Down-regulation of inhibition following unilateral deafening.

Physiological and neurochemical experiments described here suggest that unilateral deafening causes a reduction in inhibition in the adult gerbil inferior colliculus (IC) contralateral to the deafened ear. Multiple-unit recordings were made from single electrode penetrations in the IC prior to and directly after contralateral cochlear ablation. These recordings showed up to 60% increases in the proportion of sampled loci at which neural activity excited by ipsilateral stimulation was observed after the ablation. Novel excitatory responses were evident within minutes of the ablation. Western blotting for glutamic acid decarboxylase protein levels showed significant decreases in the IC contralateral to cochlear ablation, relative to those in the ipsilateral IC, at 24 h and 7 days survival after the ablation. Four hour or 1 year survival post-ablation did not produce significant contralateral/ipsilateral differences in relation to the control group. Taken together, these results suggest the presence of at least two, short-term mechanisms involved in the central response to cochlear removal, both of which appear to implicate a decreased inhibitory influence. One is a very rapid, stimulus-related, functional unmasking. The other is a more delayed reduction in the capacity of gamma-aminobutyric acid synthesis in the IC.

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.

Lateralization of large interaural delays.

Two experiments explored the limits of listeners' abilities to interpret large interaural time delays (ITDs) in terms of laterality. In experiment 1, just-noticeable differences (jnd's) were measured, using an adaptive procedure, for various reference ITDs of Gaussian noise between 0 and 3000 microseconds. The jnd's increased gradually with reference ITD for reference ITDs between 0 microsecond and 700 microseconds, then rose sharply to plateau at much higher jnd's for the remainder of the standard ITDs tested (1000-3000 microseconds). The second experiment tested left/right discrimination of Gaussian noise that was interaurally delayed up to 10,000 microseconds, and high-pass filtered to cutoff frequencies between 0 Hz (broadband) and 3000 Hz. There was good discrimination (62%; significantly above chance, p < 0.05) for broadband and 500-Hz high-pass cutoff stimuli for all ITDs up to 10,000 microseconds, and for ITDs up to at least 3000 microseconds for higher high-pass cutoff frequencies. These results indicate that laterality cues are discriminable at much larger ITDs than are experienced in free-field listening, even in the absence of energy below 3 kHz.

Response properties of neurons in the inferior colliculus of the monaurally deafened ferret to acoustic stimulation of the intact ear.

Response properties of neurons in the central nucleus of the inferior colliculus (ICC) were investigated after unilateral cochlear removal at various ages during infancy. Nineteen ferrets had the right cochlea surgically ablated, either in adulthood or on postnatal day (P) 5, 25, or 40, 3-18 mo before recording. Adult ablations were made on the same day as ("acute," n = 3), or 2-3 mo before ("chronic," n = 3), recording. Two ferrets were left binaurally intact. Single-unit (n = 702) and multiunit (n = 1,819) recordings were made in the ICC of barbiturate-anesthetized ferrets ipsilateral (all ages) or contralateral (P5 and acute adult only) to the intact ear. In binaurally intact animals, tonal stimulation of the contralateral ear evoked excitatory activity at the majority (94%) of recording loci, whereas stimulation of the ipsilateral ear evoked activity at only 33% of recording loci. In acutely ablated animals, the majority of contralateral (90%) and ipsilateral (70%) loci were excited by tonal stimulation of the intact ear. In chronically ablated animals, 80-90% of loci were excited by ipsilateral stimulation. Single-unit thresholds were generally higher for low-best frequency (BF) than for high-BF units, and higher in the ipsilateral than in the contralateral ICC. Analysis of covariance showed highly significant differences between all of the ipsilateral and contralateral groups, but no effects of age at ablation or survival time following ablation, other than that the group ablated at P25 had higher mean ipsilateral thresholds than the groups ablated at P5 or, acutely, in adulthood. Cochlear ablation at P5, 25, or 40 resulted in a significant increase in dynamic ranges of ipsilateral ICC unit rate-intensity functions relative to acutely ablated animals. Dynamic ranges of units in the contralateral ICC of P5-ablated ferrets were also significantly increased compared with those of acutely ablated animals. Cochlear ablation at P5, 25, or 40 resulted in a significant increase in single-unit spontaneous discharge rates in the ICC ipsilateral but not contralateral (P5 only) to the intact ear. These data show that unilateral cochlear removal in adult ferrets leads to a rapid and dramatic increase in the proportion of neurons in the ICC ipsilateral to the intact ear that is excited by acoustic stimulation of that ear. In addition, the data confirm that, in ferrets, cochlear removal in infancy leads to a further increase in responsiveness of individual neurons in the ipsilateral ICC. Finally, the data show that responses in the ICC contralateral to the intact ear are largely but not completely unchanged by unilateral cochlear removal.

Detection of silent intervals between noises activating different perceptual channels: some properties of "central" auditory gap detection.

This article describes four experiments on gap detection by normal listeners, with the general goal being to examine the consequences of using noises in different perceptual channels to delimit a silent temporal gap to be detected. In experiment 1, subjects were presented with pairs of narrow-band noise sequences. The leading element in each pair had a center frequency of 2 kHz and the trailing element's center frequency was parametrically varied. Gap detection thresholds became increasingly poor, sometimes by up to an order of magnitude, as the spectral disparity was increased between the noise bursts that marked the gap. These data suggested that gap-detection performance is impoverished when the underlying perceptual timing operation requires a comparison of activity in different perceptual channels rather than a discontinuity detection within a given channel. In experiment 2, we assessed the effect of leading-element duration in within-channel and between-channel gap detection tasks. Gap detection thresholds rose when the duration of the leading element was less than about 30 ms, but only in the between-channel case. In experiment 3, the gap-detection stimulus was redesigned so that we could probe the perceptual mechanisms that might be involved in stop consonant discrimination. The leading element was a wideband noise burst, and the trailing element was a 300-ms bandpassed noise centered on 1.0 kHz. The independent variable was the duration of the leading element, and the dependent variable was the smallest detectable gap between the elements. When the leading element was short in duration (5-10 ms), gap thresholds were close to 30 ms, which is close to the voice onset time that parses some voiced from unvoiced stop consonants. In experiment 4, the generality of the leading-element duration effect in between-channel gap detection was examined. Spectrally identical noises defining the leading and trailing edges of the gap were presented to the same or to different ears. There was a leading-element duration effect only for the between channel case. The mean gap threshold was again close to 30 ms for short leading-element durations. Taken together, the data suggest that gap detection requiring a temporal correlation of activity in different perceptual channels is a fundamentally different task to the discontinuity detection used to execute gap detection performance in the traditional, within-channel paradigm.