Sensitivity of single neurons in auditory cortex of cat to binaural tonal stimulation; effects of varying interaural time and intensity

Modular organization of frequency integration in primary auditory cortex

Two fundamental aspects of frequency analysis shape the functional organization of primary auditory cortex. For one, the decomposition of complex sounds into different frequency components is reflected in the tonotopic organization of auditory cortical fields. Second, recent findings suggest that this decomposition is carried out in parallel for a wide range of frequency resolutions by neurons with frequency receptive fields of different sizes (bandwidths). A systematic representation of the range of frequency resolution and, equivalently, spectral integration shapes the functional organization of the iso-frequency domain. Distinct subregions, or "modules," along the iso-frequency domain can be demonstrated with various measures of spectral integration, including pure-tone tuning curves, noise masking, and electrical cochlear stimulation. This modularity in the representation of spectral integration is expressed by intrinsic cortical connections. This organization has implications for our understanding of psychophysical spectral integration measures such as the critical band and general cortical coding strategies.

Neural sensitivity to interaural time differences: beyond the Jeffress model

Interaural time differences (ITDs) are a major cue for localizing the azimuthal position of sounds. The dominant models for processing ITDs are based on the Jeffress model and predict neurons that fire maximally at a common ITD across their responsive frequency range. Such neurons are indeed found in the binaural pathways and are referred to as "peak-type." However, other neurons discharge minimally at a common ITD (trough-type), and others do not display a common ITD at the maxima or minima (intermediate-type). From recordings of neurons in the auditory cortex of the unanesthetized rabbit to low-frequency tones and envelopes of high-frequency sounds, we show that the different response types combine to form a continuous axis of best ITD. This axis extends to ITDs well beyond that allowed by the head width. In Jeffress-type models, sensitivity to large ITDs would require neural delay lines with large differences in path lengths between the two ears. Our results suggest instead that sensitivity to large ITDs is created with short delay lines, using neurons that display intermediate- and trough-type responses. We demonstrate that a neuron's best ITD can be predicted from (1) its characteristic delay, a rough measure of the delay line, (2) its characteristic phase, which defines the response type, and (3) its best frequency for ITD sensitivity. The intermediate- and trough-type neurons that have large best ITDs are predicted to be most active when sounds at the two ears are decorrelated and may transmit information about auditory space other than sound localization.

Response magnitude and timing of auditory response initiation in the inferior colliculus of the awake chinchilla

Recent single-unit studies in anesthetized cats have revealed that the latency and strength of transient responses to tone burst stimuli are determined largely by stimulus events in the first few ms of the signal. The present study sought to extend these findings by studying the inferior colliculus potential (ICP) in unanesthetized chinchillas. The ICP magnitude and latency were studied as a function of the plateau amplitude and rise time of noise burst stimuli. ICP amplitude increased with stimulus amplitude and decreased with stimulus rise time. ICP latency decreased with stimulus amplitude and increased with stimulus rise time. The absolute values of the ICP latencies confirmed that it is only the first few ms of the stimulus which determine the timing of response initiation, and therefore, that it is not the plateau level of the stimulus that directly determines the latent period. These data constitute a direct link between earlier single-unit studies in anesthetized animals and brainstem-evoked potential data in animals and man.

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.

Modeling of auditory spatial receptive fields with spherical approximation functions

A spherical approximation technique is presented that affords a mathematical characterization of a virtual space receptive field (VSRF) based on first-spike latency in the auditory cortex of cat. Parameterizing directional sensitivity in this fashion is much akin to the use of difference-of-Gaussian (DOG) functions for modeling neural responses in visual cortex. Artificial neural networks and approximation techniques typically have been applied to problems conforming to a multidimensional Cartesian input space. The problem with using classical planar Gaussians is that radial symmetry and consistency on the plane actually translate into directionally dependent distortion on spherical surfaces. An alternative set of spherical basis functions, the von Mises basis function (VMBF), is used to eliminate spherical approximation distortion. Unlike the Fourier transform or spherical harmonic expansions, the VMBFs are nonorthogonal, and hence require some form of gradient-descent search for optimal estimation of parameters in the modeling of the VSRF. The optimization equations required to solve this problem are presented. Three descriptive classes of VSRF (contralateral, frontal, and ipsilateral) approximations are investigated, together with an examination of the residual error after parameter optimization. The use of the analytic receptive field model in computational models of population coding of sound direction is discussed, together with the importance of quantifying receptive field gradients. Because spatial hearing is by its very nature three dimensional or, more precisely, two dimensional (directional) on the sphere, we find that spatial receptive field models are best developed on the sphere.

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.

Orienting asymmetries in rhesus monkeys: the effect of time-domain changes on acoustic perception

Humans exhibit left-hemisphere dominance for processing spoken language, a species-specific acoustic signal characterized by a suite of spectro-temporal parameters. Some nonhuman primates (genus Macaca) also exhibit left-hemisphere dominance for processing their species-specific vocalizations, as evidenced by right-ear biases in orienting and reaction-time studies, and more damaging effects from left- than right-hemisphere lesions. Little, however, is known about the acoustic features underlying such biases. We conducted field playback experiments on adult rhesus monkeys, Macaca mulatta, to determine whether asymmetries in perception (measured as an orienting bias) are sensitive to changes in the temporal characteristics of their calls. If the observed right-ear bias for perceiving conspecific calls (Hauser & Andersson 1994, Proceedings of the National Academy of Sciences, U.S.A., 91, 3946-3948) depends upon particular acoustic parameters, then experimental manipulations beyond the species-typical range of signal variation will cause a change in perceptual asymmetry, either reversing the pattern (i.e. right to left ear ) or wiping it out (i.e. no asymmetry). We presented manipulated and unmanipulated exemplars of three pulsatile call types within the rhesus repertoire: an affiliative signal 'grunt', an alarm signal 'shrill bark', and a mating signal 'copulation scream'. Signal manipulations involved either (1) a reduction of the interpulse interval to zero or the population minimum or (2) an expansion of the interpulse interval to the population maximum, or two times the maximum. For the grunt and shrill bark, manipulations of interpulse interval outside the range of natural variation either eliminated the orienting bias or caused a shift from right- to left-ear bias. For the copulation scream, however, a right-ear bias was observed in response to all stimuli, manipulated and unmanipulated. Results show that for some call types within the repertoire, temporal properties such as interpulse interval provide significant information to listeners about whether the signal is from a conspecific or not. We interpret the orienting bias as evidence that hemispheric asymmetries underly this perceptual effect.Copyright 1998 The Association for the Study of Animal Behaviour.

Factors shaping the response latencies of neurons in the cat's auditory cortex

This article addresses two issues. Firstly, the hypothesis that response latency might be a neural code for tone frequency was examined in single-neuron data from the primary auditory cortex of anesthetized cats. Minimal response latencies for characteristic frequency (CF) tones were independent of neural CF. Mean response latencies for a constant amplitude CF tone were also independent of CF. These data, and the fact that cortical neurons do not have an obvious independent referent for stimulus onset time, do not support the view that latency is a code for frequency. Secondly, to investigate a simple threshold model of spike initiation time, we describe the prolongations of response latency with increases in stimulus rise time and their dependence on the peak amplitude of the stimulus. These data show that in cortical neurons, it is not the peak stimulus intensity which determines first-spike latency, and second, that the response latencies are systematically not those expected on the basis of simple threshold model.

Role of synaptic inhibition in processing of dynamic binaural level stimuli

We have recently discovered a paradoxical aftereffect associated with inhibition in the gerbil auditory midbrain. Single neurons in the inferior colliculus (IC) were assessed for sensitivity to a virtual motion stimulus produced by modulating the interaural level difference (ILD), a major cue for sound localization. The class of neuron studied was predominantly excited by contralateral stimulation and inhibited by ipsilateral stimulation. Sound pressure level was modulated trapezoidally at the ipsilateral "inhibitory" ear, whereas the contralateral "excitatory" level remained constant. When the inhibitory stimulus was decreased within a range of sound levels that maintained suppression under static conditions, an unexpected discharge was often elicited, apparently because of an aftereffect of synaptic inhibition. In contrast, when the inhibitory stimulus was increased within a range of sound levels that produced only modest suppression under static conditions, neuronal discharge was often profoundly suppressed. In many cases the "conditioned enhancement" or "conditioned suppression" persisted for several seconds after the modulation of ILD, and such conditioned responses were influenced by the modulation depth and rate. To test the effect of inhibition in the IC directly, glycine and GABA were pulsed from a glass recording pipette during a constant monaural excitatory stimulus. The acoustically elicited discharge rate was potentiated markedly if preceded immediately by the brief (0.5-10 sec) application of inhibitory transmitter. Collectively, these results revealed unusually long-lasting effects of inhibition that may establish a new range of acoustic cues to which the neuron responds best. This may have broad implications for processing ensuing auditory stimuli.

First-spike timing of auditory-nerve fibers and comparison with auditory cortex

First-spike timing of auditory-nerve fibers and comparison with auditory cortex. J. Neurophysiol. 78: 2438-2454, 1997. The timing of the first spike of cat auditory-nerve (AN) fibers in response to onsets of characteristic frequency (CF) tone bursts was studied and compared with that of neurons in primary auditory cortex (AI), reported previously. Tones were shaped with cosine-squared rise functions, and rise time and sound pressure level were parametrically varied. Although measurement of first-spike latency of AN fibers was somewhat compromised by effects of spontaneous activity, latency was an invariant and inverse function of the maximum acceleration of peak pressure (i.e., a feature of the 2nd derivative of the stimulus envelope), as previously found in AI, rather than of tone level or rise time. Latency-acceleration functions of all AN fibers were of very similar shape, similar to that observed in AI. As in AI, latency-acceleration functions of different fibers were displaced along the latency axis, reflecting differences in minimum latency, and along the acceleration axis, reflecting differences in sensitivity to acceleration [neuronal transient sensitivity (S)]. S estimates increased with spontaneous rate (SR), but values of high-SR fibers exceeded those in AI. This suggests that S estimates are biased by SR per se, and that unbiased true S values would be less tightly correlated with response properties covarying with SR, such as firing threshold. S estimates varied with CF in a fashion similar to the cat's audiogram and, for low- and medium-SR fibers, matched those for AI neurons. Minimum latency decreased with increasing SR and CF. As in AI, the standard deviation of first-spike timing (SD) in AN was also an inverse function of maximum acceleration of peak pressure. The characteristics of the increase of SD with latency in a given AN fiber/AI neuron and across AN fibers/AI neurons revealed that the precision of first-spike timing to some stimuli can actually be higher in AI than in AN. The data suggest that the basic characteristics of the latency-acceleration functions of transient onset responses seen in cortex are generated at inner hair cell-AN fiber synapses. Implications for signal processing in the auditory system and for first-spike generation and adaptation in AN are discussed.

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.

Responses of neurons in the inferior colliculus to binaural masking level difference stimuli measured by rate-versus-level functions

The psychophysical detection threshold of a low-frequency tone masked by broadband noise is reduced by < or = 15 dB by inversion of the tone in one ear (called the binaural masking level difference: BMLD). The contribution of 120 low-frequency neurons (best frequencies 168-2,090 Hz) in the inferior colliculus (ICC) of the guinea pig to binaural unmasking of 500-Hz tones masked by broadband noise was examined. We measured rate-level functions of the responses to identical signals (So) and noise (No) at the two ears (NoSo) and to identical noise but with the signal inverted at one ear (NoS pi): the noise was 7-15 dB suprathreshold. The masked threshold was estimated by the standard separation, "D". The neural BMLD was estimated as the difference between the masked thresholds for NoSo and NoS pi. The presence of So and S pi tones was indicated by discharge rate increases in 55.3% of neurons. In 36.4% of neurons, the presence of So tones was indicated by an increase in discharge rate and S pi tones by a decrease. In 6.8% of neurons, both So and S pi tones caused a decrease in discharge rate. In only 1.5% of neurons was So indicated by a decrease and S pi by an increase in discharge rate. Responses to the binaural configurations were consistent with the neuron's interaural delay sensitivities; 34.4% of neurons showing increases in discharge rate to both So and S pi tones gave positive BMLDs > or = 3 dB (S pi tones were detected at lower levels than So), whereas 37.3% gave negative BMLDs > or = 3 dB. For neurons in which So signals caused an increase in the discharge rate and S pi a decrease, 72.7% gave positive BMLDs > or = 3 dB and only 4.5% gave negative BMLDs > or = 3 dB. The results suggest that the responses of single ICC neurons are consistent with the psychophysical BMLDs for NoSo versus NoS pi at 500 Hz, and with current binaural interaction models based on coincidence detection. The neurons likely to contribute to the psychophysical BMLD are those with BFs near 500 Hz, but detection of So and S pi tones may depend on different populations of neurons.

Auditory cortical onset responses revisited. I. First-spike timing

Sound onsets are salient and behaviorally relevant, and most auditory neurons discharge spikes locked to such transients. The acoustic parameters of sound onsets that shape such onset responses are unknown. In this paper is analyzed the timing of spikes of single neurons in the primary auditory cortex of barbiturate-anesthetized cats to the onsets of tone bursts. By parametric variation of sound pressure level, rise time, and rise function (linear or cosine-squared), the time courses of peak pressure, rate of change of peak pressure, and acceleration of peak pressure during the tones' onsets were systematically varied. For cosine-squared rise function tones of a given frequency and laterality, any neuron's mean first-spike latency was an invariant and inverse function of the maximum acceleration of peak pressure occurring at tone onset. For linear rise function tones, latency was an invariant and inverse function of the rate of change of peak pressure. Thus latency is independent of rise time or sound pressure level per se. Latency-acceleration functions, obtained with cosine-squared rise function tones under different stimulus conditions (frequency, laterality) from any given neuron and across the neuronal pool, were of strikingly similar shape. The same was true for latency-rate of change of peak pressure functions obtained with linear rise function tones. Latency-acceleration/rate of change of peak pressure functions could differ in their extent and in their position within the coordinate system. The positional differences reflect neuronal differences in minimum latency Lmin and in a sensitivity S to acceleration and rate of change of peak pressure (transient sensitivity), a hitherto unrecognized neuronal property that is distinctly different from firing threshold. Estimates of Lmin and S, which were derived by fitting a simple function to the neuronal latency-acceleration/rate of change of peak pressure functions, were independent of rise function. On average, Lmin decreased with increasing characteristic frequency (CF), but varied widely for neurons with the same CF. S varied with CF in a fashion similar to the cat's audiogram and, for a given neuron, varied with frequency. SD of first-spike latency was roughly proportional to the slope of the functions relating latency to acceleration/rate of change of peak pressure. Thus SD increased exponentially, rather than linearly, with mean latency, and did so at about twice the rate for linear than for cosine-squared rise function tones. The proportionality coefficients were quite similar across the neuronal pool and similar for both rise functions. Minimum SD increased nonlinearly with increasing Lmin. These findings suggest a peripheral origin of S and a peripheral establishment of latency-acceleration/rate of change of peak pressure functions. Because of the striking similarity in the shapes of such functions across the neuronal pool, sound onsets will produce orderly and predictable spatiotemporal patterns of first-spike timing, which could be used to instantaneously track rapid transients and to represent transient features by partly scale-invariant temporal codes.

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.

Sound lateralization in patients with lesions including the auditory cortex: comparison of interaural time difference (ITD) discrimination and interaural intensity difference (IID) discrimination

We examined sound lateralization using dichotic presentation of noises in 15 patients with left unilateral (12 patients) or bilateral (3 patients) temporal lobe lesions, that included the auditory cortex, and evaluated their abilities to discriminate interaural time and intensity difference (ITD, IID) separately. On the ITD discrimination test, discrimination thresholds in patients with left unilateral lesions were significantly higher than those in normal subjects, but all patients with left unilateral lesions could detect ITD. However, none of 3 patients with bilateral lesions could detect ITD. On the IID discrimination test, all patients with either unilateral or bilateral lesions could detect IID. IID discrimination thresholds in these patients were significantly higher than those in normal subjects. The auditory cortex plays an important role in discriminating both cues, but appears to be necessary for discriminating ITD.

Azimuthal sensitivity of rat pinna reflex: EMG recordings from cervicoauricular muscles

Electromyographic (EMG) responses of the cervicoauricular muscles (CAM) to free-field sounds were recorded in two groups of rats whose brainstems were dissected transversely either at a pretectal or transtectal level. After the rat recovered from anesthesia, wide-band noise pulses were presented and speaker positions were varied systematically in azimuth. Sound levels were set at 10-15 dB above empirically determined threshold for an EMG response to a sound from 0 degree azimuth. In both animal groups, transient CAM EMGs with short latency were produced and three main types of azimuthal sensitivity of CAM EMG response were observed. (1) For the majority of the cases, an inverted "U' type of azimuthal sensitivity was identified: the maximum activity occurred around 0 degree azimuth, but as the speaker was moved toward either the ipsilateral or contralateral fields, the sound-evoked activity declined systematically. This directional tuning is quite different from the passive pinna directionality which is very lateral in the resting positions used in this study. (2) In a small number of cases, the spatial sensitivity curves were not symmetrical about the midline (0 degree azimuth): the EMG response was vigorous in one hemifield and dropped off systematically as the speaker was moved toward extreme positions of the other hemifield. Regardless of shapes of EMG spatial tuning curves, obstruction of either the ipsilateral or contralateral meatus reduced the sound-elicited response dramatically and eliminated the spatial sensitivity. (3) Some cases exhibited an omnidirectional function: the EMG spike rate had no or minor systematical variation as the speaker position was changed in azimuth. The results of this study indicate that with either pretectal or transtectal decerebrate preparations, the acoustically evoked CAM EMG can exhibit an azimuthal sensitivity which is based on binaural processing.

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.

Acoustic features and acoustic changes are represented by different central pathways

The central processing of acoustic stimulus changes can be observed neurophysiologically in the mismatch negativity auditory evoked potential (MMN). Stimuli differing in interaural phase were used to investigate the contributions of the primary and non-primary auditory pathways to the encoding of binaural stimuli and to investigate passively elicited measures of binaural processing in experimental animals. In guinea pigs, the MMN was obtained in response to 1000 Hz tones embedded in white noise (S:N = 2 dB). Using a modified oddball paradigm (that is, two stimuli presented in a series, each with a different probability of occurrence), stimuli were presented binaurally with both the tone and noise in-phase to the two ears (S0N0) as the standard stimulus ans the tone 180 degrees out-of-phase (S(PI)N0) as the deviant stimulus. The MMN, by definition, should occur only in response to a change, or 'mismatch,' between the standard and deviant stimuli. The response to the deviant stimulus in the oddball paradigm was compared to the response to the same stimulus when presented in a series alone. The responses to S0N0 and S(PI)N0 collected in a series alone, termed the intrinsic responses, were also compared. Responses were recorded from two surface epidural electrodes - one at the posterior midline and one over the left temporal lobe. AEPs from these locations have been shown to reflect the activity of primary and non-primary thalamo-cortical pathways respectively. A significant MMN was observed at the midline electrode, but no MMN was observed over the temporal lobe.(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory aftereffects following simulated motion produced by varying interaural intensity or time

Simulated auditory motion, ie step-ramp modulated interaural intensity (delta I) or time (delta t) was presented via headphones as an adapting stimulus (narrow-band signal of 1 kHz mean frequency). After adaptation, settings of a stationary test stimulus were systematically shifted in the opposite direction when the experimental parameter was delta I, but not when it was delta t. Further studies with delta t motion with the use of mean frequencies of 100 Hz or 6 kHz showed an aftereffect only at 6 kHz. Unlike visual motion aftereffects, no counter-motion was observed; rather the test stimulus appeared stationary, but settings of its interaural midline were displaced in a direction opposite to the direction of adaptation (on average by 1.2 dB or 30 microseconds for delta I-stimulated and delta t-stimulated motion, respectively). This displacement effect decayed with time after adaptation. The frequency dependence found for delta t motion suggests that the low-frequency mechanism of directional hearing that uses interaural ongoing-time (phase) differences is not able to adapt. The observed auditory aftereffects may be analogous to visual motion aftereffects since they are direction specific; however, because they lack apparent motion they also resemble disparity-specific stereoscopic aftereffects.

Anatomy and projection patterns of the superior olivary complex in the Mexican free-tailed bat, Tadarida brasiliensis mexicana

The superior olivary complex (SOC) is the first station in the ascending auditory pathway that receives binaural projections. Two of the principal nuclei, the lateral superior olive (LSO) and the medial superior olive (MSO), are major sources of ascending projections to the inferior colliculus. Whereas almost all mammals have an LSO, it has traditionally been thought that only animals that hear low frequencies have an MSO. Recent reports, however, suggest that the medial part of the SOC in bats is highly variable and that at least some bats have a well-developed MSO. Thus, the main goal of this study was to evaluate the cytoarchitecture and connections of the principal superior olivary nuclei of the Mexican free-tailed bat, with specific attention directed at the MSO. Cell and fiber stained material revealed that the LSO and the medial nucleus of the trapezoid body (MNTB) are similar to those described for other mammals. There are two medial nuclei we refer to as dorsomedial periolivary nucleus (DMPO) and MSO. Tracer experiments exhibited that the DMPO receives bilateral projections from the cochlear nucleus, and additional projections from the ipsilateral MNTB. The DMPO sends a strong projection to the ipsilateral inferior colliculus. Positive staining for acetylcholinesterase indicates that the DMPO is a part of the olivocochlear system, as it is in other animals. The MSO in the free-tailed bat meets many of the criteria that traditionally define this nucleus. These include the presence of bipolar and multipolar principal cells, bilateral innervation from the cochlear nucleus, a strong projection from the ipsilateral MNTB, and the absence of cholinergic cells. The major difference from traditional MSO features is that it projects bilaterally to the inferior colliculus. Approximately 30% of its cells provide collateral projections to the colliculi on both sides. Functional implications of the MSO for the free-tailed bat are considered in the Discussion.

Sound localization in hemispherectomized patients

In order to precisely evaluate the consequences of cortical damage on free-field sound localization in humans, the present study examined response accuracy to auditory targets in three hemispherectomized patients and IQ-matched controls. Listeners reported sound location by pointing with their dominant hand to the apparent sound location in an anechoic chamber. Two conditions were tested: (i) localization of a fixed-sound source and (ii) localization of the beginning and the end of a simulated moving stimulus. In both conditions, the responses of the patients were less accurate than those of the controls in the hemifield contralateral to their removed hemisphere. Moreover, the single-case analyses revealed that the performances obtained with fixed sources were generally more precise than those obtained with moving sources. This result is discussed in terms of a differential involvement of cortical and subcortical pathways in the processing of stationary and moving sounds. Finally, the age at surgery and the post-surgical interval were related with the magnitude of the deficits, suggesting the possible influences of functional reorganization and cerebral plasticity.

Binaural processing in the dorsal nucleus of the lateral lemniscus

We studied the binaural properties of 72 neurons in the dorsal nucleus of the lateral lemniscus (DNLL) of the mustache bat. There are six main findings: 1) Conventional EI neurons that were excited by stimulation of the contralateral ear and inhibited by ipsilateral stimulation, comprise the majority (80%) of binaural DNLL cells. 2) For most EI neurons the quantitative features of their interaural intensity disparity (IID) functions, maximum inhibition, dynamic range and 50% point IIDs, were largely unaffected by the absolute intensity at the contralateral ear. 3) Although the net effect of the inhibition evoked by ipsilateral stimulation was to suppress discharges evoked by contralateral stimulation, our results indicate that the inhibitory inputs can act in three different ways. The first was a time-intensity trade, where increasing the intensity at the ipsilateral ear evoked inhibitory effects with progressively shorter latencies. The second way was that the latency of inhibition did not appear to decrease with ipsilateral intensity, but rather increasing ipsilateral intensity appeared only to increase the strength of the inhibition. The third way was that the lowest effective ipsilateral intensity suppressed the first spikes evoked by the contralateral stimulus and higher ipsilateral intensities then suppressed the later discharges of the train. Each of these inhibitory patterns was seen in about a third of the cells. 4) Neurons that had more complex binaural properties, such as the facilitated EI neurons (EI/F) and neurons that were driven by sound to either ear (EE neurons), represented about 20% of the binaural population. There were two types of EE neurons; those in which there was a simple summation of discharges evoked with certain IIDs, and those in which the spike-counts to binaural stimulation at certain IIDs were greater than a summation of the monaural counts and thus were facilitated. 5) All binaural neurons were strongly inhibited with IIDs that favored the ipsilateral ear. Our findings indicate that the more complex binaural types, the facilitated EI neurons (EI/F) as well as the two types of EE neurons, may be constructed from conventional EI neurons by adding inputs from several sources that impart the more complex features to these neurons. We propose four circuits that could account for the different binaural response properties that we observed. The circuits are based on the known connections of the DNLL and the neurochemistry of those connections. Finally, we compared the binaural properties of neurons in the mustache bat DNLL with those of neurons in the mustache bat inferior colliculus and lateral superior olive.(ABSTRACT TRUNCATED AT 400 WORDS)

Human auditory cortical mechanisms of sound lateralization: I. Interaural time differences within sound

Neuromagnetic responses to 600-ms binaural click trains, presented once every 1.1 s, were recorded with a 24-channel gradiometer from 6 healthy humans. During the first 300 ms, the left-ear stimulus led the right by 0.7 ms and the sound was lateralized to the left ear. At 300 ms, the interaural time difference (ITD) changed and the lateralization moved to one of 5 different locations between the ears. An N100m response peaked about 110 ms after the sound onset and an N130mc response (c to stress a response to the change) about 135 ms after the ITD change. The source locations of N100m and N130mc agreed with activity in the supratemporal auditory cortex; this was confirmed in one subject by superimposing MEG results on MR images. The sources of N100m and N130mc did not differ statistically significantly from each other, nor were there differences in N130mc sources to various lateralization changes. N130mc grew larger when the ITD change increased, in parallel with the increase in the change of the perceived location. We suggest that N130mc is analogous to N100m, but is delayed due to postmasking induced by the early part of the sound.

Effects of superior olivary complex lesions on binaural responses in rat auditory cortex

Unilateral or bilateral lesions of the superior olivary complex (SOC) were made by local injection of kainic acid through a micropipette lowered stereotaxically into the rat's auditory brain stem. The lesions had the effect of destroying cell bodies in the superior olive without disrupting fibers of passage. After a recovery period of approximately one month physiological recordings were made with tungsten microelectrodes from the auditory cortex of normal animals and animals with SOC lesions. For animals with unilateral SOC lesions recordings were made either ipsilateral or contralateral to the site of damage. Monaural and binaural tone bursts (110 ms duration) were presented dichotically through a sealed sound delivery system and binaural interaction patterns were determined by comparison of monaural and binaural responses. Some cells were excited by stimulation of either ear and facilitated by binaural stimulation (binaural summation or EE cells), whereas others were excited by contralateral stimulation and inhibited by simultaneous ipsilateral stimulation (binaural suppression or EI cells). Both binaural summation and suppression responses were still present following unilateral SOC lesions. Interaural intensity difference thresholds were within the normal range and no differences were found between animals with lesions placed ipsilateral or contralateral to the recording site. Following a bilateral lesion in one case, both binaural summation and suppression responses were still present. The bilateral lesion had the effect of shifting the average interaural intensity difference thresholds in favor of the contralateral ear. These data suggest that binaural interactions above the level of the superior olive contribute to physiological responses in auditory cortex and that binaural responses in the rat's auditory cortex are shaped by both olivary and supraolivary interactions.

Processing of frequency-modulated stimuli in the chick auditory cortex analogue: evidence for topographic representations and possible mechanisms of rate and directional sensitivity

Responses of units in the auditory forebrain (field L/hyperstriatum ventrale-complex) of awake domestic chicks were studied to frequency-modulated (FM) signals and isointensity tone bursts, presented to the ear contralateral to the recording sites. FM signals, linear frequency sweeps in the range of 50 Hz to 10.25 kHz, differed in the rate of change of frequency (RCF) and in the direction of modulation. The majority of RCF response functions obtained could be classified as predominantly ascending and bell shaped. Best rates of change of frequency (BRCFs), assigned to these functions, covered a range of nearly 3 orders of magnitude. BRCFs of the same units for upward (positive BRCFs) and for downward modulations (negative BRCFs) were correlated. The lowest BRCF encountered among all units for a given isointensity ON-response bandwidth (delta FON) increased as a function of delta FON. delta FON was derived from the responses to tone bursts of various frequencies at 70 dB SPL. As delta FON tended to increase with the best frequency (BF) of units the lowest BRCF encountered among all units for a given BF also increased as a function of BF. Positive and negative BRCFs of a unit were also correlated with the slopes of onset latency-frequency relationships below and above BF, respectively. FM responses were optimal, when the frequency-specific latency differences at a given unit were compensated by the direction and rate of frequency change in the signal. FM-directional sensitivity varied with BF. Most units with BFs below about 2 kHz preferred upward modulations, while those with BFs above 2 kHz preferred downward modulations. Directional preference and sensitivity correlated with asymmetric distributions of inhibitory sidebands around BF, as derived from the analysis of OFF-responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensitivity of neurons in cat primary auditory cortex to tones and frequency-modulated stimuli. I: Effects of variation of stimulus parameters

In the primary auditory cortex (AI) of barbiturate-anesthetized cats multi-unit responses to tones and to frequency-modulated (FM) tonal stimuli were analyzed. Characteristic frequency (CF), sharpness of tuning, minimum threshold, and dynamic range of spike count--intensity functions were determined. Minimum threshold and dynamic range were positively correlated. The response functions to unidirectional FM sweeps of varying linear rate of change of frequency (RCF) that traversed the excitatory frequency response areas (FRAs) displayed a variety of shapes. Preferences for fast RCFs (> 1000 kHz/s) were most common. Best RCF was not correlated with measures of sharpness of tuning. Directional preference and sensitivity were quantified by a DS index which varied with RCF. About two-thirds of the multi-unit responses showed a preference for downward sweeps. Directional sensitivity was independent of CF and independent of best RCF. Measurements of latencies of phasic responses to unidirectional FM sweeps of different RCF demonstrated that the discharges of a given multi-unit over its effective RCF range were initiated at the same instantaneous frequency (effective Fi), independent of RCF. Effective Fis fell within the excitatory FRA of a given multi-unit. The relationships of effective Fis to CF show that responses were evoked only when the frequency of the signal was modulated towards CF and not when modulated away from it, and that responses were initiated before the modulation reached CF. Changes in the range and depth of modulation had only minor, if any, effects on RCF response characteristics, FM directional sensitivity, and effective Fis, as long as the beginning and ending frequencies of FM sweeps fell outside a multi-unit's FRA. Stimulus intensity also had only moderate effects on RCF response characteristics and DS. However, effective Fis were influenced in systematic fashions; with increases in intensity, effective Fis to upward and downward sweeps decreased and increased, respectively. Thus, for higher intensities FM responses were initiated at instantaneous frequencies occurring earlier in the signal. The results are compared with previous data on tone and FM sensitivity of auditory neurons in cortical and subcortical structures, and mechanisms of FM rate and directional sensitivity are discussed. The topographic representations of these neuronal properties in AI are reported in the companion report.

Effects of superior olivary complex lesions on binaural responses in rat inferior colliculus

Unilateral or bilateral lesions of the superior olivary complex (SOC) were made by local injection of kainic acid through a micropipette lowered stereotaxically into the rat's auditory brainstem. The lesions had the effect of destroying cell bodies in the superior olive without disrupting fibers of passage. After a recovery period of approximately one month, physiological recordings were made with tungsten micro-electrodes from the central nucleus of the inferior colliculus of animals with SOC lesions. For animals with unilateral lesions recordings were made either ipsilateral or contralateral to the site of damage. Results were compared with data from normal control cases. Monaural and binaural tone bursts were presented through a sealed sound delivery system and binaural interaction patterns were determined. Both binaural summation and suppression responses were still present following unilateral SOC lesions. Interaural intensity difference thresholds were within the normal range and no differences were found between animals with lesions placed ipsilateral or contralateral to the recording site. Binaural summation and suppression responses were also present following bilateral lesions. Bilateral lesions had the effect of shifting the average interaural intensity difference thresholds in favor of the contralateral ear. But, in general, binaural interactions were remarkably little affected by bilateral destruction of the SOC.

Neural selectivity for interaural frequency disparity in cat primary auditory cortex

Single-unit responses to interaural frequency disparities (IFDs) were examined in 74 neurons in cat primary auditory cortex (AI). Thirty-three of these cells were classified as EE (binaural facilitators), 39 were classified as EI (binaural inhibitors), and 2 were classified as EO (binaural occluders). The best frequency (BF) was presented to the dominant (usually the contralateral) ear while tones of the same or different frequency (either higher or lower than BF) were presented simultaneously to the nondominant (usually the ipsilateral) ear. Most cells displayed sensitivity to IFDs and thus were classified according to the IFD condition that elicited the strongest facilitatory or inhibitory response. The stimulus condition which evoked the strongest binaural response is referred to as the best IFD. For 50 cells (68%), the best IFD response was obtained when tones of different frequency were presented to each ear. Across the entire sample, binaural IFD responses of cortical neurons were categorized into one of three groups: Those preferring a lower frequency than BF in the ipsilateral ear (referred to as the 'lower IFD group'), those preferring a frequency equal to BF (the 'zero IFD group'), or those preferring a frequency higher than BF (the 'higher IFD group'). Among EE cells, approximately one third were maximally facilitated when the ipsilateral ear frequency was lower than BF, one third when it was equal to BF, and one third when it was higher than BF. Among EI cells, 50% exhibited deepest inhibition for higher IFDs with relatively fewer cells showing inhibition for zero or lower IFDs. Overall, EI cells responded over a broader range of IFD conditions than EE cells. Finally, approximately 50% of all units exhibited bimodal responses such that cells classified as EE displayed some inhibitory response characteristics when stimulated with certain IFD conditions and vice versa.

Rate and synchronization measures of periodicity coding in cat primary auditory cortex

Periodicity coding was studied in primary auditory cortex of the ketamine anesthetized cat by simultaneously recording with two electrodes from up to 6 neural units in response to one second long click trains presented once per 3 s. Trains with click rates of 1, 2, 4, 8, 16 and 32/s were used and the responses of the single units were quantified by both rate measures (entrainment and rate modulation transfer function, rMTF) and synchronization measures (vector strength VS and temporal modulation transfer functions, tMTF). The rate measures resulted in low-pass functions of click rate and the synchrony measures resulted in band-pass functions of click rate. Limiting rates (-6 dB point of maximum response) were in the range of 3-24 Hz depending on the measure used. Best modulating frequencies were in the range of 5-8 Hz again depending on the synchrony measure used. It appeared that especially the VS was highly sensitive to spontaneous firing rate, duration of the post click suppression and the size of the rebound response after the suppression. These factors were dominantly responsible for the band-pass character of the VS-rate function and the peak VS frequency was nearly identical to the inverse of the suppression period. It is concluded that the use of the VS and to a lesser extent also the tMTF as the sole measure for the characterization of periodicity coding is not recommended in cases where there is a strong suppression of spontaneous activity. The combination of entrainment and tMTF appeared to characterize the periodicity coding in an unambiguous way.

Separate mechanisms control spike numbers and inter-spike intervals in transient responses of cat auditory cortex neurons

In the anesthetized cat, some cortical auditory neurons discharge a train of up to 5 spikes in response to the onset of a characteristic frequency tone pulse. This report provides the first description of the inter-spike intervals (ISIs) in these responses. The ISIs were typically close to 2.0 ms in length, and, as indexed by the standard deviation of the interval length, were very regular. Except at threshold levels of stimulation, mean ISIs were relatively insensitive to both tone amplitude and repetition rate. This was true even over ranges of those variables that exerted dramatic effects on spike numbers and first spike latency. These data suggest that the relative timing of discharges within the spike burst is controlled by a mechanism which is separable from that which determines the number of spikes in them. The brevity of the ISIs suggest that they may be a means of enhancing the salience of the transient response against a background of spontaneous discharges.

The anterior ectosylvian sulcal auditory field in the cat: I. An electrophysiological study of its relationship to surrounding auditory cortical fields

The extent of a region containing acoustically responsive neurons within the anterior ectosylvian sulcus and its relationship to surrounding gyral auditory cortical fields was examined in chloralose-anaesthetized cats. Multiple microelectrode penetrations were made orthogonal to the middle and anterior ectosylvian gyral surfaces, and longer penetrations were made into the dorsal and ventral banks and fundus of the anterior ectosylvian sulcus. The quantitative and qualitative auditory response characteristics of neurons and neuron clusters in the sulcal banks and surrounding regions were mapped in detail, and the degree of overlap of auditory and visual neurons within the sulcus was determined by routinely testing for responsiveness to a gross light flash. The detailed results from three animals and a summary of all penetrations into the sulcus are presented. The anterior ectosylvian sulcal field (Field AES) lay deep within the banks and fundus of the posterior three quarters of the sulcus. A combination of changes in the auditory response characteristics of neurons (i.e., in optimal stimulus, latency, and frequency tuning), and the presence of visually responsive cells, distinguished this field from surrounding fields. The distinction between the anterior ectosylvian field and extensions of the nearby tonotopic fields (i.e., primary and anterior auditory fields) into the dorsal and ventral banks of the dorsoposterior sector of the sulcus was readily made on the basis of these characteristics. The distinction between the anterior ectosylvian field and extensions of the second auditory field into the ventral bank of the middle sector of the sulcus was more difficult and there were differences between animals in the transition between these fields. Anterior ectosylvian sulcal field responses did not extend into the dorsal bank in anterior parts of the sulcus but were restricted to fundal regions, an observation consistent with the presence of the fourth somatosensory field in the dorsal bank of this sector of the sulcus. The majority of penetrations into the sulcus revealed coextensive auditory and visual activity, an observation apparently at variance with the identification of a purely visual field in this region. Barbiturate anaesthesia, which has been used in experiments demonstrating an anterior ectosylvian visual area, was found to have a depressing effect on auditory responses within the anterior ectosylvian sulcal field.

Neural representation of sound amplitude in the auditory cortex: effects of noise masking

Single auditory cortical neurons express their sensitivity to the amplitude of a preferred-frequency tone pulse as either a monotonic, saturating intensity profile or as a non-monotonic, bell-shaped intensity function. In the presence of continuous, wideband noise masking, the tone intensity profile is displaced toward higher tone levels. The magnitude of the tone threshold adjustments brought about by increments in noise level very closely match the elevations in noise amplitude. The mechanisms underlying the threshold adjustments likely include neural adaptation. This is because the tone threshold shifts seen in the spike count data are paralleled by spike latency data, and because recovery of tonal sensitivity following noise offset proceeds in a negatively-accelerating fashion. In some instances, the slope of the masked tone intensity profile is greater than that for unmasked tones. For masked tone levels evoking submaximal responses, this has the consequence that cortical responses to masked tones are somewhat more salient than those for unmasked tones of comparable suprathreshold level. These observations bolster our understanding of the psychophysics of noise-masking in normal listeners, and they provide a partial explanation of the difficulty shown by patients with temporal lobe lesions in discriminating signals in noise.

Timing of spike discharges in cat auditory cortex neurons: implications for encoding of stimulus periodicity

Neurons at more central stations in the central auditory pathway show progressively poorer responses to high-frequency stimulus periodicities. This has been attributed to the relatively poorer spike timing in forebrain auditory neurons. This study directly examined the timing of spikes evoked by brief tone pulses which were varied in peak level and repetition rate. The experiments revealed that at tone repetition rates which produced progressively poorer entrainment, the timing of spike discharges remained sufficiently precise to support entrained responses. The fact that responses were poor indicates that imprecision in spike timing may not be the only factor limiting the encoding of temporal frequency. The data are discussed in relation to evidence on the temporal tuning of central neurons seen in studies using continuous amplitude modulations.

Coding of tone-pulse amplitude by single neurons in auditory cortex of albino rats (Rattus norvegicus)

We examined the neural representation of tone pulse amplitude in the auditory cortex of anesthetized albino rats. Rate-level functions for monaural, contralateral CF tones were obtained from single neurons. Most of these functions were saturating and monotonic in shape. The dynamic ranges of these functions were typically 5 to 35 dB in breadth, although the tail of this distribution extended to beyond 60 dB. The neurons with the widest dynamic ranges were usually those with the lowest CF thresholds. Nonmonotonic neurons were uncommon, and the nonmonotonicity was not as well developed as that seen in the cortex of cats and monkeys. For two individual rats, data are presented for neurons tuned to the same tone frequency in a single cerebral hemisphere. The CF thresholds of these neurons varied over a 50 dB range, and their collective dynamic ranges spanned at least 70 dB. These data provide a minimum estimate of the dynamic range of the cortical code for tone pulse amplitude in the rat. They suggest that there may be no serious mismatch between the neural and behavioral amplitude dynamic ranges.

Time is traded for intensity in the bat's auditory system

Disparities in time and intensity are the two chief cues animals use for localizing a sound source in space. Echolocating bats belonging to the family Molossidae emit brief, ultrasonic signals for orientation that sweep downward about an octave over the duration of the pulse. Due to acoustic shadowing and the directional properties of the ears, pronounced interaural intensity disparities are created that vary as a function of azimuth. However, due to the small headwidth of these animals, azimuthal changes create small interaural time disparities that are at most 30 microseconds. The experiments in this report are concerned with the binaural processing of time and intensity disparities using brief FM signals that simulate the animal's natural echolocation calls. Binaural neurons receiving excitation from one ear and inhibition from the other (E-I neurons) were recorded from the inferior colliculus of Mexican free-tailed bats. The majority of units sampled were highly sensitive for temporal disparities of 100-300 microseconds, and a few had significant changes in discharge probability when interaural time was changed by 10-20 microseconds. However, all E-I neurons were also sensitive to intensity disparities. With only one exception, all E-I neurons traded time for intensity. On the average, each decibel difference in intensity could be compensated for by advancing or delaying the inhibitory sound by 47 microseconds. The main conclusion is that the auditory system processes interaural disparities by transforming level differences at the two ears into latency differences. Thus the discharge probability of each binaural neuron is determined largely by the arrival times of the discharges from the excitatory and inhibitory ears. In view of the substantial time-intensity trading ratios, the small interaural time disparities produced by azimuthal locations off the midline play no role in shaping the response properties of these neurons. Specific examples of how time-intensity trades can translate into a high spatial selectivity are presented.

Sensitivity of auditory cortical neurons of kittens to monaural and binaural high frequency sound

The experiments reported here describe the abilities of young auditory cortical neurons to encode information about tone bursts having frequencies above 2.5 kHz. The studies were carried out in anesthetized kittens ranging from 8 to 44 days of age. At all ages studied, stimulation of the contralateral ear was most effective in evoking spikes. Typically the response was confined to stimulus onset. Thresholds were comparatively high and response latencies were comparatively long in the youngest kittens studied. The time course of threshold development was very similar to that of the auditory nerve and cochlear nuclei indicating that most, if not all, age related thresholds and threshold changes at the cortical level are accounted for by mechanisms operating at the level of the cochlea and auditory nerve. Response latency shortened progressively over the first month of postnatal life and while the absolute change in response latency differed considerably from that of cells in the cochlear nuclei the proportional changes were very similar. These data indicate that the comparatively long response latency and latency changes recorded at the cortex are imposed by underdeveloped central auditory processes. Response areas of kitten cortical neurons resembled those of the adult. At all ages studied, binaural interactions were robust and similar in kind to those recorded in adult cats. We conclude that cortical neurons of kittens preserve the results of interactions occurring at lower brainstem levels and that the development of the circuits of which these neurons are a part develop as a functional unit.

Primary auditory cortex in the ferret (Mustela putorius): neural response properties and topographic organization

Microelectrode recording studies were made of the crown of the ectosylvian auditory cortex of barbiturate-anesthetized ferrets, using calibrated, sealed acoustic stimulus delivery systems. We confirmed our previous finding using free-field stimuli that this region of the ferret's cerebral cortex contains a tonotopically organized field in which neurons are briskly excited by the onset of tonal stimuli. The vast majority of neurons in this field were narrowly tuned to tone pulse frequency, with Q factors comparable to those described for cortical cells in other species. The distribution of minimum tone thresholds across the frequency representation of this field paralleled the behavioral audiogram in the same species. The majority of neurons received input from both ears, and the natures of the influences exerted by tonal stimuli at the two ears were similar in form to those described for other carnivores. The various binaural cell types had characteristic frequencies that spanned the entire tonal spectrum represented in the cortex. Preliminary mapping studies revealed that there may be a topographic segregation of cells according to their binaural interactions. The basic properties of cells in this region of the ferret's cerebrum are similar to those previously described for the cat's primary auditory cortex. The data on the binaural properties of these cells, and the spatial distribution of those cells, provide the first evidence in a species other than the cat for segregated binaural representation in the auditory cortex.

Electrophysiology of the auditory system

This review has attempted to summarise the properties of electro physiological responses in the auditory system. The treatment was broad and consequently somewhat sketchy. For a more detailed recent treatment of the physiology of the auditory system the reader is referred to Pickles (1982), Møller (1983), or Altschuller et al (1986). The data on acoustic injury have been reviewed recently by Schmiedt (1984). Discussions of a number of topics such as development, hair cell function and speech encoding are found in Berlin (1984).

Age-related changes in auditory temporal perception

The discrimination of signal and silence duration was evaluated in 6-month-old infants, 5 1/2-year-old children, and adults. Listeners were tested with a conditioned-discrimination procedure in which they were presented a sequence of 18 white-noise bursts and trained to discriminate a change in duration of the middle 6 signal or silence elements. There were no differential effects on performance for changes in signal compared to silence duration. At each age, performance varied only as a function of magnitude of duration change. Infants discriminated duration changes of 20 ms or greater, children discriminated 15 ms, and adults discriminated changes as small as 10 ms. These findings are consistent with other research in revealing age-related improvements in auditory temporal perception.

Maps of auditory cortex in cats reared after unilateral cochlear ablation in the neonatal period

The responses of many neurons recorded in the high best-frequency region of primary auditory cortical field, AI, of the normal adult cat depend upon intensity differences of the sounds arriving at the two ears. These binaural interactions are exhibited early in postnatal life, well before structural maturation of the auditory pathways from the ear to the cortex is complete. The aim of the present work was to study certain aspects of the functional development of the auditory cortex in adult cats unilaterally deaf from birth. In adult animals reared with a neonatal cochlear ablation, field AI ipsilateral to the non-operated ear showed a normal tonotopic map, which was derived from single neurons and neuron clusters driven securely by best-frequency tonal stimulation in virtually every electrode penetration. The acoustic thresholds at many recording sites were as low as those obtained in AI contralateral to the non-operated ear. These findings are in marked contrast to those from control experiments on normal adult cats where only about 65% of AI neurons were excited by a sound delivered to the ipsilateral ear and where thresholds to ipsilateral ear stimulation were significantly higher than contralateral thresholds. The spatial distribution of cortical neurons based on acoustic thresholds also appeared to be different in cats unilaterally deaf from birth when compared to control cats. Closely spaced electrode penetrations in AI ipsilateral to the non-operated ear suggested that neurons were separated into low-threshold regions and high-threshold regions. There was no evidence for this type of non-random segregation in control experiments.

Responses of single neurons in cat auditory cortex to time-varying stimuli: linear amplitude modulations

Single neurons in the auditory cortex of anesthetized cats were examined quantitatively for their sensitivity to the sound pressure level of characteristic frequency (CF) tone pulses, and to 6 dB, linear modulations in the amplitude of a continuous CF carrier tone. The direction and rate of amplitude modulation (AM), and the carrier level on which it was imposed, were manipulated parametrically. Studied with amplitude modulations, the majority of neurons responded only to intensity increments. The minimum carrier level upon which an amplitude modulation was able to evoke spike discharges was typically comparable to the tone pulse threshold SPL. For many neurons, an "intensity increment response area", i.e., the domain of AM rate and carrier level conjunctions within which a 6 dB AM was able to evoke discharges, could be delimited. For many neurons, preferred rate of AM drifted from high to low with increases in the carrier level on which the modulation was imposed. The most vigorous responses to AM stimuli often occurred when the carrier levels were associated with the rising slope or the peak of the tone pulse rate intensity function. It may be possible to understand the general form of AM response areas in terms of short-term adaptation, the disposition of excitatory and inhibitory tone pulse response areas, and the spectra of the AM stimuli used.

A comparison of two methods for the measurement of neural sensitivity to interaural intensity differences

Two alternative methods for the measurement of neural sensitivity to interaural intensity differences (IIDs) were used to obtain IID-sensitivity functions for samples of excitatory-inhibitory (EI) neurons from the central nucleus of the inferior colliculus and the primary auditory cortex of the cat. In one, the EMI-constant method, intensity was held constant in the ear providing excitatory input and varied above and below that level in the other ear. In the alternative (ABI-constant) method, intensity at the two ears was varied symmetrically about a constant base intensity, in a manner roughly approximating the pattern of changes that occur when a free-field stimulus is moved in azimuth from the median sagittal plane. For neurons with monotonic or near-monotonic rate-intensity functions for the excitatory ear, the two methods generated IID-sensitivity functions that were identical or near-identical by a number of quantitative criteria. For neurons with non-monotonic rate-intensity functions, however, the IID functions generated by the two methods were very different: those produced by the EMI-constant method were monotonic, whereas those generated by the ABI-constant method were non-monotonic and sharply peaked. The advantages and disadvantages of the two methods, and the implications of the results for the neural encoding of IIDs and for the azimuthal sensitivity of EI neurons with non-monotonic rate-intensity functions, are discussed.

Pentobarbital and ketamine alter the pattern of 2-deoxyglucose uptake in the central auditory system of the gerbil

Relative 2-deoxyglucose (2-DG) uptake was investigated during pentobarbital and/or ketamine anesthesia, when animals were either kept in silence or stimulated with wide band noise at 85 dB SPL. In the absence of anesthesia, noise stimulation produced a large increase in relative 2-DG uptake, when compared to silence, in all auditory nuclei up to and including the inferior colliculus. Much more modest noise-induced increases were seen in the medial geniculate nucleus and auditory cortex. These effects were markedly altered by anesthesia. Pentobarbital, and especially pentobarbital plus ketamine, enhanced stimulus-evoked increases in relative 2-DG uptake in lower auditory nuclei: the cochlear nuclei, superior olivary complex and ventral nucleus of the lateral lemniscus. At the same time, stimulus-evoked increases were decreased in the dorsal nucleus of the lateral lemniscus and inferior colliculus, and virtually eliminated in the medial geniculate and auditory cortex. The results of this study permit more meaningful comparison of 2-DG techniques with electrophysiological measures of central auditory activity, and illuminate the utility and limitations of each method. The data indicate that 2-DG observations from barbiturate-anesthetized preparations should be interpreted with some caution. They further suggest that the 2-DG technique is inappropriate for the study of stimulus-evoked activity in the medial geniculate and auditory cortex of barbiturate-anesthetized animals.

Neurons accumulating [3H]gamma-aminobutyric acid (GABA) in supragranular layers of cat primary auditory cortex (AI)

The classes of neurons accumulating exogenously injected, tritiated gamma-aminobutyric acid [( 3H]GABA) were studied in the supragranular layers in the primary auditory field of the adult cat. The size, laminar locus, and somatodendritic profiles of labeled neurons were studied light microscopically in frozen- or Vibratome-sectioned, 30 micron thick material, and in semithin, 1-2 micron thick, plastic-embedded high-resolution autoradiographic preparations. The chief goals of the study were to determine which types of cells could be identified as accumulating [3H]GABA in layers I, II and III, and to establish possible relationships between these cells and neurons described in Golgi studies of these layers, and the neurons found, in parallel investigations of the connections of the primary auditory field, to participate as ipsilateral corticocortical and commissural cells of origin. The principal findings are: that neurons in every layer in the primary auditory field take up tritiated gamma-aminobutyric acid; that their Nissl-counterstained somata have a smaller average area, and a smaller range of areas, than do the unlabeled cells; that more than one type of labeled neuron-as defined by somatic size and shape, height:width ratios, and nuclear membrane morphology-could be identified in each layer; that none of the labeled neurons had a soma with a pyramidal configuration; that the labeled cells are comparable in size, shape, and laminar distribution to some populations of non-pyramidal ipsilateral corticocortical cells of origin in layers II and III, and perhaps to certain classes of commissurally projecting, layer III non-pyramidal neurons; and finally, that only a rather small proportion-perhaps 10% or less, except in layer I-of the supragranular cells appear to accumulate labeled material. With regard to the identity of particular classes of neurons accumulating silver grains above background in the individual layers, in layer I, 2 of the 4 types of neurons characterized in Golgi preparations take up gamma-aminobutyric acid and the remaining 2 types may also, and the relative number of labeled cells appears to be higher than in the other layers; in layer II, 2 of the 9 varieties are labeled, and 4 other types may also be; and in layer III, 2 of the 11 types take up gamma-aminobutyric acid, and 5 other varieties may as well. Three types of non-pyramidal layer II cells that project ipsilaterally from AI to the second auditory cortical field, AII, possibly accumulate gamma-aminobutyric acid; 3 types of commissural non-pyramidal cells of origin linking AI to AI appear to be labeled by gamma-aminobutyric acid.(ABSTRACT TRUNCATED AT 400 WORDS)