Reorganization of receptive fields following hearing loss in inferior colliculus neurons.

We explored frequency and intensity encoding in the inferior colliculus (IC) of the C57 mouse model of sensorineural hearing loss. Consistent with plasticity reported in the IC of other models of hearing loss, frequency response areas (FRAs) in hearing-impaired (HI) mice were broader with fewer high-frequency units than normal-hearing (NH) mice. The broad FRAs recorded from HI mice had lower cutoffs on the low frequency edge of the FRA. Characteristic frequency (CF) and sharpness of tuning (Q10) calculated from the FRA were used to divide the sample into four categories: low-CF sharp-FRA, low-CF broad-FRA, high-CF sharp-FRA, and high-CF broad-FRA units. Rate-intensity functions (RIFs) for CF tones and noise were used to determine the minimum and maximum response counts as well as the sound pressure levels resulting in 10%, 50%, and 90% of the maximum spike count. Tone RIFs of broad FRA units were shifted to the right of tone RIFs of sharp FRA units in both NH and HI mouse IC, regardless of the unit CF. The main effects of hearing loss were seen in the noise RIFs. The low-CF broad-FRA units in HI mice had elevated responses to noise, and the high-CF sharp-FRA units in HI mice had lower maximum rates, as compared with the units recorded from NH mice. These results suggest that, as the IC responds to peripheral hearing loss with changes in the representation of frequency, an altered balance between inhibitory and excitatory inputs to the neurons recorded from the HI mice alters aspects of the units' intensity encoding. This altered balance likely occurs, at least in part, outside of the IC.

Background noise differentially effects temporal coding by tonic units in the mouse inferior colliculus.

In natural environments, temporally complex signals often occur in a background of noise. The neural mechanisms underlying the preservation of temporal sensitivity in background noise are poorly understood. In the present study, we examined the ability of inferior colliculus (IC) units with primary-like and sustained response patterns ('tonic units') to encode silent gaps in quiet and in background noise. Minimum gap thresholds (MGTs), the shortest silent gap in a noise burst evoking a neural response, were measured in quiet and background noise for 34 IC units. Units were classified as background noise resistant (BNR; MGT did not change in background noise) or background noise sensitive (BNS; MGTs became elevated in background noise). In quiet, the MGTs of BNR and BNS units were comparable and both types of units encoded the gap by a cessation of activity during the gap. The addition of background noise had little effect on the response rate of BNR units either during or after the gap stimulus. In contrast, for BNS units, background noise reduced the response rate during the gap stimulus while increasing the response rate after the gap stimulus. Background noise also altered the first spike latency of BNS units. For BNS units, the mean first spike latency was no longer inversely related to BF, but this relationship was maintained in BNR units. These results suggest that the response of BNS units to background noise obliterates their response to the gap stimulus.

Gap encoding by inferior collicular neurons is altered by minimal changes in signal envelope.

Neural correlates of temporal resolution in the central auditory system are currently under intense investigation. The gap detection paradigm offers a simple, yet important, test of temporal acuity because changes in behavioral gap thresholds have been correlated with deficits in complex stimulus processing, such as speech perception. In gap detection studies, silent gaps are typically shaped by rapid (< 1.0 ms) rise/fall (R/F) times, i.e., rapid decreases and increases in sound intensity. However, in nature, the envelopes surrounding silent periods can vary significantly in R/F time. Therefore, we investigated whether changes in the R/F time surrounding the silent gap affect neural processing by inferior collicular (IC) neurons. Gap R/F times were varied between 0.5 and 16 ms and the discharge pattern, response rate, and first spike latency of IC neurons were measured for gap widths up to 100 ms. Neurons were classified into phasic or tonic discharge patterns based on peri-stimulus time histograms elicited to 100 ms noise carriers. The results indicate that (1) minimal gap thresholds increased with R/F time regardless of response type, (2) first spike latency variance increased systematically with R/F time for units which had small first spike standard deviations at short R/F times, and (3) the response rate of some units (called 'gap-tuned') changed as a function of both R/F time and gap width. Gap-tuned units responded strongly to a particular gap width only when the envelope of the gap was shaped by a particular R/F time. For gap-tuned units, increases in R/F time shifted the tuning to larger gap widths and also broadened the response profile. These results show that temporal acuity of neurons in the IC, as measured by the gap detection paradigm, is sensitive to the envelope surrounding gaps embedded in noise carriers.

Auditory pattern perception: the effect of tone location on the discrimination of tonal sequences.

This study examined the effect of tonal location on the discrimination of sequences differing in the order of the tones and on their perceived grouping. The frequency range between the sequential tones was held constant. When all tones came from the same location, the sequence was rated as integrated, but when the higher frequency tones came from a different location than the lower frequency tones, the sequence was rated as segregated. Listeners discriminated the sequences in a 3IFC task. Discrimination performance was impaired when the sequence was split between two locations and tonal order was changed in only one location, even though the order of tones in different locations was changed. This result suggests that listeners have difficulty relating tones across locations or in different perceptual groups. Performance in this experiment is generally better than that observed by Barsz (1988), and it is suggested that the level of stimulus uncertainty explains this difference.

Laterality effects in response to the offset of visual stimuli.

In a study of simple reaction time to visual stimuli it was found that the offset (or termination) of stimuli presented in the right visual field elicits significantly later responses than does the offset of stimuli presented in the left visual field. No such difference was observed for the responses to stimulus onset. A similar effect has been reported for responses to tonal stimuli. The results do not support the view that hemispheric asymmetries arise at higher stages of information processing than those which mediate simple reaction time.