Responses of single neurons in physiologically defined area AI of cat cerebral cortex: sensitivity to interaural intensity differences

Stimulus-frequency-dependent dominance of sound localization cues across the cochleotopic map of the inferior colliculus

The horizontal direction of a sound source (i.e., azimuth) is perceptually determined in a frequency-dependent manner: low- and high-frequency sounds are localized via differences in the arrival time and intensity of the sound at the two ears, respectively, called interaural time and level differences (ITDs and ILDs). In the central auditory system, these binaural cues to direction are thought to be separately encoded by neurons tuned to low and high characteristic frequencies (CFs). However, at high sound levels a neuron often responds to frequencies far from its CF, raising the possibility that individual neurons may encode the azimuths of both low- and high-frequency sounds using both binaural cues. We tested this possibility by measuring auditory-driven single-unit responses in the central nucleus of the inferior colliculus (ICC) of unanesthetized female Dutch Belted rabbits with a multitetrode drive. At 70 dB SPL, ICC neurons across the cochleotopic map transmitted information in their firing rates about the direction of both low- and high-frequency noise stimuli. We independently manipulated ITD and ILD cues in virtual acoustic space and found that sensitivity to ITD and ILD, respectively, shaped the directional sensitivity of ICC neurons to low (<1.5 kHz)- and high (>3 kHz)-pass stimuli, regardless of the neuron's CF. We also found evidence that high-CF neurons transmit information about both the fine-structure and envelope ITD of low-frequency sound. Our results indicate that at conversational sound levels the majority of the cochleotopic map is engaged in transmitting directional information, even for sources with narrowband spectra.NEW & NOTEWORTHY A "division of labor" has previously been assumed in which the directions of low- and high-frequency sound sources are thought to be encoded by neurons preferentially sensitive to low and high frequencies, respectively. Contrary to this, we found that auditory midbrain neurons encode the directions of both low- and high-frequency sounds regardless of their preferred frequencies. Neural responses were shaped by different sound localization cues depending on the stimulus spectrum-even within the same neuron.

Neurons in primary auditory cortex represent sound source location in a cue-invariant manner

Auditory cortex is required for sound localisation, but how neural firing in auditory cortex underlies our perception of sound sources in space remains unclear. Specifically, whether neurons in auditory cortex represent spatial cues or an integrated representation of auditory space across cues is not known. Here, we measured the spatial receptive fields of neurons in primary auditory cortex (A1) while ferrets performed a relative localisation task. Manipulating the availability of binaural and spectral localisation cues had little impact on ferrets’ performance, or on neural spatial tuning. A subpopulation of neurons encoded spatial position consistently across localisation cue type. Furthermore, neural firing pattern decoders outperformed two-channel model decoders using population activity. Together, these observations suggest that A1 encodes the location of sound sources, as opposed to spatial cue values.

The brain's auditory cortex is involved not just in detection of sounds, but also in localizing them. Here, the authors show that neurons in ferret primary auditory cortex (A1) encode the location of sound sources, as opposed to merely reflecting spatial cues.

Processing complexity increases in superficial layers of human primary auditory cortex

The layers of the neocortex each have a unique anatomical connectivity and functional role. Their exploration in the human brain, however, has been severely restricted by the limited spatial resolution of non-invasive measurement techniques. Here, we exploit the sensitivity and specificity of ultra-high field fMRI at 7 Tesla to investigate responses to natural sounds at deep, middle, and superficial cortical depths of the human auditory cortex. Specifically, we compare the performance of computational models that represent different hypotheses on sound processing inside and outside the primary auditory cortex (PAC). We observe that while BOLD responses in deep and middle PAC layers are equally well represented by a simple frequency model and a more complex spectrotemporal modulation model, responses in superficial PAC are better represented by the more complex model. This indicates an increase in processing complexity in superficial PAC, which remains present throughout cortical depths in the non-primary auditory cortex. These results suggest that a relevant transformation in sound processing takes place between the thalamo-recipient middle PAC layers and superficial PAC. This transformation may be a first computational step towards sound abstraction and perception, serving to form an increasingly more complex representation of the physical input.

Evidence for cue-independent spatial representation in the human auditory cortex during active listening

Few auditory functions are as important or as universal as the capacity for auditory spatial awareness (e.g., sound localization). That ability relies on sensitivity to acoustical cues-particularly interaural time and level differences (ITD and ILD)-that correlate with sound-source locations. Under nonspatial listening conditions, cortical sensitivity to ITD and ILD takes the form of broad contralaterally dominated response functions. It is unknown, however, whether that sensitivity reflects representations of the specific physical cues or a higher-order representation of auditory space (i.e., integrated cue processing), nor is it known whether responses to spatial cues are modulated by active spatial listening. To investigate, sensitivity to parametrically varied ITD or ILD cues was measured using fMRI during spatial and nonspatial listening tasks. Task type varied across blocks where targets were presented in one of three dimensions: auditory location, pitch, or visual brightness. Task effects were localized primarily to lateral posterior superior temporal gyrus (pSTG) and modulated binaural-cue response functions differently in the two hemispheres. Active spatial listening (location tasks) enhanced both contralateral and ipsilateral responses in the right hemisphere but maintained or enhanced contralateral dominance in the left hemisphere. Two observations suggest integrated processing of ITD and ILD. First, overlapping regions in medial pSTG exhibited significant sensitivity to both cues. Second, successful classification of multivoxel patterns was observed for both cue types and-critically-for cross-cue classification. Together, these results suggest a higher-order representation of auditory space in the human auditory cortex that at least partly integrates the specific underlying cues.

Sensitivity to an Illusion of Sound Location in Human Auditory Cortex

Human listeners place greater weight on the beginning of a sound compared to the middle or end when determining sound location, creating an auditory illusion known as the Franssen effect. Here, we exploited that effect to test whether human auditory cortex (AC) represents the physical vs. perceived spatial features of a sound. We used functional magnetic resonance imaging (fMRI) to measure AC responses to sounds that varied in perceived location due to interaural level differences (ILD) applied to sound onsets or to the full sound duration. Analysis of hemodynamic responses in AC revealed sensitivity to ILD in both full-cue (veridical) and onset-only (illusory) lateralized stimuli. Classification analysis revealed regional differences in the sensitivity to onset-only ILDs, where better classification was observed in posterior compared to primary AC. That is, restricting the ILD to sound onset—which alters the physical but not the perceptual nature of the spatial cue—did not eliminate cortical sensitivity to that cue. These results suggest that perceptual representations of auditory space emerge or are refined in higher-order AC regions, supporting the stable perception of auditory space in noisy or reverberant environments and forming the basis of illusions such as the Franssen effect.

Tuning to Binaural Cues in Human Auditory Cortex

Interaural level and time differences (ILD and ITD), the primary binaural cues for sound localization in azimuth, are known to modulate the tuned responses of neurons in mammalian auditory cortex (AC). The majority of these neurons respond best to cue values that favor the contralateral ear, such that contralateral bias is evident in the overall population response and thereby expected in population-level functional imaging data. Human neuroimaging studies, however, have not consistently found contralaterally biased binaural response patterns. Here, we used functional magnetic resonance imaging (fMRI) to parametrically measure ILD and ITD tuning in human AC. For ILD, contralateral tuning was observed, using both univariate and multivoxel analyses, in posterior superior temporal gyrus (pSTG) in both hemispheres. Response-ILD functions were U-shaped, revealing responsiveness to both contralateral and—to a lesser degree—ipsilateral ILD values, consistent with rate coding by unequal populations of contralaterally and ipsilaterally tuned neurons. In contrast, for ITD, univariate analyses showed modest contralateral tuning only in left pSTG, characterized by a monotonic response-ITD function. A multivoxel classifier, however, revealed ITD coding in both hemispheres. Although sensitivity to ILD and ITD was distributed in similar AC regions, the differently shaped response functions and different response patterns across hemispheres suggest that basic ILD and ITD processes are not fully integrated in human AC. The results support opponent-channel theories of ILD but not necessarily ITD coding, the latter of which may involve multiple types of representation that differ across hemispheres.

Age-related changes in sound localisation ability

Auditory spatial processing is an important ability in everyday life and allows the processing of omnidirectional information. In this review, we report and compare data from psychoacoustic and electrophysiological experiments on sound localisation accuracy and auditory spatial discrimination in infants, children, and young and older adults. The ability to process auditory spatial information changes over lifetime: the perception of the acoustic space develops from an initially imprecise representation in infants and young children to a concise representation of spatial positions in young adults and the respective performance declines again in older adults. Localisation accuracy shows a strong deterioration in older adults, presumably due to declined processing of binaural temporal and monaural spectro-temporal cues. When compared to young adults, the thresholds for spatial discrimination were strongly elevated both in young children and older adults. Despite the consistency of the measured values the underlying causes for the impaired performance might be different: (1) the effect is due to reduced cognitive processing ability and is thus task-related; (2) the effect is due to reduced information about the auditory space and caused by declined processing in auditory brain stem circuits; and (3) the auditory space processing regime in young children is still undergoing developmental changes and the interrelation with spatial visual processing is not yet established. In conclusion, we argue that for studying auditory space processing over the life course, it is beneficial to investigate spatial discrimination ability instead of localisation accuracy because it more reliably indicates changes in the processing ability.

Responses of neurons in the marmoset primary auditory cortex to interaural level differences: comparison of pure tones and vocalizations

Interaural level differences (ILDs) are the dominant cue for localizing the sources of high frequency sounds that differ in azimuth. Neurons in the primary auditory cortex (A1) respond differentially to ILDs of simple stimuli such as tones and noise bands, but the extent to which this applies to complex natural sounds, such as vocalizations, is not known. In sufentanil/N2O anesthetized marmosets, we compared the responses of 76 A1 neurons to three vocalizations (Ock, Tsik, and Twitter) and pure tones at cells' characteristic frequency. Each stimulus was presented with ILDs ranging from 20 dB favoring the contralateral ear to 20 dB favoring the ipsilateral ear to cover most of the frontal azimuthal space. The response to each stimulus was tested at three average binaural levels (ABLs). Most neurons were sensitive to ILDs of vocalizations and pure tones. For all stimuli, the majority of cells had monotonic ILD sensitivity functions favoring the contralateral ear, but we also observed ILD sensitivity functions that peaked near the midline and functions favoring the ipsilateral ear. Representation of ILD in A1 was better for pure tones and the Ock vocalization in comparison to the Tsik and Twitter calls; this was reflected by higher discrimination indices and greater modulation ranges. ILD sensitivity was heavily dependent on ABL: changes in ABL by ±20 dB SPL from the optimal level for ILD sensitivity led to significant decreases in ILD sensitivity for all stimuli, although ILD sensitivity to pure tones and Ock calls was most robust to such ABL changes. Our results demonstrate differences in ILD coding for pure tones and vocalizations, showing that ILD sensitivity in A1 to complex sounds cannot be simply extrapolated from that to pure tones. They also show A1 neurons do not show level-invariant representation of ILD, suggesting that such a representation of auditory space is likely to require population coding, and further processing at subsequent hierarchical stages.

The discrimination of interaural level difference sensitivity functions: development of a taxonomic data template for modelling

Background

A major cue for the position of a high-frequency sound source in azimuth is the difference in sound pressure levels in the two ears, Interaural Level Differences (ILDs), as a sound is presented from different positions around the head. This study aims to use data classification techniques to build a descriptive model of electro-physiologically determined neuronal sensitivity functions for ILDs. The ILDs were recorded from neurons in the central nucleus of the Inferior Colliculus (ICc), an obligatory midbrain auditory relay nucleus. The majority of ICc neurons (~ 85%) show sensitivity to ILDs but with a variety of different forms that are often difficult to unambiguously separate into different information-bearing types. Thus, this division is often based on laboratory-specific and relatively subjective criteria. Given the subjectivity and non-uniformity of ILD classification methods in use, we examined if objective data classification techniques for this purpose. Our key objectives were to determine if we could find an analytical method (A) to validate the presence of four typical ILD sensitivity functions as is commonly assumed in the field, and (B) whether this method produced classifications that mapped on to the physiologically observed results.

Methods

The three-step data classification procedure forms the basic methodology of this manuscript. In this three-step procedure, several data normalization techniques were first tested to select a suitable normalization technique to our data. This was then followed by PCA to reduce data dimensionality without losing the core characteristics of the data. Finally Cluster Analysis technique was applied to determine the number of clustered data with the aid of the CCC and Inconsistency Coefficient values.

Results

The outcome of a three-step analytical data classification process was the identification of seven distinctive forms of ILD functions. These seven ILD function classes were found to map to the four “known” ideal ILD sensitivity function types, namely: Sigmoidal-EI, Sigmoidal-IE, Peaked, and Insensitive, ILD functions, and variations within these classes. This indicates that these seven templates can be utilized in future modelling studies.

Conclusions

We developed a taxonomy of ILD sensitivity functions using a methodological data classification approach. The number and types of generic ILD function patterns found with this method mapped well on to our electrophysiologically determined ILD sensitivity functions. While a larger data set of the latter functions may bring a more robust outcome, this good mapping is encouraging in providing a principled method for classifying such data sets, and could be well extended to other such neuronal sensitivity functions, such as contrast tuning in vision.

The three-channel model of sound localization mechanisms: interaural time differences

Previous psychophysical work on sound localization in humans has proposed that a midline channel be added to the current two-channel model of mammalian sound localization mechanisms. Evidence for this third channel has been found in interaural time difference (ITD) studies with low-frequency tones, and interaural level difference (ILD) studies with both high- and low-frequency tones. The latter is interesting because it suggests that, despite the fact that low frequencies do not generate significant ILDs for humans in natural settings, there is a constancy of ILD coding mechanisms across the frequency domain. To complement this finding, the present study sought to determine whether the three-channel model holds for ITDs at high frequencies. In three experiments, a selective adaptation paradigm was used in combination with transposed tones to probe for the existence of three (left, right, and midline) perceptual channels for sound source azimuth. The experiments provided evidence for lateral hemifield ITD channels but little evidence for a midline ITD channel at high frequencies.

Evidence for opponent process analysis of sound source location in humans

Research with barn owls suggested that sound source location is represented topographically in the brain by an array of neurons each tuned to a narrow range of locations. However, research with small-headed mammals has offered an alternative view in which location is represented by the balance of activity in two opponent channels broadly tuned to the left and right auditory space. Both channels may be present in each auditory cortex, although the channel representing contralateral space may be dominant. Recent studies have suggested that opponent channel coding of space may also apply in humans, although these studies have used a restricted set of spatial cues or probed a restricted set of spatial locations, and there have been contradictory reports as to the relative dominance of the ipsilateral and contralateral channels in each cortex. The current study used electroencephalography (EEG) in conjunction with sound field stimulus presentation to address these issues and to inform the development of an explicit computational model of human sound source localization. Neural responses were compatible with the opponent channel account of sound source localization and with contralateral channel dominance in the left, but not the right, auditory cortex. A computational opponent channel model reproduced every important aspect of the EEG data and allowed inferences about the width of tuning in the spatial channels. Moreover, the model predicted the oft-reported decrease in spatial acuity measured psychophysically with increasing reference azimuth. Predictions of spatial acuity closely matched those measured psychophysically by previous authors.

Mechanisms underlying azimuth selectivity in the auditory cortex of the pallid bat

This study focused on mechanisms underlying azimuth selectivity in the primary auditory cortex (A1) of pallid bats. The pallid bat listens to prey-generated noise (5-35 kHz) to localize and hunt terrestrial prey. The region of A1 tuned between 5 and 35 kHz consists of two clusters of neurons distinguished by interaural intensity difference (IID) selectivity: binaurally inhibited (EI) and peaked. The first aim of this study was to use sequential dichotic/free-field stimulation to test the hypothesis that IID is the primary cue underlying azimuth selectivity in neurons tuned in the prey-generated noise frequency band. IID selectivity and ear directionality at the neuron's characteristic frequency (CF) were used to predict azimuth selectivity functions. The predicted azimuth selectivity was compared with the actual azimuth selectivity from the same neurons. Prediction accuracy was similarly high for EI neurons and peaked neurons with low CF, whereas predictions were increasingly inaccurate with increasing CF among the peaked neurons. The second aim of this study was to compare azimuth selectivity obtained with noise and CF tones to determine the extent to which stimulus bandwidth influences azimuth selectivity in neurons with different binaural properties. The azimuth selectivity functions were similar for the two stimuli in the majority of EI neurons. A greater percentage of peaked neurons showed differences in their azimuth selectivity for noise and tones. This included neurons with multiple peaks when tested with tones and a single peak when tested with noise. Taken together, data from the two aims suggest that azimuth tuning of EI neurons is primarily dictated by IID sensitivity at CF. Peaked neurons, particularly those with high CF, may integrate IID sensitivity across frequency to generate azimuth selectivity for broadband sound. The data are consistent with those found in cat and ferret A1 in that binaurally facilitated neurons depend to a greater extent (compared to EI neurons) on spectral integration of binaural properties to generate azimuth selectivity for broadband stimuli.

Systematic representation of sound locations in the primary auditory cortex

The primary auditory cortex (A1) is involved in sound localization. A consistent observation in A1 is a clustered representation of binaural properties, but how spatial tuning varies within binaural clusters is unknown. Here, this issue was addressed in A1 of the pallid bat, a species that relies on passive hearing (as opposed to echolocation) to localize prey. Evidence is presented for systematic representations of sound azimuth within two binaural clusters in the pallid bat A1: the binaural inhibition (EI) and peaked (P) binaural interaction clusters. The representation is not a "point-to-point" space map as seen in the superior colliculus, but is in the form of a systematic increase in the area of activated cortex as azimuth changes from ipsilateral to contralateral locations. The underlying substrate in the EI cluster is a systematic representation of the medial boundary of azimuth receptive fields. The P cluster is activated mostly for sounds near the midline, providing a spatial acoustic fovea. Activity in the P cluster falls off systematically as the sound is moved to more lateral locations. Sensitivity to interaural intensity differences predicts azimuth tuning in the vast majority of neurons. Azimuth receptive field properties are relatively stable across intensity over a moderate range (20-40 dB above threshold) of intensities. This suggests that the maps will be similar across the intensities tested. These results challenge the current view that no systematic representation of azimuth is present in A1 and show that such representations are present locally within individual binaural clusters.

Stability of central binaural sound localization mechanisms in mammals, and the Heffner hypothesis

Heffner (2004) provided an overview of data on the evolutionary pressures on sound localization acuity in mammals. Her most important finding was that sound localization acuity was most strongly correlated with width of field of best vision. This correlation leaves unexplained the mechanism through which evolutionary pressures affect localization acuity in different mammals. A review of the neurophysiology of binaural sound localization cue coding, and the behavioural performance it supports, led us to two hypotheses. First, there is little or no evidence that the neural mechanisms for coding binaural sound location cues, or the dynamic range of the code, vary across mammals. Rather, the neural coding mechanism is remarkably constant both across species, and within species across frequency. Second, there is no need to postulate that evolutionary pressures are exerted on the cue coding mechanism itself. We hypothesize instead that the evolutionary pressure may be on the organism's ability to exploit a 'lower envelope principle' (after Barlow, 1972).

Interplay of excitation and inhibition elicited by tonal stimulation in pyramidal neurons of primary auditory cortex

Tonal responses of neurons in the primary auditory cortex are a function of frequency, intensity and ear of stimulation. These responses occasionally display suppression. This review discusses how excitatory and inhibitory synaptic inputs interact to form suppressive responses and how changes in stimulus attributes affect the magnitude and timing of those responses. Stimulation at the characteristic frequency evokes a stereotyped sequence of depolarization (excitatory) and then hyperpolarization (inhibitory), as predicted from the canonical circuitry. Some neurons stimulated at higher sound intensities display a prominent increase in the magnitude of hyperpolarization or a decrease in its latency, both enabling counteraction with the preceding excitation. These interactions, in part, underlie the non-monotonic suppression. Furthermore, monaural non-dominant ear stimulation elicits such a powerful hyperpolarization as to cancel out the depolarization elicited at dominant ear stimulation, suggesting a linear mechanism for the binaural suppression. Alternatively, it elicits a depolarization almost equal in magnitude and time course to that elicited at binaural stimulation, suggesting a nonlinear interaction responsible for the suppression. Laminar differences are also noted for these inhibitory interactions.

Evidence for opponent-channel coding of interaural time differences in human auditory cortex

In humans, horizontal sound localization of low-frequency sounds is mainly based on interaural time differences (ITDs). Traditionally, it was assumed that ITDs are converted into a topographic (or rate-place) code, supported by an array of neurons with parametric tuning to ITDs within the behaviorally relevant range. Although this topographic model has been confirmed in owls, its applicability to mammals has been challenged by recent physiological results suggesting that, at least in small-headed species, ITDs are represented by a nontopographic population rate code, which involves only two opponent (left and right) channels, broadly tuned to ITDs from the two auditory hemifields. The current study investigates which of these two models of ITD processing is more likely to apply to humans. For that, evoked responses to abrupt changes in the ITDs of otherwise continuous sounds were measured with electroencephalography. The ITD change was either away from ("outward" change) or toward the midline ("inward" change). According to the opponent-channel model, the response to an outward ITD change should be larger than the response to the corresponding inward change, whereas the topographic model would predict similar response sizes for both conditions. The measured response sizes were highly consistent with the predictions of the opponent-channel model and contravened the predictions of the topographic model, suggesting that, in humans, ITDs are coded nontopographically. The hemispheric distributions of the ITD change responses suggest that the majority of ITD-sensitive neurons in each hemisphere are tuned to ITDs from the contralateral hemifield.

GABA shapes a systematic map of binaural sensitivity in the auditory cortex

A consistent organizational feature of auditory cortex is a clustered representation of binaural properties. Here we address two questions. What is the intrinsic organization of binaural clusters and to what extent does intracortical processing contribute to binaural representation. We address these issues in the auditory cortex of the pallid bat. The pallid bat listens to prey-generated noise transients to localize and hunt terrestrial prey. As in other species studied, binaural clusters are present in the auditory cortex of the pallid bat. One cluster contains neurons that require binaural stimulation to be maximally excited, and are commonly termed predominantly binaural (PB) neurons. These neurons do not respond to monaural stimulation of either ear but show a peaked sensitivity to interaural intensity differences (IID) centered near 0 dB IID. We show that the peak IID varies systematically within this cluster. The peak IID is also correlated with the best frequency (BF) of neurons within this cluster. In addition, the IID selectivity of PB neurons is shaped by intracortical GABAergic input. Iontophoresis of GABA(A) receptor antagonists on PB neurons converts a majority of them to binaurally inhibited (EI) neurons that respond best to sounds favoring the contralateral ear. These data indicate that the cortex does not simply inherit binaural properties from lower levels but instead sharpens them locally through intracortical inhibition. The IID selectivity of the PB cluster indicates that the pallid bat cortex contains an increased representation of the frontal space that may underlie increased localization accuracy in this region.

A midline azimuthal channel in human spatial hearing

Neurophysiological and psychophysical evidence has driven the formulation of a hemifield model of mammalian sound localization in which the perceived location of a stimulus is based on the relative activity of two hemifield-tuned azimuthal channels that are broadly responsive to contralateral auditory space and have overlapping medial borders. However, neurophysiological work in mammals has consistently found neurons selective for sound sources at the midline, which may indicate the existence of a third, 'midline', perceptual channel. In three experiments, the existence of three (left, right, midline) perceptual channels for azimuth in man was examined using auditory selective adaptation paradigms. If no midline channel exists, exposure to highly lateralized, symmetrical adaptor frequencies should not result in a shift in the perceived intracranial location of subsequent test tones away from the adaptors because the relative activation of the two hemifield channels will remain the same. Rather, our results indicate a shift in perceived test tones towards the azimuthal midline. This result can best be explained by a perceptual/neural channel tuned to sounds located along the midline. The present study gives the first psychophysical evidence of a midline channel serving human auditory localization, adding to the earlier evidence on the same point from animal neurophysiological studies.

Cochlear implant use following neonatal deafness influences the cochleotopic organization of the primary auditory cortex in cats

Electrical stimulation of spiral ganglion neurons in a deafened cochlea, via a cochlear implant, provides a means of investigating the effects of the removal and subsequent restoration of afferent input on the functional organization of the primary auditory cortex (AI). We neonatally deafened 17 cats before the onset of hearing, thereby abolishing virtually all afferent input from the auditory periphery. In seven animals the auditory pathway was chronically reactivated with environmentally derived electrical stimuli presented via a multichannel intracochlear electrode array implanted at 8 weeks of age. Electrical stimulation was provided by a clinical cochlear implant that was used continuously for periods of up to 7 months. In 10 long-term deafened cats and three age-matched normal-hearing controls, an intracochlear electrode array was implanted immediately prior to cortical recording. We recorded from a total of 812 single unit and multiunit clusters in AI of all cats as adults using a combination of single tungsten and multichannel silicon electrode arrays. The absence of afferent activity in the long-term deafened animals had little effect on the basic response properties of AI neurons but resulted in complete loss of the normal cochleotopic organization of AI. This effect was almost completely reversed by chronic reactivation of the auditory pathway via the cochlear implant. We hypothesize that maintenance or reestablishment of a cochleotopically organized AI by activation of a restricted sector of the cochlea, as demonstrated in the present study, contributes to the remarkable clinical performance observed among human patients implanted at a young age.

Latency represents sound frequency in mouse IC

Frequency is one of the fundamental parameters of sound. The frequency of an acoustic stimulus can be represented by a neural response such as spike rate, and/or first spike latency (FSL) of a given neuron. The spike rates/frequency function of most neurons changes with different acoustic amplitudes, whereas FSL/frequency function is highly stable. This implies that FSL might represent the frequency of a sound stimulus more efficiently than spike rate. This study involved representations of acoustic frequency by spike rate and FSL of central inferior colliculus (IC) neurons responding to free-field pure-tone stimuli. We found that the FSLs of neurons responding to characteristic frequency (CF) of sound stimulus were usually the shortest, regardless of sound intensity, and that spike rates of most neurons showed a variety of function according to sound frequency, especially at high intensities. These results strongly suggest that FSL of auditory IC neurons can represent sound frequency more precisely than spike rate.

The effects of lateralized adaptors on lateral position judgements of tones within and across frequency channels

Two experiments examined the effect of highly lateralized adaptor tone pulses on the perceived intracranial location of subsequent test tones. In Experiment 1, adaptor tones of each of two frequencies, highly lateralized to opposite sides by a quarter-period interaural time difference (ITD), were found to shift the perceived intracranial location of test tones of each adaptor frequency away from the side of the adaptor. The shift in perceived location was seen for all test tone ITDs with the same sign as the adaptor tone, and sometimes extended to include test tones with small ITDs favoring the opposite ear. The generality of the effect across test tone ITDs of the same sign as the adaptor suggests that the human auditory lateralization system is built of two (left, right) hemifield-tuned azimuthal channels, and that perceived lateral location depends on the relative outputs of those two channels. In Experiment 2, the perceived location of test tones lateralized by ITD was studied in the same listeners at each of the same two frequencies, but after selective adaptation with tone pulses of only one frequency and laterality. The perceived lateral position of test tones with the same frequency as that of the adaptor underwent the same changes as seen in Experiment 1. The perceived lateral position of test tones of the nonadapted frequency usually shifted weakly in the opposite direction, i.e., in the direction expected if the second adaptor from Experiment 1 had actually been present. These data have implications both for the processes mediating selective adaptation using contingent stimuli, and for the azimuthal tuning of auditory spatial channels in man.

Electrophysiological mapping of cat primary auditory cortex with multielectrode arrays

The present study employs simultaneous multielectrode recording techniques to study the feline primary auditory cortex (AI) to characterize its functional architecture. High electrode-count microelectrode arrays provide a high spatial and temporal view of AI, but at the potential cost of significant cortical insult. However, the number of electrodes that record single- and multiunit action potentials shown in this study suggest that the implantation of high electrode-count microelectrode arrays allows for reliable recordings from the cortex and that the neurons abutting the electrode tips appear to be spared from significant insult. Using these recordings, we have constructed a functional model of AI that best specifies the distribution of characteristic frequencies (CF's), and have reaffirmed that CF is logarithmically distributed across the cortical surface with a principal CF axis perpendicular to generally straight isofrequency contours. In four cats, we found that the average CF gradient was 0.53 +/- 0.08 octave per millimeter. This study demonstrates the use of high electrode count, microelectrode array recordings in characterizing the spatial distribution of acoustic information in the feline AI.

Interaction in the perceptual processing of interaural time and level differences

Phillips and Hall [Psychophysical evidence for adaptation of central auditory processors for interaural differences in time and level, Hear. Res., 202 (2005) 188-199.] recently described the frequency-specific, selective adaptation of perceptual channels for interaural differences in level (ILD) and time (ITD). Psychometric functions for laterality based on ITD or ILD were obtained before and after exposure to adaptor tones of two frequencies presented alternately and highly lateralized to opposite sides. Following adaptation, points of perceived centrality (PPCs) were displaced towards the sides of the adaptor tones, and in opposite directions for the two frequencies. That is, laterality judgements showed a shift away from the adapted side, particularly for test cue values near the middle of the range. These data were congruent with a two-channel, opponent-process model of sound laterality coding. The present study used the same general paradigm to explore the independence of perceptual ITD and ILD processing. Psychometric functions for laterality based on ITD or ILD were obtained for each of two frequencies concurrently, before and after exposure to adaptor tones lateralized using the complementary cue. Once again, PPCs derived from the psychometric functions were displaced towards the sides of the adaptor tones, consistent with an opponent-process account of sound laterality coding. The size of the adaptation effect was at least as great as that described in the earlier study. Thus, a quarter cycle ITD adapting stimulus effected a 3 dB shift in the mean ILD-based PPC, and a 12 dB ILD adapting stimulus effected a 100 micros shift in the mean ITD-based PPC. These data offer new evidence concerning interaction in the processing of ITDs and ILDs.

Adaptive stimulus optimization for auditory cortical neurons

Despite the extensive physiological work performed on auditory cortex, our understanding of the basic functional properties of auditory cortical neurons is incomplete. For example, it remains unclear what stimulus features are most important for these cells. Determining these features is challenging given the considerable size of the relevant stimulus parameter space as well as the unpredictable nature of many neurons' responses to complex stimuli due to nonlinear integration across frequency. Here we used an adaptive stimulus optimization technique to obtain the preferred spectral input for neurons in macaque primary auditory cortex (AI). This method uses a neuron's response to progressively modify the frequency composition of a stimulus to determine the preferred spectrum. This technique has the advantage of being able to incorporate nonlinear stimulus interactions into a "best estimate" of a neuron's preferred spectrum. The resulting spectra displayed a consistent, relatively simple circumscribed form that was similar across scale and frequency in which excitation and inhibition appeared about equally prominent. In most cases, this structure could be described using two simple models, the Gabor and difference of Gaussians functions. The findings indicate that AI neurons are well suited for extracting important scale-invariant features in sound spectra and suggest that they are designed to efficiently represent natural sounds.

Limited segregation of different types of sound localization information among classes of units in the inferior colliculus

The auditory system uses three cues to decode sound location: interaural time differences (ITDs), interaural level differences (ILDs), and spectral notches (SNs). Initial processing of these cues is done in separate brainstem nuclei, with ITDs in the medial superior olive, ILDs in the lateral superior olive, and SNs in the dorsal cochlear nucleus. This work addresses the nature of the convergence of localization information in the central nucleus of the inferior colliculus (ICC). Ramachandran et al. (1999) argued that ICC neurons of types V, I, and O, respectively, receive their predominant inputs from ITD-, ILD-, and SN-sensitive brainstem nuclei, suggesting that these ICC response types should be differentially sensitive to localization cues. Here, single-unit responses to simultaneous manipulation of pairs of localization cues were recorded, and the mutual information between discharge rate and individual cues was quantified. Although rate responses to cue variation were generally consistent with those expected from the hypothesized anatomical connections, the differences in information were not as large as expected. Type I units provide the most information, especially about SNs in the physiologically useful range. Type I and O units provide information about ILDs, even at low frequencies at which actual ILDs are very small. ITD information is provided by a subset of all low-frequency neurons. Type V neurons provide information mainly about ITDs and the average binaural intensity. These results are the first to quantify the relative representation of cues in terms of information and suggest a variety of degrees of cue integration in the ICC.

Psychophysical evidence for adaptation of central auditory processors for interaural differences in time and level

Human listeners were studied for their ability to lateralize single target tones of each of two frequencies relative to midline clicks. They did so before and after exposure to adaptor tones of the same frequencies. The adaptor tones were strongly lateralized, and in opposite directions for each frequency, by either an interaural time difference (ITD, Experiment 1) or interaural level difference (ILD, Experiment 2). Following adaptation, psychometric functions for ITD (Exp. 1) and ILD (Exp. 2) were obtained for target tones for the two frequencies separately. These were found to be shifted in the direction of the fatigued side. In the case of ILD, this was in the absence of a shift in monaural sensitivity sufficient to account for the effect. For both ITD and ILD studies, shifts in perceived laterality were induced in opposite directions at two frequencies concurrently. This effect was induced with only seconds of intermittent exposure to the adaptor tones. The fact that it could be induced at two frequencies in opposite directions at the same time, suggests (a), that these data constitute new psychophysical evidence for the frequency specificity of ITD and ILD coding in the human brain, and (b), that the effect was not due to the introduction of some response bias at the decision level of perceptual judgement. The data are interpreted in terms of a two- or three-channel opponent process model.

Location coding by opponent neural populations in the auditory cortex

Although the auditory cortex plays a necessary role in sound localization, physiological investigations in the cortex reveal inhomogeneous sampling of auditory space that is difficult to reconcile with localization behavior under the assumption of local spatial coding. Most neurons respond maximally to sounds located far to the left or right side, with few neurons tuned to the frontal midline. Paradoxically, psychophysical studies show optimal spatial acuity across the frontal midline. In this paper, we revisit the problem of inhomogeneous spatial sampling in three fields of cat auditory cortex. In each field, we confirm that neural responses tend to be greatest for lateral positions, but show the greatest modulation for near-midline source locations. Moreover, identification of source locations based on cortical responses shows sharp discrimination of left from right but relatively inaccurate discrimination of locations within each half of space. Motivated by these findings, we explore an opponent-process theory in which sound-source locations are represented by differences in the activity of two broadly tuned channels formed by contra- and ipsilaterally preferring neurons. Finally, we demonstrate a simple model, based on spike-count differences across cortical populations, that provides bias-free, level-invariant localization—and thus also a solution to the “binding problem” of associating spatial information with other nonspatial attributes of sounds.

A model relying on properties of auditory cortical neurons recorded in the cat can account for the accurate localization of sounds

Processing of frequency-modulated sounds in the lateral auditory belt cortex of the rhesus monkey

Single neurons were recorded from the lateral belt areas, anterolateral (AL), mediolateral (ML), and caudolateral (CL), of nonprimary auditory cortex in 4 adult rhesus monkeys under gas anesthesia, while the neurons were stimulated with frequency-modulated (FM) sweeps. Responses to FM sweeps, measured as the firing rate of the neurons, were invariably greater than those to tone bursts. In our stimuli, frequency changed linearly from low to high frequencies (FM direction "up") or high to low frequencies ("down") at varying speeds (FM rates). Neurons were highly selective to the rate and direction of the FM sweep. Significant differences were found between the 3 lateral belt areas with regard to their FM rate preferences: whereas neurons in ML responded to the whole range of FM rates, AL neurons responded better to slower FM rates in the range of naturally occurring communication sounds. CL neurons generally responded best to fast FM rates at a speed of several hundred Hz/ms, which have the broadest frequency spectrum. These selectivities are consistent with a role of AL in the decoding of communication sounds and of CL in the localization of sounds, which works best with broader bandwidths. Together, the results support the hypothesis of parallel streams for the processing of different aspects of sounds, including auditory objects and auditory space.

Response Patterns Along an Isofrequency Contour in Cat Primary Auditory Cortex (AI) to Stimuli Varying in Average and Interaural Levels

The topographical response of a portion of an isofrequency contour in primary cat auditory cortex (AI) to a series of monaural and binaural stimuli was studied. Responses of single neurons to monaural and a matrix of binaural characteristic frequency tones, varying in average binaural level (ABL) and interaural level differences (ILD), were recorded. The topography of responses to monaural and binaural stimuli was appreciably different. Patches of cells that responded monotonically to increments in ABL alternated with patches that responded nonmonotonically to ABL. The patches were between 0.4 and 1 mm in length along an isofrequency contour. Differences were found among monotonic patches and among nonmonotonic patches. Topographically, activated and silent populations of neurons varied with both changes in ILD and changes in ABL, suggesting that the area of responsive units may underlie the coding of sound level and sound location.

Directional sensitivity of neurons in the primary auditory (AI) cortex: effects of sound-source intensity level

Transient sounds were delivered from different directions in virtual acoustic space while recording from single neurons in primary auditory cortex (AI) of cats under general anesthesia. The intensity level of the sound source was varied parametrically to determine the operating characteristics of the spatial receptive field. The spatial receptive field was constructed from the onset latency of the response to a sound at each sampled direction. Spatial gradients of response latency composing a receptive field are due partially to a systematic co-dependence on sound-source direction and intensity level. Typically, at any given intensity level, the distribution of response latency within the receptive field was unimodal with a range of approximately 3-4 ms, although for some cells and some levels, the spread could be as much as 20 or as little as 2 ms. Response latency, averaged across directions, differed among neurons for the same intensity level, and also differed among intensity levels for the same neuron. Generally, increases in intensity level resulted in decreases in the mean and variance, which follows an inverse Gaussian distribution. Receptive field models, based on response latency, are developed using multiple parameters (azimuth, elevation, intensity), validated with Monte Carlo simulation, and their spatial filtering described using spherical harmonic analysis. Observations from an ensemble of modeled receptive fields are obtained by linking the inverse Gaussian density to the probabilistic inverse problem of estimating sound-source direction and intensity. Upper bounds on acuity is derived from the ensemble using Fisher information, and the predicted patterns of estimation errors are related to psychophysical performance.

Redefining the tonotopic core of rat auditory cortex: physiological evidence for a posterior field

Previous physiological studies have identified a tonotopically organized primary auditory cortical field (AI) in the rat. Some of this prior research suggests that the rat, like other mammals, may have additional fields surrounding AI. We, therefore, recorded in the Sprague-Dawley rat extracellular responses of single neurons throughout AI, and continued posteriorly to verify the existence of a posterior field (P) and to compare the neuronal properties in the two regions. Acoustic stimuli, including tones, bandpass noise, broadband noise, and temporally modulated stimuli, were delivered dichotically via sealed systems. Consistent with previous findings, AI was characterized by an anterior-to-posterior tonotopic progression from high to low frequencies (ranging from >40 kHz to <1 kHz). A frequency reversal at the posterior border of AI marked entry into a second core tonotopic region, P, with progressively higher frequencies encountered further posteriorly, up to a point (approximately 8 kHz) where cells were no longer tone responsive. Nevertheless, bandpass noise was an effective stimulus in P, enabling characterization of cells up to 15 kHz. Compared with AI, the frequency tuning of response areas was relatively broader in P, the response latency was often longer and more variable, and the response magnitude was more commonly a nonmonotonic function of stimulus level. In both fields, most neurons were binaurally influenced. The presence of multiple auditory cortical fields in the rat is consistent with auditory cortical organization in other mammals. Moreover, the response properties of P relative to AI in the rat also resemble those found in other mammals. Finally, the physiological data suggest that core auditory cortex (temporal area TE1) is composed not only of AI as previously thought, but also of at least two other subdivisions, P and an anterior field (A). Furthermore, our physiological characterization of TE1 reveals that it is larger than suggested by previous anatomical characterizations.

Heschl's gyrus is more sensitive to tone level than non-primary auditory cortex

Previous neuroimaging studies generally demonstrate a growth in the cortical response with an increase in sound level. However, the details of the shape and topographic location of such growth remain largely unknown. One limiting methodological factor has been the relatively sparse sampling of sound intensities. Additionally, most studies have either analysed the entire auditory cortex without differentiating primary and non-primary regions or have limited their analyses to Heschl's gyrus (HG). Here, we characterise the pattern of responses to a 300-Hz tone presented in 6-dB steps from 42 to 96 dB sound pressure level as a function of its sound level, within three anatomically defined auditory areas; the primary area, on HG, and two non-primary areas, consisting of a small area lateral to the axis of HG (the anterior lateral area, ALA) and the posterior part of auditory cortex (the planum temporale, PT). Extent and magnitude of auditory activation increased non-linearly with sound level. In HG, the extent and magnitude were more sensitive to increasing level than in ALA and PT. Thus, HG appears to have a larger involvement in sound-level processing than does ALA or PT.

Gamma-aminobutyric acid circuits shape response properties of auditory cortex neurons

Neurons containing gamma aminobutyric acid (GABA) are widely distributed throughout the primary auditory cortex (AI). We investigated the effects of endogenous GABA by comparing response properties of 110 neurons in chinchilla AI before and after iontophoresis of bicuculline, a GABA(A) receptor antagonist, and/or CGP35348, a GABA(B) receptor antagonist. GABA(A) receptor blockade significantly increased spontaneous and driven discharge rates, dramatically decreased the thresholds of many neurons, and constricted the range of thresholds across the neural population. Some neurons with 'non-onset' temporal discharge patterns developed an onset pattern that was followed by a long pause. Interestingly, the excitatory response area typically expanded on both sides of the characteristic frequency; this expansion exceeded one octave in a third of the sample. Although GABA(B) receptor blockade had little effect alone, the combination of CGP35348 and bicuculline produced greater increases in driven rate and expansion of the frequency response area than GABA(A) receptor blockade alone, suggesting a modulatory role of local GABA(B) receptors. The results suggest that local GABA inhibition contributes significantly to intensity and frequency coding by controlling the range of intensities over which cortical neurons operate and the range of frequencies to which they respond. The inhibitory circuits that generate nonmonotonic rate-level functions are separate from those that influence other response properties of AI neurons.

Tonal response patterns of primary auditory cortex neurons in alert cats

The firing rates of primary auditory cortex (A1) neurons are known to be modulated only at the onset, offset, and change of a tonal stimulus in anesthetized animals. The tonal response pattern has been rarely investigated in alert animals. We investigated the time-course of A1 neuron responses to a steady tonal stimulus in alert cats. We found four types of firing responses based on statistical evaluation of the time course of the firing rate. The tonic cells (38 cells) showed a significant (P<0.05) firing increase throughout the stimulus period after a relatively long latency (mean, 25.3 ms) with little tendency of adaptation. The phasic-tonic cells (22 cells) showed a significant firing increase throughout the stimulus period after a medium latency (19.8 ms) with tendency of adaptation to less than a half of the maximum excitation level. Phasic cells (15 cells) responded, after a short latency (10.2 ms), at onset and offset of the stimuli. The unresponsive cells (26 cells) did not show a significant firing increase during stimuli. The findings suggest that there is a functional difference between each type of cells: the tonic cells encode information of static auditory signals in their firing rates; the phasic-tonic cells, of the changing auditory signal during the stimulus period; and the phasic cells, of rapid change of the auditory signal at onset and offset.

Spontaneous and stimulus-evoked intrinsic optical signals in primary auditory cortex of the cat

Spontaneous and tone-evoked changes in light reflectance were recorded from primary auditory cortex (A1) of anesthetized cats (barbiturate induction, ketamine maintenance). Spontaneous 0.1-Hz oscillations of reflectance of 540- and 690-nm light were recorded in quiet. Stimulation with tone pips evoked localized reflectance decreases at 540 nm in 3/10 cats. The distribution of patches "activated" by tones of different frequencies reflected the known tonotopic organization of auditory cortex. Stimulus-evoked reflectance changes at 690 nm were observed in 9/10 cats but lacked stimulus-dependent topography. In two experiments, stimulus-evoked optical signals at 540 nm were compared with multiunit responses to the same stimuli recorded at multiple sites. A significant correlation (P < 0.05) between magnitude of reflectance decrease and multiunit response strength was evident in only one of five stimulus conditions in each experiment. There was no significant correlation when data were pooled across all stimulus conditions in either experiment. In one experiment, the spatial distribution of activated patches, evident in records of spontaneous activity at 540 nm, was similar to that of patches activated by tonal stimuli. These results suggest that local cerebral blood volume changes reflect the gross tonotopic organization of A1 but are not restricted to the sites of spiking neurons.

Organisation of binaural interactions in the primary and dorsocaudal fields of the guinea pig auditory cortex

This study investigated the nature and topography of binaural interactions in the primary auditory field (AI) and dorsocaudal field (DC) of the urethane anaesthetised guinea pig auditory cortex. Single and multi-units were classified by their responses to monaural and binaural stimulation. In both AI and DC, units displayed binaural facilitation, binaural inhibition, or a level dependent mixture of facilitation and inhibition. There was a significant difference in the distribution of binaural response types between the two fields. Facilitated units predominated in DC (facilitated: 58%; inhibited: 24%; mixed: 6%; non-interacting: 12%), while inhibited units were the most common class in AI (facilitated: 15%; inhibited: 44%; mixed: 18%; non-interacting: 22%). It has previously been suggested that inhibited and facilitated units are concerned with processing different areas of space suggesting a possible separation of function between the two core fields. Topographically, the binaural response properties in AI and DC varied along isofrequency bands, with neurones displaying similar interactions aggregating in clusters. These clusters were similar in size for the two fields and often overlapped neighbouring isofrequency bands. However, their shape and position varied between different animals. This clustered organisation of binaural interactions is similar to that reported in recent studies of AI in other mammals.

Correlation between the activity of single auditory cortical neurons and sound-localization behavior in the macaque monkey

Lesion studies have indicated that the auditory cortex is crucial for the perception of acoustic space, yet it remains unclear how these neurons participate in this perception. To investigate this, we studied the responses of single neurons in the primary auditory cortex (AI) and the caudomedial field (CM) of two monkeys while they performed a sound-localization task. Regression analysis indicated that the responses of approximately 80% of neurons in both cortical areas were significantly correlated with the azimuth or elevation of the stimulus, or both, which we term "spatially sensitive." The proportion of spatially sensitive neurons was greater for stimulus azimuth compared with stimulus elevation, and elevation sensitivity was primarily restricted to neurons that were tested using stimuli that the monkeys also could localize in elevation. Most neurons responded best to contralateral speaker locations, but we also encountered neurons that responded best to ipsilateral locations and neurons that had their greatest responses restricted to a circumscribed region within the central 60 degrees of frontal space. Comparing the spatially sensitive neurons with those that were not spatially sensitive indicated that these two populations could not be distinguished based on either the firing rate, the rate/level functions, or on their topographic location within AI. Direct comparisons between the responses of individual neurons and the behaviorally measured sound-localization ability indicated that proportionally more neurons in CM had spatial sensitivity that was consistent with the behavioral performance compared with AI neurons. Pooling the responses across neurons strengthened the relationship between the neuronal and psychophysical data and indicated that the responses pooled across relatively few CM neurons contain enough information to account for sound-localization ability. These data support the hypothesis that auditory space is processed in a serial manner from AI to CM in the primate cerebral cortex.

Neuronal sensitivity to interaural time differences in the sound envelope in the auditory cortex of the pallid bat

Interaural time differences in the envelope of a sound (envelope ITDs) can potentially provide spatial information at high frequencies where interaural phase differences (IPDs) are not available. Interaural intensity differences (IIDs) also provide important spatial information at high frequencies. Both IIDs and envelope ITDs can influence spatial perception at high frequencies, but behavioral and physiological studies suggest that IIDs dominate perception. This study examines envelope ITD sensitivity in the auditory cortex of the pallid bat, a species that uses passive sound localization at the low end of its audible range to find prey. Its auditory system is entirely 'high-frequency' in that phase-locking does not occur at the low end of its audible range. If the bat uses ITDs, they must be derived from the envelope of the signal. A previous study of envelope ITD sensitivity in its inferior colliculus (IC) reported that neurons are sensitive to the small +/-70 micros range of available ITDs. This study extends these findings to the cortical level to assess the transformation of ITD sensitivity and the binaural response properties that underlie this sensitivity. Two measures of sensitivity were used. The dynamic ITD range measures the range of ITDs over which the maximum response of a neuron decreases by 80%. When presented with square-wave amplitude-modulated tones statically delayed in arrival time, the average dynamic ITD range in the IC is 304 micros, but dropped to 175 micros in auditory cortex. IC neurons average a 38% change in maximum response over the relevant ITD range, while cortical neurons average a 67% change. Also measured were time-intensity trading ratios, which index the extent to which a change in IID can cause a shift the dynamic ITD range. Average trading ratios are approximately the same in the IC and auditory cortex (17.9 micros/dB vs. 16.7 micros/dB, respectively). Binaural interactions changed from the IC to auditory cortex. In IC, ITD sensitivity is an inhibitory, subtractive process in which ITDs reduce the response evoked by contralateral monaural stimulation. In the auditory cortex, both binaural inhibition and facilitation occur. In the majority of cortical neurons, IID and ITD functions were remarkably similar in shape, having stepped, step-peaked or peaked functions. The binaural interactions (inhibition and/or facilitation) evoked by ITDs and IIDs were also typically similar. These results suggest that IIDs and envelope ITDs are having similar effects on output of the same binaural comparator system.

Azimuthal tuning of human perceptual channels for sound location

Human sound localization is acute for frontal locations, but relatively poor in the lateral hemifields. Previous studies in man have not, however, provided evidence on the tuning of the perceptual channels for auditory space that subserve this pattern of acuity. The spatial tuning of perceptual channels used in human azimuthal sound localization was determined using a between-channel auditory temporal gap detection paradigm. In this paradigm, gap thresholds are low when the markers bounding the silent period (gap) activate the same perceptual channel but are elevated when the two markers activate different channels. To determine the tuning of spatial channels, gap thresholds were obtained in an anechoic room with white noise markers coming from each combination of 12 leading marker locations and 18 trailing marker locations throughout the full 360 degrees of azimuth in the horizontal plane through the interaural axis. Gap thresholds remained low (2-4 ms) for all combinations of leading and trailing markers between 30 degrees and 150 degrees in both lateral hemifields. When the leading marker was located deep in one hemifield, and the trailing marker was in the opposite hemifield, gap thresholds rose to 8-16 ms. For leading marker locations at 30 degrees from the midline, gap thresholds were low for all trailing marker locations in the ipsilateral hemifield and locations near the midline in the contralateral hemifield, and were elevated (6-8 ms) only near the contralateral pole. Finally, for leading marker locations at 0 degree or 180 degrees, gap thresholds were low for any trailing location within 30 degrees of the midline at the front or back, and thresholds were elevated for trailing locations at the lateral poles. These data are accountable in terms of two broadly tuned perceptual channels, occupying the left and right auditory hemifields, respectively, each extending 30 degrees across the midline. These channels have widths and locations similar to the spatial receptive fields previously described for central auditory neurons in animals. The data suggest a model of spatial acuity based on the rates of activation of two spatially overlapping channels, rather than the selective activation of members of a large population of finely tuned channels.

Maps versus clusters: different representations of auditory space in the midbrain and forebrain

The auditory system determines the location of stimuli based on the evaluation of specific cues. The analysis begins in the tonotopic pathway, where these cues are processed in parallel, frequency-specific channels. This frequency-specific information is processed further in the midbrain and in the forebrain by specialized, space-processing pathways that integrate information across frequency channels, creating high-order neurons tuned to specific locations in space. Remarkably, the results of this integrative step are represented very differently in the midbrain and forebrain: in the midbrain, space is represented in maps, whereas, in the forebrain, space is represented in clusters of similarly tuned neurons. We propose that these different representations reflect the different roles that these two brain areas have in guiding behavior.

Azimuth coding in primary auditory cortex of the cat. I. Spike synchrony versus spike count representations

The neural representation of sound azimuth in auditory cortex most often is considered to be average firing rate, and azimuth tuning curves based thereupon appear to be rather broad. Coincident firings of simultaneously recorded neurons could provide an improved representation of sound azimuth compared with that contained in the firing rate in either of the units. In the present study, a comparison was made between local field potentials and several measures based on unit firing rate and coincident firing with respect to their azimuth-tuning curve bandwidth. Noise bursts, covering a 60-dB intensity range, were presented from nine speakers arranged in a semicircular array with a radius of 55 cm in the animal's frontal half field. At threshold intensities, all local field potential (LFP) recordings showed preferences for contralateral azimuths. Multiunit recordings showed in 74% a threshold for contralateral azimuths, in 16% for frontal azimuths, and in only 5% showed an ipsilateral threshold. The remaining 5% were not spatially tuned. Representations for directionally sensitive units based on coincident firings provided significantly sharper tuning (50-60 degrees bandwidth at 25 dB above the lowest threshold) than those based on firing rate (bandwidths of 80-90 degrees). The ability to predict sound azimuth from the directional information contained in the neural population activity was simulated by combining the responses of the 102 single units. Peak firing rates and coincident firings with LFPs at the preferred azimuth for each unit were used to construct a population vector. At stimulus levels of >/=40 dB SPL, the prediction function was sigmoidal with the predicted frontal azimuth coinciding with the frontal speaker position. Sound azimuths >45 degrees from the midline all resulted in predicted values of -90 or 90 degrees, respectively. No differences were observed in the performance of the prediction based on firing rate or coincident firings for these intensities. This suggests that although coincident firings produce narrower azimuth tuning curves, the information contained in the overall neural population does not increase compared with that contained in a firing rate representation. The relatively poor performance of the population vector further suggests that primary auditory cortex does not code sound azimuth by a globally distributed measure of peak firing rate or coincident firing.

Neuronal coding of interaural transient envelope disparities

Onsets are salient and important transient (i.e. dynamic) features of acoustic signals, and evoke vigorous responses from most auditory neurons, but paradoxically these onset responses have most often been analysed with respect to steady-state stimulus features, e.g. the sound pressure level (SPL). In nearly all studies concerned with the coding of differences in SPL at the two ears (interaural level differences; ILDs), which provide a major cue for the azimuthal location of high frequency sound sources, interaural onset disparities were covaried with ILD, but the possibly confounding effects of this covariation on neuronal responses have been entirely neglected. Therefore, dichotic stimulus paradigms were designed here in which onset and steady-state features were varied independently. Responses were recorded from single neurons in the inferior colliculus of rats, anaesthetized with pentobarbital and xylazine. It is demonstrated that onset responses, or the onset response components of neurons with more complex temporal response patterns, are dependent on the binaural combination of dynamic envelope features associated with conventional ILD stimulus paradigms, but not on the binaural combination of steady-state SPLs reached after the onset. In contrast, late or sustained response components appear more sensitive to the binaural combination of steady-state SPLs. These data stress the general necessity for a separate analysis of onset and late response components, with respect to different stimulus features, and suggest a need for re-evaluation of existing studies on ILD coding. The sensitivity of onset responses to the binaural combination of envelope transients, rather than to steady-state ILD, is in line with their sensitivity to other interaural envelope disparities, created by stationary or moving sounds.

Processing of frequency-modulated sounds in the cat's posterior auditory field

Single-neuron activity was recorded from the posterior auditory field (PAF) in the cortex of gas-anesthetized cats. Tone bursts and broadband complex sounds were used for auditory stimulation. Responses to frequency-modulated (FM) sounds, in particular, were studied systematically. Linear FM sweeps were centered around the best frequency (BF) of a neuron and had an excursion large enough to cover its whole frequency tuning range. Rate and direction of change of the FM sweeps were varied. In the majority of PAF neurons (75%) the FM response seemed not to be linear, i.e., their best instantaneous frequency (BIF) varied by more than one octave at different FM rates (FMR). When the difference between BIF and BF at each FMR was used as a measure of linearity, it was within one-third octave only at five or fewer FMR in most PAF neurons (74%). The majority of PAF neurons (70%) preferred moderate FM rates (<200 Hz/ms). Fifty-four percent of all neurons in this area showed band-pass behavior with a clear preference in the middle range of FM rates in at least one direction. Overall, neurons with high-pass behavior in both directions made up only a minor portion (22%) of PAF neurons. When both directions of an FM sweep (low-to-high and high-to-low frequency) were tested, 50% of the neurons were clearly selective for one direction, i.e., the response to one FM direction was at least twice as large as that to the other direction. This selectivity was not necessarily present at the preferred FM rate. In general, FM direction selectivity was equally distributed over FM rates tested. The selectivity of PAF neurons for the rate and direction of FM sounds makes these neurons suitable for the detection and analysis of communication sounds, which often contain FM components with a moderate sweep rate in a particular direction.

Positional, directional and speed selectivities in the primary auditory cortex of the cat

Responses of high-frequency primary auditory cortex (A1) neurons of the cat to noise stimulation were obtained in a quasianechoic chamber using a static and an apparently moving stimulus presented at similar azimuths. Simulated motion toward right or left as well as different simulated velocities were used. Under static stimulation, most units were contralateral-preferring followed by ipsilateral- and midline-preferring. Some were omnidirectional and a few were unclassifiable. Width of tuning was similar for contralateral-, ipsilateral- and midline-preferring units. Overall, about 25% were finely tuned (< 20 degrees) and the remaining were broadly tuned (> or =20 degrees). All cells sampled with static stimulation responded to apparent motion. About one quarter of the units were sensitive to the direction of the simulated moving noise in that they responded at least twice as much to one direction as to the other. Almost all directional contralateral-preferring units responded more when the apparent motion was directed toward ipsilateral azimuths, whereas all directional ipsilateral-preferring units responded preferentially to contralaterally oriented motion. In some units, up to five apparent speeds were tested. About half the units were not speed-selective (46%). The other cells were tuned to a preferential speed (40%), decreased their response as the apparent speed increased (10%) or displayed direction-dependent speed selectivity (4%). These results indicate that moving-sound sources are processed by some A1 single units.

Auditory cortical onset responses revisited. II. Response strength

Most neurons of the auditory pathway discharge spikes locked to the onset of an acoustic stimulus, but it is largely unknown in which way the acoustic parameters of sound onsets shape the neuronal responses. In this paper is analyzed the number of spikes discharged by single neurons in primary auditory cortex of barbiturate-anesthetized cats to the onsets of tones of characteristic frequency. The time course of the peak pressure (i.e., the envelope) was altered by parametrically varying sound pressure level (SPL), rise time, and rise function (linear or cosine-squared). For both rise functions, rise time had manifold, and in some cases dramatic, effects on conventional spike count-level functions. In general, threshold SPL, dynamic range, and the lowest SPL at which monotonic spike count functions saturated increased with prolongation of the rise time. In neurons with mostly nonmonotonic spike count-level functions, "best SPL" increased and the descending high-SPL arms flattened, so that functions obtained with long rise times were often monotonic whereas those obtained with shorter rise times were highly nonmonotonic. Consequently, the "tuning" to SPL was less sharp for longer rise time tones, and spike count versus rise time functions changed from "short-pass" to "long-pass" with an increase in SPL. Systematic effects of rise time persisted when spike counts were plotted against the rate of change of peak pressure or against the maximum acceleration of peak pressure. However, when spike counts were plotted as a function of the instantaneous peak pressure at the time of response initiation, the functions obtained with different rise times, and even with different rise functions, were in close register. This suggests that the stimulus-dependent component of first-spike latency can be viewed as an integration window, during which rate of change of peak pressure is integrated. The window commences with tone onset and its duration is inversely related to the maximum acceleration (or, for linear rise functions, the rate of change) of peak pressure and the neuron's transient sensitivity. The present findings seriously question, for onset responses, the usefulness of the spike count-level function and measures derived from it, such as threshold SPL, dynamic range, best SPL, or degree of nonmonotonicity. They further cast doubt onto the validity of current concepts of intensity coding at cortical levels, because most neurons' onset responses are not indicative of a signal's steady-state SPL. However, they suggest a mechanism by which a neuronal population will sample a given transient in an orderly, sensitivity-dependent, temporal sequence. The sampling rate is automatically adjusted to, and adjusted by, the rapidity of the signal's change. And the instantaneous properties of the transient could be represented by the ratios and spatial distribution of responses across the simultaneously active subpopulation. Such a mechanism could provide the basis for the demonstrated capability of discrimination of rapid transients.

The structure of spatial receptive fields of neurons in primary auditory cortex of the cat

Transient broad-band stimuli that mimic in their spectrum and time waveform sounds arriving from a speaker in free space were delivered to the tympanic membranes of barbiturized cats via sealed and calibrated earphones. The full array of such signals constitutes a virtual acoustic space (VAS). The extra-cellular response to a single stimulus at each VAS direction, consisting of one or a few precisely time-locked spikes, was recorded from neurons in primary auditory cortex. Effective sound directions form a virtual space receptive field (VSRF). Near threshold, most VSRFs were confined to one quadrant of acoustic space and were located on or near the acoustic axis. Generally, VSRFs expanded monotonically with increases in stimulus intensity, with some occupying essentially all of the acoustic space. The VSRF was not homogeneous with respect to spike timing or firing strength. Typically, onset latency varied by as much as 4-5 msec across the VSRF. A substantial proportion of recorded cells exhibited a gradient of first-spike latency within the VSRF. Shortest latencies occupied a core of the VSRF, on or near the acoustic axis, with longer latency being represented progressively at directions more distant from the core. Remaining cells had VSRFs that exhibited no such gradient. The distribution of firing probability was mapped in those experiments in which multiple trials were carried out at each direction. For some cells there was a positive correlation between latency and firing probability.

Responses of neurons in the inferior colliculus of the rat to interaural time and intensity differences in transient stimuli: Implications for the latency hypotheses

Although the sensitivity to interaural intensity differences (IIDs) of neurons receiving excitatory - inhibitory binaural input (EI neurons) has been examined in numerous studies, the mechanisms underlying this sensitivity remain unclear. According to the 'latency hypotheses' neuronal sensitivity to IIDs reflects sensitivity to differences in the timing of ipsilateral and contralateral inputs that are produced as a consequence of the effects of intensity upon latency. If the latency hypothesis is correct, a neuron's responses over any given IID range should be predicted by its responses to the interaural time differences (ITDs) that are 'equivalent' to the IIDs tested, in the sense that they produce the same changes in the relative timing of inputs. This prediction from the latency hypotheses were examined by determining the sensitivity of EI neurons in the inferior colliculus of anesthetized rats to IIDs and ITDs in click stimuli, under conditions that allowed 'equivalent' ITDs to be estimated. In approximately 10% of the 41 neurons tested, the IID-sensitivity function was a perfect or near-perfect match to the equivalent-ITD function, indicating that IID sensitivity could be entirely accounted for in terms of sensitivity to intensity-produced neural time differences, as asserted by the latency hypothesis. For the majority of neurons, however, sensitivity to equivalent ITDs accounted only partially for the characteristics of the IID-sensitivity function; other features of the function in these cases appeared to reflect the operation of an additional factor, most probably the relative magnitude of the inputs from the two ears. Although the conclusions are qualified by the fact that one of the assumptions on which the estimation of equivalent ITDs was based was probably not satisfied for some neurons, the results suggest that intensity-produced changes in both the magnitude and the timing of excitatory and inhibitory inputs shape the IID sensitivity of most EI neurons.