditory pathways have been identified with the ventral medial geniculate body (MGB) vs. the dorsal and medial MGB, respectively. Lemniscal neurons have narrow frequency tuning and provide highly specific frequency information to the auditory cortex whereas non-lemniscal neurons generally have broader tuning and greater response lability, including plasticity of frequency receptive fields during learning. To determine if frequency selectivity is related to temporal fidelity of response, we measured both the breadth of tuning and neuronal excitability in a paired tone paradigm for single neurons throughout the MGB. Excitability to the second tone of a pair was directly correlated with frequency selectivity: the narrower the frequency tuning, the greater the excitability. Cells with broad tuning based on multiple-peak response areas also were less excitable than cells with single-peak RAs. Cells in the ventral MGB showed greater temporal fidelity of response (greater excitability) than cells in the dorsal and medial MGB. These findings show that high degrees of both frequency selectivity and temporal response fidelity are characteristic of the lemniscal, but not the non-lemniscal, thalamocortical auditory system.

Measuring and modelling the response of auditory midbrain neurons in the grassfrog to temporally structured binaural stimuli

The combined selectivity for amplitude modulation frequency (AMF) and interaural time difference (ITD) was investigated for single units in the auditory midbrain of the grassfrog. Stimuli were presented by means of a closed sound system. A large number of units was found to be selective for AMF (95%) or ITD (85%) and mostly, these selectivities were intricately coupled. At zero ITD most units showed a band-pass (54%) or bimodal (24%) AMF-rate histogram. At an AMF of 36 Hz, which is equal to the pulse repetition rate of the mating call, 70% of the units possessed an asymmetrical ITD-rate histogram, whereas about 15% showed a symmetrically peaked histogram. With binaural stimulation more units appeared to be selective for AMF (95%) as was the case with monaural stimulation (85%). A large fraction of the units appeared to be most selective for ITD at AMFs of 36 and 72 Hz, whereas units seldomly exhibited ITD selectivity with unmodulated tones. Based upon previous papers (Melssen et al., 1990; Van Stokkum, 1990) a binaural model is proposed to explain these findings. An auditory midbrain neuron is modelled as a third order neuron which receives excitatory input from second order neurons. Furthermore the model neuron receives inputs from the other ear, which may be either excitatory or inhibitory. Spatiotemporal integration of inputs from both ears, followed by action potential generation, produces a combined selectivity for AMF and ITD. In particular the responses of an experimentally observed EI neuron to a set of stimuli are reproduced well by the model.

Functional organization of the avian auditory cortex analogue. II. Topographic distribution of latency

Onset latencies of units were measured at 70 dB SPL in the auditory forebrain area (field L/Hv-complex) of awake domestic chicks. Latencies ranged from 8.8 to 75 ms. Latencies of units averaged for octave bands of best frequencies (BF) declined with increasing BF. Latencies were topographically distributed in the radial but not in the longitudinal dimension of frequency band laminae (FB laminae). Latencies were shortest in the input-layer L2 and increased systematically towards the postsynaptic layers L3 and L1/Hv, respectively. This topography visualizes the spatiotemporal spread of onset excitation and reflects the hierarchical processing within the structure. It also indicates a topographical representation of temporal resolution.

Sensitivity for interaural time and intensity difference of auditory midbrain neurons in the grassfrog

The sensitivity for interaural time (ITD) and intensity (IID) difference was investigated for single units in the auditory midbrain of the grassfrog. A temporally structured stimulus was used which was presented by means of a closed sound system. At best frequency (BF) the majority of units was selective for ITD as indicated by an asymmetrically (73%) or symmetrically (7%) shaped ITD-rate histogram. About 20% appeared to be nonselective. Units with a symmetrical rate histogram had BFs well above 0.9 kHz, whereas for the other categories no relationship with BF was observed. Most units had a selectivity for ITD which was rather independent from frequency and absolute intensity level. In 62% of the units interaural time difference could be traded by interaural intensity difference. In most cases this so-called time-intensity trading could be explained by the intensity-latency characteristics of auditory nerve fibres. About 20% was sensitive to IID only and 5% to ITD only. A binaural model is proposed which is based on the intensity-rate and intensity-latency characteristics of auditory nerve fibres, the linear summation of excitatory and inhibitory post synaptic potentials in second order neurons, and spatiotemporal integration at the level of third order neurons. By variation of only a small number of parameters, namely strengths and time constants of the connectivities, the range of experimentally observed response patterns could be reproduced.

Covariation of latency and temporal resolution in the inferior colliculus of the cat

Onset latency of single-unit and multiunit responses was measured in the central nucleus of the inferior colliculus (ICC) of barbiturate anesthetized cats. The relationship of the units' onset latency with their characteristic frequency (CF), their best modulation frequency (BMF), and their location within the ICC was studied. Latencies were significantly correlated to BMF; they were only weakly correlated to CF. The contribution of CF to the delays can be attributed to the traveling wave mechanism; covariation of latency and temporal resolution (BMF) likely manifests neuronal mechanisms underlying periodicity coding.

Single-unit characteristics in the auditory midbrain of the immobilized grassfrog

The anuran auditory midbrain of the grassfrog (Rana temporaria L.) was studied by a combined spectro-temporal analysis of sound preceding neural events. From the spectro-temporal sensitivities (STS) estimates of best frequencies (BF) and latencies (LT) were derived. Several types of STSs were observed: monomodal excitatory STSs comprised about half of the cases. Bimodal excitatory STSs, i.e. STSs with two discrete excitation regions, were observed in about 25%. Trimodal and broadly tuned STSs comprised about 5%. The remaining 20% of the STSs were characterized by inhibitory phenomena such as pure inhibition, sideband inhibition and post-activation inhibition. The distribution of best frequencies matches the frequency spectrum of the animal's vocalizations. A relative absence of monomodal units was noted in the mid frequency range. The distribution of latencies was bimodal over the range 7-108 ms. For each unit 6 functional parameters were determined; besides BF and LT these were: form of the STS (i.e. monomodality versus multimodality), spontaneous activity, binaural interaction, and firing mode (i.e. sustained versus transient) upon continuous noises stimulation. In addition, two structural parameters were considered: location in the torus and action potential waveform. Large correlations appeared between LT and action potential waveform, and between BF and binaural interaction type. Tonotopy was not found. A comparison was made between results from this study with a previous study on lightly anesthetized grassfrogs, using the same stimulus paradigms (D.J. Hermes et al. (1981): Hearing Res. 5, 147-178; D.J. Hermes et al. (1982): Hearing Res. 6, 103-126). Spontaneous activity, inhibitory phenomena and complex STSs were common using immobilization, whereas these have hardly been observed using anesthesia. Furthermore, interdependencies between the neural characteristics are substantially weaker for the immobilized preparation.

Neural correlates of cubic difference tones in the medial geniculate body of the cat

Single unit responses to the cubic difference tone CDT (2f1 - f2 = CF) and the difference tone DT (f2 - f1 = CF) were studied in the medial geniculate body (MGB) of the cat. Out of 66 units tested with CDT stimuli and having characteristic frequencies (CF) below 10 kHz, 77% gave a response to the two-tone combination stimulus. The component tones when presented alone evoked no responses, or in some cases a response pattern that was different from the one observed for the combination tone. The CDT response pattern was always similar to that seen for a pure tone at the CF. The threshold of response for the CDT was 10-70 dB higher than for a pure tone stimulus at the CF. The few units which were phase-locked could be synchronised with the CF, CDT, or DT, depending on the particular stimulus conditions. The index of synchrony was in many cases found to be higher for CDT responses than for a pure tone at CF.

Intensity functions of single unit responses to tone in the medial geniculate body of cat

Extracellular spike activity was recorded from single units in the medial geniculate body (MGB) of nitrous oxide anaesthetized cats. The responses of 291 units to tone bursts at the characteristic frequency (CF) were studied as a function of stimulus intensity, covering a range from 10 to 100 dB SPL. The proportion of MGB units characterized by a monotonic or a non-monotonic discharge rate--intensity function was 26% and 74%, respectively. In addition, changes of response latency as a function of tone levels were demonstrated to be either monotonic (38% of units) or non-monotonic (62% of units). One third Of MGB units showed a change of response pattern with increasing intensities, in similar proportion towards either prevailing excitatory or inhibitory components. The monotonic units tended to differ from non-monotonic ones in addition to their intensity function by showing shorter response latencies, a higher response probability to broad-band stimuli and simpler response patterns. The mean dynamic range of the monotonic unit population was 60 dB, with thresholds ranging from 10 to 90 dB SPL; most discharge rate--intensity functions did not saturate at sound levels of 100 dB SPL. In the population of non-monotonic units, the 'best' intensity, defined as th intensity giving the strongest response, ranged between 10 and 100 dB SPL. The present results suggest that the intensity could be signaled by the mean firing rate of a restricted population of monotonic units or place coded by the distribution of maximally activated non-monotonic units which are broadly tuned to different intensities.

Neural coding of repetitive clicks in the medial geniculate body of cat

The activity of 418 medial geniculate body (MGB) units was studied in response to repetitive acoustic pulses in 35 nitrous oxide anaesthetized cats. The proportion of MGB neurons insensitive to repetitive clicks was close to 30%. On the basis of their pattern of discharge, the responsive units were divided into three categories. The majority of them (71%), classified as "lockers', showed discharges precisely time-locked to the individual clicks of the train. A few units (8%), called "groupers', had discharges loosely synchronized to low-rate repetitive clicks. When the spikes were not synchronized, the cell had transient or sustained responses for a limited frequency range and was classified as a "special responder' (21%). Responses of "lockers' were time-locked up to a limiting rate, which varied between 10 and 800 Hz; half of the "lockers' had a limiting rate of locking equal to or higher than 100 Hz. The degree of entrainment, defined as the probability that each click evokes at least one spike, regularly decreases for increasing rates; on the other hand, the precision of locking increasing increases with frequency. The time jitter observed at 100 Hz might be as small as 0.2 ms and was 1.2 ms on average. The population of "lockers' can mark with precision the transients of complex sounds and has response properties still compatible with a temporal coding of the fundamental frequency of most animal vocalizations.