Cat Inferior Colliculus Research Articles

Provision of interaural time difference information in chronic intracochlear electrical stimulation enhances neural sensitivity to these differences in neonatally deafened cats

Although performance with bilateral cochlear implants is superior to that with a unilateral implant, bilateral implantees have poor performance in sound localisation and in speech discrimination in noise compared to normal hearing subjects. Studies of the neural processing of interaural time differences (ITDs) in the inferior colliculus (IC) of long-term deaf animals, show substantial degradation compared to that in normal hearing animals. It is not known whether this degradation can be ameliorated by chronic cochlear electrical stimulation, but such amelioration is unlikely to be achieved using current clinical speech processors and cochlear implants, which do not provide good ITD cues. We therefore developed a custom sound processor to deliver salient ITDs for chronic bilateral intra-cochlear electrical stimulation in a cat model of neonatal deafness, to determine if long-term exposure to salient ITDs would prevent degradation of ITD processing. We compared the sensitivity to ITDs in cochlear electrical stimuli of neurons in the IC of cats chronically stimulated with our custom ITD-aware sound processor with sensitivity in acutely deafened cats with normal hearing development and in cats chronically stimulated with a clinical stimulator and sound processor. Animals that experienced stimulation with our custom ITD-aware sound processor had significantly higher neural sensitivity to ITDs than those that received stimulation from clinical sound processors. There was no significant difference between animals received no stimulation and those that received stimulation from clinical sound processors, consistent with findings from clinical cochlear implant users. This result suggests that development and use of clinical ITD-aware sound processing strategies from a young age may promote ITD sensitivity in the clinical population.

An ecological approach to neural computation

In traditional theories of perception, primitives are extracted from sensory signals, and ecologically relevant stimuli are described in terms of complex combinations of these primitives. Recognizing these combinations requires learning. James Gibson criticized this view, for the concept of primitive is a mathematical abstraction that may have no ecological meaning. What is for a sensory system may not be simple in terms of mathematical primitives, and conversely. Instead, Gibson argued that objects in an animal's environment produce sensory signals that are highly structured, and the invariant is what is meaningful about the environment. He proposed that sensory systems directly extract this invariant structure. I will address two questions, focusing on the auditory system: 1) what invariant can be considered for a sensory system? 2) how can the nervous system extract this structure? I define elementary structure as the set of identities between the time-varying sensory signals produced by different receptors, up to a delay. That is, Si(t) = Sj(t+T), where i and j are receptor indexes. In the cochlea, this corresponds to a regularity in the spatio-temporal pattern of vibration of the basilar membrane. In a monaural context, sounds that produce such an are periodic sounds, that is, sounds that elicit a pitch percept. I postulate that the pitch of a sound is precisely the that it produces. When taking into account the physiological constraints in the identification of this (essentially, that conduction delays are limited), it explains many phenomena that were previously unexplained by any single model: lower and upper limits of pitch, the distinction between resolved and unresolved harmonic complexes, the topology of pitch (e.g. the octave similarity), and slight level dependences of pitch. In a binaural context, stimuli that produce an across the two cochleas are binaural sounds produced by a single source in the environment. The depends on source location but not on the signal emitted by the source. This is much more complex, mathematically, than often described. In particular, the interaural time difference (ITD) depends on frequency. I will show evidence from single unit recordings in the inferior colliculus of cats that binaural neurons do indeed encode this structure, and not the mathematically simpler ITD. How can this be extracted by the nervous system? Since neurons are highly sensitive to coincident inputs, a natural question to ask is for what signals the presynaptic neurons produce synchronous spikes. This yields to the concept of the synchrony receptive field of a group of neurons, defined as the set of stimuli that elicit synchronous responses in these neurons. When applied to sensory neurons, the synchrony receptive field is precisely the of sensory signals, and it can be decoded by postsynaptic neurons by coincidence detection. I will demonstrate that simple spiking models based on these ideas can indeed decode the of auditory signals corresponding to pitch and sound location. This is from the environmental viewpoint, but not from the mathematical viewpoint. Therefore, I suggest that this approach provides simple practical solutions to non-trivial sensory problems.

A physiologically constrained van Bergeijk model of interaural time difference coding predicts bandwidth‐dependent lateralization.

Recent physiological data have revived the notion that ITD is encoded centrally by the relative activity of coarsely tuned channels that pool the firing rates of many neurons [McAlpine et al., Nat. Neurosci. 4, 396–401 (2001), van Bergeijk, J. Acoust. Soc. Am. 34, 1431–1437 (1962)]. Here we implement a model of the van Bergeijk‐type and demonstrate that it predicts the dependence of perceived laterality on stimulus bandwidth [Stern et al., J. Acoust. Soc. Am. 84, 156–165 (1988)]. Each model channel comprises a large number of neurons, whose characteristic frequencies and best ITDs (BDs) are distributed to mimic physiological data from cat inferior colliculus. Each neuron cross‐correlates the bandpass‐filtered stimuli to the two ears, and the channel output is simply the summed firing rate of its neurons. The model predicts lateralization as a function of bandwidth, if four channels are used, one representing each medial and lateral superior olive. The outputs of the van Bergeijk‐type code are appropriate for controlling phonotaxis. Its lack of labeled lines minimizes the precision required in the BD distribution and may explain how a viable ITD representation is formed despite the existence of several factors that determine the BD of individual neurons.

Microvascular organization of the cat inferior colliculus

Brain neural activity depends critically on the blood supply to a given structure. The blood supply can differ within and between divisions, which may have functional significance. We analyzed the microvascular organization of the cat inferior colliculus (IC) to determine if the capillary distribution is homogenous throughout. The IC consists of the central nucleus (CN), the dorsal cortex (DC), and the lateral cortex (LC), each with different roles in auditory behavior and perception. Plastic-embedded tissue was studied from adult cats in 1-μm thick semi-thin sections stained with toluidine blue; tissue was sampled from the IC in a caudal–rostral series of sections. The architectonic subdivisions were drawn independently based on Golgi impregnations.We used the nearest neighbor distance (NND) method to quantify capillary density between subdivisions. Overall, the distribution of capillary density was non-homogenous across the IC. We found significant capillary NND differences between the CN and LC (Mann–Whitney test; p⩽0.05), CN and DC (Mann–Whitney test; p⩽0.05), and LC and DC (Mann–Whitney test; p⩽0.05). The CN had the lowest NND values among all three divisions, indicating the highest capillary density. NND values changed gradually as analysis moved from the center of the IC towards the periphery.The significantly higher microvascular density in the CN may imply that the lemniscal auditory pathway has higher levels of blood flow and metabolic activity than non-lemniscal areas of the IC. The non-homogenous microvascular organization of the IC supports parcellation schemes that delineate three major subdivisions and confirms that the borders between the three regions are not sharp.

Accurate Sound Localization in Reverberant Environments Is Mediated by Robust Encoding of Spatial Cues in the Auditory Midbrain

In reverberant environments, acoustic reflections interfere with the direct sound arriving at a listener's ears, distorting the spatial cues for sound localization. Yet, human listeners have little difficulty localizing sounds in most settings. Because reverberant energy builds up over time, the source location is represented relatively faithfully during the early portion of a sound, but this representation becomes increasingly degraded later in the stimulus. We show that the directional sensitivity of single neurons in the auditory midbrain of anesthetized cats follows a similar time course, although onset dominance in temporal response patterns results in more robust directional sensitivity than expected, suggesting a simple mechanism for improving directional sensitivity in reverberation. In parallel behavioral experiments, we demonstrate that human lateralization judgments are consistent with predictions from a population rate model decoding the observed midbrain responses, suggesting a subcortical origin for robust sound localization in reverberant environments.

Auditory Awareness Due to Synchronous Neural Activity in Inferior Colliculus of Awake Cats

The previous report on the dynamics of spontaneous and evoked activities of the neural net within the cat inferior colliculus (IC) has been extended in relation to today's issues in the study of behavioral measures such as attention and sensory awareness in terms of functional state of the local neural network. As for the consciousness, the cerebral cortex may not be the only place; however, synchronization in oscillatory activities within the IC which is influenced by experience and expectation reflected in the internal state of the animal might be the source of the simple form of auditory awareness that may accompany the execution of attentive behavior.

Comparison of Bandwidths in the Inferior Colliculus and the Auditory Nerve. II: Measurement Using a Temporally Manipulated Stimulus

To localize low-frequency sounds, humans rely on an interaural comparison of the temporally encoded sound waveform after peripheral filtering. This process can be compared with cross-correlation. For a broadband stimulus, after filtering, the correlation function has a damped oscillatory shape where the periodicity reflects the filter's center frequency and the damping reflects the bandwidth (BW). The physiological equivalent of the correlation function is the noise delay (ND) function, which is obtained from binaural cells by measuring response rate to broadband noise with varying interaural time delays (ITDs). For monaural neurons, delay functions are obtained by counting coincidences for varying delays across spike trains obtained to the same stimulus. Previously, we showed that BWs in monaural and binaural neurons were similar. However, earlier work showed that the damping of delay functions differs significantly between these two populations. Here, we address this paradox by looking at the role of sensitivity to changes in interaural correlation. We measured delay and correlation functions in the cat inferior colliculus (IC) and auditory nerve (AN). We find that, at a population level, AN and IC neurons with similar characteristic frequencies (CF) and BWs can have different responses to changes in correlation. Notably, binaural neurons often show compression, which is not found in the AN and which makes the shape of delay functions more invariant with CF at the level of the IC than at the AN. We conclude that binaural sensitivity is more dependent on correlation sensitivity than has hitherto been appreciated and that the mechanisms underlying correlation sensitivity should be addressed in future studies.

Tonotopy and inhibition in the midbrain inferior colliculus shape spectral resolution of sounds in neural critical bands

Frequency resolution and spectral integration in acoustic perception is investigated psychacoustically by measuring critical bands (CBs) or equivalent quantities. In general, CB bandwidths increase with increasing sound frequency but remain constant over a large range of sound pressure levels (SPL; intensity independence). These CB properties have previously been found, on average, in responses of midbrain inferior colliculus neurons. Here, we use single-neuron recordings from the central nucleus of mouse inferior colliculus (ICC) to study neurons' excitatory and inhibitory frequency receptive fields together with neural critical bands (NCBs) measured in a narrowband noise-masking paradigm at SPLs up to 85 dB. We aim to clarify whether and how neurons with very different shapes of excitatory and inhibitory receptive fields express CB properties, whether and how inhibition contributes to set boundaries of NCBs, and where these boundaries are located in the excitatory-inhibitory receptive fields. The main results are: the above-mentioned general CB properties exist in neurons independent of the shapes of their receptive fields, that is, frequency filtering related to single tones (tuning curves) and frequency resolution related to complex sounds (NCBs) are different neuronal properties; NCB boundaries match the boundaries of an area devoid of inhibition around the characteristic frequencies in 67% of the neurons, that is, the inhibitory influence is adjusted to frequency resolution in part of the neurons; filter bandwidths of NCBs are, relative to their centre frequencies, about on average 1/3 octave wide, equaling the average frequency distance between frequency-band laminae as found in the cat ICC.

External inferior colliculus integrates trigeminal and acoustic information: Unit responses to trigeminal nucleus and acoustic stimulation in the guinea pig

The external nucleus of the inferior colliculus (ICx) receives ascending projections from both auditory and somatosensory nuclei [L.M. Aitkin, H. Dickhaus, W. Schult, M. Zimmermann, External nucleus of inferior colliculus: auditory and spinal somatosensory afferents and their interactions, J. Neurophysiol. 41 (1978) 837–847; L.M. Aitkin, C.E. Kenyon, P. Philpott, The representation of the auditory and somatosensory systems in the external nucleus of the cat inferior colliculus, J. Comp. Neurol. 196 (1981) 25–40]. In the guinea pig, both the spinal trigeminal nucleus (TN) and the cochlear nucleus converge in the ventrolateral region of ICx [J. Zhou, S.E. Shore, Convergence of spinal trigeminal and cochlear nucleus projections in the inferior colliculus of the guinea pig, J. Comp. Neurol., in press]. We investigated the function of trigeminal-collicular pathways by electrically stimulating the TN while recording unit responses from ICx. Pairing electrical stimulation with acoustic stimuli allowed us to investigate the function of converging auditory and somatosensory inputs, i.e. multisensory integration. Unit responses were recorded from ICx using a multi channel, single shank electrode. Electrical stimulation of the TN produced small changes above or below spontaneous rate, but resulted in significant suppression or enhancement of sound-evoked responses. Multisensory integration has been demonstrated in the dorsal cochlear nucleus (DCN) [S.E. Shore, Multisensory integration in the dorsal cochlear nucleus: unit responses to acoustic and trigeminal ganglion stimulation, Eur. J. Neurosci. 21 (2005) 3334–3348], superior colliculus [M.A. Meredith, B.E. Stein, Visual, auditory, and somatosensory convergence on cells in superior colliculus results in multisensory integration, J. Neurophysiol. 56 (1986) 640–662] and sensory cortices [P.J. Laurienti, M.T. Wallace, J.A. Maldjian, C.M. Susi, B.E. Stein, J.H. Burdette, Cross-modal sensory processing in the anterior cingulate and medial prefrontal cortices, Hum. Brain Mapp. 19 (2003) 213–223] and may play a role in plasticity that occurs after sensory deprivation.

The Contribution of Spike Threshold to Acoustic Feature Selectivity, Spike Information Content, and Information Throughput

Hypotheses of sensory coding range from the notion of nonlinear "feature detectors" to linear rate coding strategies. Here, we report that auditory neurons exhibit a novel trade-off in the relationship between sound selectivity and the information that can be communicated to a postsynaptic cell. Recordings from the cat inferior colliculus show that neurons with the lowest spike rates reliably signal the occurrence of stereotyped stimulus features, whereas those with high response rates exhibit lower selectivity. The highest information conveyed by individual action potentials comes from neurons with low spike rate and high selectivity. Surprisingly, spike information is inversely related to spike rates, following a trend similar to that of feature selectivity. Information per time interval, however, was proportional to measured spike rates. A neuronal model based on the spike threshold of the synaptic drive accurately accounts for this trade-off: higher thresholds enhance the spiking fidelity at the expense of limiting the total communicated information. Such a constraint on the specificity and throughput creates a continuum in the neural code with two extreme forms of information transfer that likely serve complementary roles in the representation of the auditory environment.

Quantitative assessment of developing afferent patterns in the cat inferior colliculus revealed with calbindin immunohistochemistry and tract tracing methods

The central nucleus of the inferior colliculus (CNIC) is comprised of an orderly series of fibrodendritic layers. These layers include integrative circuitry for as many as 13 different ascending auditory pathways, each tonotopically ordered. Calcium-binding proteins, such as calbindin-D28k (CB), may be useful neurochemical markers for specific subsets of afferent input in these layers and their spatial organization that are developmentally regulated. In this study, CB-immunohistochemistry was used to examine 1–42 postnatal-day-old kitten and adult cat CNIC and anterograde tracers were used to label afferent projections from the lateral superior olivary nucleus (LSO) to the CNIC at similar ages. A distinct axonal plexus that is CB-immunopositive is described. This CB-afferent compartment is present at birth and persists throughout the ages examined. Already at birth, the CB-immunostained plexus in kitten CNIC is organized into discrete bands that are approximately 75μm thick and 500μm long. In adult CNIC, the periodic banded pattern of CB-immunostained fibers is similar to that in kittens albeit bands are thicker (145μm) and longer (700μm). Growth in band thickness in adult cat appears proportional to growth of the IC, whereas length of the dense CB-immunostained bands is somewhat more focused in the central region of fibrodendritic layers. The banded pattern of the CB-immunostained plexus is well correlated with the location and dimension of afferent projections from the LSO in newborn kitten labeled with carbocyanine dye, 1,1′-dioctodecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate and in adult cat labeled with wheat germ agglutinin conjugated with horseradish peroxidase. The results reveal a neurochemical marker for one type of synaptic compartment in CNIC layers, banding, that is organized before hearing onset in kittens, but that may undergo some postnatal pruning.

Evoked Potentials in the Cat Inferior Colliculus During Exposure to Signals Simulating Movement of Sound Sources at Different Speeds in Opposite Directions

The relationships between changes in the amplitudes of evoked potentials (EP) in the inferior colliculus of anesthetized adult cats were studied during presentation of acoustic signals simulating sound sources moving in the azimuthal plane at different speeds and in opposite directions, as well as stationary sound sources. Movement was created by changing the interaural time differences in stimuli between clicks in binaurally presented series of clicks. These studies showed that the amplitude of EP arising as a result of presentation of these signals depended on the speed of movement. The upper border of the range of speeds in which responses to "moving" and stationary signals were different was 320 degrees/sec. Different experiments showed that the greatest difference in responses was seen at movement rates of 67-320 degrees/sec, though most were recorded at speeds of 125 and 170 degrees/sec. Responses to movement in the lateromedial direction, regardless of speed, had greater magnitudes than responses to movement at the same speed in the opposite direction.

Parvalbumin-like immunostaining in the cat inferior colliculus. Light and electron microscopic investigation

The presence of the calcium-binding protein parvalbumin (PV) was studied in neuronal elements of the cat's inferior colliculus (IC) by means of light and electron microscopic immunocytochemistry. Immunostaining of PV was detected in all three main parts of the IC. Several subtypes of large neurons that differed in size and shape were immunostained, comprising approx. 15% of the total number of PV-containing neurons. Approx. half of the labeled neurons were medium sized. Two types of small neurons were found to be PV synthesizing, and comprised approx. 35% of the total PV-containing population. Ultrastructurally, many dendrites were heavily immunolabeled, and the reaction product was present in dendritic spines as well. Several types of synaptic boutons contained reaction product, and terminated on both labeled and unlabeled postsynaptic targets forming asymmetric and symmetric synapses. Approx. 70% of all PV-immunolabeled terminals contained round synaptic vesicles and formed asymmetric synapses. The majority of these boutons were of the “large round” type and corresponded to the terminals of cochlear nuclei. A lower number were of the “small round” type, and were probably corticotectal terminals. The remaining 30% of PV-containing terminals contained pleomorphic or elongated vesicles and formed symmetric synapses. These terminals corresponded with “P” and “F1” bouton types. Part of these boutons appeared to arise from nuclei of the lateral lemniscus and the superior olive, and a certain percentage likely represented endings of inhibitory interneurons.

Naturalistic Auditory Contrast Improves Spectrotemporal Coding in the Cat Inferior Colliculus

Statistical analysis of natural sounds and speech reveals logarithmically distributed spectrotemporal modulations that can cover several orders of magnitude. By contrast, most artificial stimuli used to probe auditory function, including pure tones and white noise, have linearly distributed amplitude fluctuations with a limited average dynamic range. Here we explore whether the operating range of the auditory system is physically matched to the statistical structure of natural sounds. We recorded single-unit and multi-unit neuronal activity from the central nucleus of the cat inferior colliculus (ICC) in response to dynamic spectrotemporal sound sequences to determine whether ICC neurons respond preferentially to linear or logarithmic spectrotemporal amplitudes. We varied the intensity, dynamic range, and contrast statistics of these sounds to mimic those of natural and artificial stimuli. ICC neurons exhibited monotonic and nonmonotonic contrast dependencies with increasing dynamic range that were independent of the stimulus intensity. Midbrain neurons had higher firing rates and higher receptive field energies and showed a net improvement in spectrotemporal encoding ability for logarithmic stimuli, with an increase in the mutual information rate of approximately 50% over linear amplitude sounds. This efficient use of logarithmic spectrotemporal modulations by auditory midbrain neurons reflects a neural adaptation to structural regularities in natural sounds and likely underlies human perceptual abilities.

Spectral shape sensitivity contributes to the azimuth tuning of neurons in the cat's inferior colliculus.

We recorded high-best-frequency single-unit responses to free-field noise bursts that varied in intensity and azimuth to determine whether inferior colliculus (IC) neurons derive directionality from monaural spectral-shape. Sixty-nine percent of the sample was directional (much more responsive at some azimuths than others). One hundred twenty-nine directional units were recorded under monaural conditions (unilateral ear plugging). Binaural directional (BD) cells showed weak monaural directionality. Monaural directional (MD) cells showed strong monaural directionality, i.e., were much more responsive at some directions than others. Some MD cells were sensitive to both monaural and binaural directional cues. MD cells were monaurally nondirectional in response to tone bursts that lack direction-dependent variation in spectral shape. MD cells were unresponsive to noise bursts at certain azimuths even at high intensities showing that particular spectral shapes inhibit their responses. Two-tone inhibition was stronger where MD cells were unresponsive to noise stimulation than at directions where they were responsive. According to the side-band inhibition model, MD cells derive monaural directionality by comparing energy in excitatory and inhibitory frequency domains and thus should have stronger inhibitory side-bands than BD cells. MD and BD cells showed differences in breadth of excitatory frequency domains, strength of nonmonotonic level tuning, and responsiveness to tones and noise that were consistent with this prediction. Comparison of these data with previous findings shows that strength of spectral inhibition increases greatly between the level of the cochlear nucleus and the IC, and there is relatively little change in strength of spectral inhibition among the IC, auditory thalamus, and cortex.

Gabor analysis of auditory midbrain receptive fields: spectro-temporal and binaural composition.

The spectro-temporal receptive field (STRF) is a model representation of the excitatory and inhibitory integration area of auditory neurons. Recently it has been used to study spectral and temporal aspects of monaural integration in auditory centers. Here we report the properties of monaural STRFs and the relationship between ipsi- and contralateral inputs to neurons of the central nucleus of cat inferior colliculus (ICC) of cats. First, we use an optimal singular-value decomposition method to approximate auditory STRFs as a sum of time-frequency separable Gabor functions. This procedure extracts nine physiologically meaningful parameters. The STRFs of approximately 60% of collicular neurons are well described by a time-frequency separable Gabor STRF model, whereas the remaining neurons exhibited obliquely oriented or multiple excitatory/inhibitory subfields that require a nonseparable Gabor fitting procedure. Parametric analysis reveals distinct spectro-temporal tradeoffs in receptive field size and modulation filtering resolution. Comparisons between an identical model used to study spatio-temporal integration areas of visual neurons further shows that auditory and visual STRFs share numerous structural properties. We then use the Gabor STRF model to compare quantitatively receptive field properties of contra- and ipsilateral inputs to the ICC. We show that most interaural STRF parameters are highly correlated bilaterally. However, the spectral and temporal phases of ipsi- and contralateral STRFs often differ significantly. This suggests that activity originating from each ear share various spectro-temporal response properties such as their temporal delay, bandwidth, and center frequency but have shifted or interleaved patterns of excitation and inhibition. These differences in converging monaural receptive fields expand binaural processing capacity beyond interaural time and intensity aspects and may enable colliculus neurons to detect disparities in the spectro-temporal composition of the binaural input.

Projections from the rostral pole of the inferior colliculus to the cat superior colliculus

The neuroanatomical data given here reveal a dense projection from the rostral pole of the cat inferior colliculus (rpIC) to the superior colliculus (SC). A portion of this pathway distributes in ‘patches’ across the ventral portion of the intermediate grey layer. These finding suggest that the rpIC input to the SC might play a role in determining the auditory receptive fields of SGI neurons and in the construction of the SC’s precise two dimensional map of auditory space.

Acute spiral ganglion lesions change the tuning and tonotopic organization of cat inferior colliculus neurons

Many studies have reported plastic changes in central auditory frequency organization after chronic cochlear lesions. These studies employed mechanical, acoustic or drug-induced disruptions of restricted regions of the organ of Corti that permanently alter its tuning and sensitivity and require an extended recovery period before central effects can be measured. In this study, mechanical lesions were made to 1 mm sectors of the spiral ganglion (SG). These lesions remove a restricted portion of the cochlear output, but leave the organ of Corti and basilar membrane intact. Multiunit mapping assessed the pre- and post-lesion tonotopic organization of the inferior colliculus (IC). Immediately after SG lesions, IC neurons previously tuned to the lesion frequencies became less sensitive to those frequencies but more sensitive to lesion edge frequencies, resulting in a shift in their characteristic frequencies (CFs). Notches in the excitatory response areas at frequencies corresponding to the lesion frequencies and expansion of spatial tuning curves were also observed. CFs of neurons tuned to unlesioned frequencies were unchanged. These results suggest that ‘plastic’ changes similar to those observed after long survival times in previous studies require little or no experience and occur within minutes to hours following the lesion.

Receptive fields and binaural interactions for virtual-space stimuli in the cat inferior colliculus.

Sound localization depends on multiple acoustic cues such as interaural differences in time (ITD) and level (ILD) and spectral features introduced by the pinnae. Although many neurons in the inferior colliculus (IC) are sensitive to the direction of sound sources in free field, the acoustic cues underlying this sensitivity are unknown. To approach this question, we recorded the responses of IC cells in anesthetized cats to virtual space (VS) stimuli synthesized by filtering noise through head-related transfer functions measured in one cat. These stimuli not only possess natural combinations of ITD, ILD, and spectral cues as in free field but also allow precise control over each cue. VS receptive fields were measured in the horizontal and median vertical planes. The vast majority of cells were sensitive to the azimuth of VS stimuli in the horizontal plane for low to moderate stimulus levels. Two-thirds showed a "contra-preference" receptive field, with a vigorous response on the contralateral side of an edge azimuth. The other third of receptive fields were tuned around a best azimuth. Although edge azimuths of contra-preference cells had a broad distribution, best azimuths of tuned cells were near the midline. About half the cells tested were sensitive to the elevation of VS stimuli along the median sagittal plane by showing either a peak or a trough at a particular elevation. In general receptive fields for VS stimuli were similar to those found in free-field studies of IC neurons, suggesting that VS stimulation provided the essential cues for sound localization. Binaural interactions for VS stimuli were studied by comparing responses to binaural stimulation with responses to monaural stimulation of the contralateral ear. A majority of cells showed either purely inhibitory (BI) or mixed facilitatory/inhibitory (BF&I) interactions. Others showed purely facilitatory (BF) or no interactions (monaural). Binaural interactions were correlated with azimuth sensitivity: most contra-preference cells had either BI or BF&I interactions, whereas tuned cells were usually BF. These correlations demonstrate the importance of binaural interactions for azimuth sensitivity. Nevertheless most monaural cells were azimuth-sensitive, suggesting that monaural cues also play a role. These results suggest that the azimuth of a high-frequency sound source is coded primarily by edges in azimuth receptive fields of a population of ILD-sensitive cells.

Monaural response properties of single neurons in the chinchilla inferior colliculus

The responses of 274 inferior colliculus (IC) central nucleus neurons from 20 chinchillas were studied. Characteristic frequency (CF) increased as the IC was traversed in the dorsal-ventral direction. Most units had little or no spontaneous activity, with a mean threshold for response of about 30 dB SPL across all units. Tuning curve width varied between units, with a significant increase in Q 20 with increasing CF. Peri-stimulus time histogram (PSTH) types were similar to those reported for cat inferior colliculus units. Transient, sustained, pauser, and buildup types were observed, with transient responses predominating. Response area (RA) types were also similar to those of cat IC units, with most units displaying stable best frequencies across a range of stimulus intensity levels. For a few units, excitatory RA regions were surrounded by inhibitory sidebands. Nonmonotonic discharge rate vs. stimulus intensity level functions were common in all CF ranges and for all PSTH and RA types. Mean first spike latencies, however, differed across PSTH groups, owing to the temporal definitions of these PSTH shapes. Latencies of sustained units were significantly longer than those of transient units, and buildup PSTHs showed significantly longer latencies than any other group.