Effects of an acute acoustic trauma on the representation of a voice onset time continuum in cat primary auditory cortex.

Here we show that hearing loss associated with an impairment of speech recognition causes a decrease in neural temporal resolution. In order to assess central auditory system changes in temporal resolution, we investigated the effect of an acute hearing loss on the representation of a voice onset time (VOT) and gap-duration continuum in primary auditory cortex (AI) of the ketamine-anesthetized cat. Multiple single-unit activity related to the presentation of a /ba/-/pa/ continuum--in which VOT was varied in 5-ms step from 0 to 70 ms-- was recorded from the same sites before and after an acoustic trauma using two 8-electrode arrays. We also obtained data for gaps, of duration equal to the VOT, embedded in noise 5 ms after the onset. We specifically analyzed the maximum firing rate (FRmax), related to the presentation of the vowel or trailing noise burst, as a function of VOT and gap duration. The changes in FRmax for /ba/-/pa/ continuum as a function of VOT match the psychometric function for categorical perception of /ba/-/pa/ modeled by a sigmoid function. An acoustic trauma made the sigmoid fitting functions shallower, and shifted them toward higher values of VOT. The less steep fitting function may be a neural correlate of an impaired psychoacoustic temporal resolution, because the ambiguity between /ba/ and /pa/ should consequently be increased. The present study is the first one in showing an impairment of the temporal resolution of neurons in AI caused by an acute acoustic trauma.