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Human and chinchilla listeners exhibit nonmonotonic temporal acuity for speech sounds differing in voice onset time (VOT). Characteristics of the neural discharge pattern or of the stimuli themselves that might account for the pattern of temporal acuity have not been described. Responses of chinchilla auditory-nerve fibers to syllables from an alveolar VOT continuum were measured. Peak discharge rates and peak response latencies elicited by the syllables with the shortest and longest VOTs were highly variable across groups of neurons with similar characteristic frequencies. For VOTs from the middle of the continuum, peak responses were larger, and response latencies were nearly constant across the same group of neurons. Overall, the magnitude and temporal variability of the responses of populations of primary auditory neurons varied nonmonotonically with VOT, consistent with the pattern of psychophysical temporal acuity for these syllables exhibited by humans and chinchillas. Spectral analyses suggested by the pattern of neural responses indicated that synchronous or correlated spectral cues were available over a wider bandwidth for those syllables from the middle of the continuum for which the neural representation was least variable.

Responses of chinchilla auditory-nerve fibers were measured for stimulus conditions analogous to those in which psychophysical release from masking has been observed in humans. The maskers were two equal power, narrow-band noise stimuli with different amplitude envelopes. The neurons in the sample fell into three groups that resolved the maskers' envelopes with varying degrees of accuracy. The boundaries of these groups were not sharply delineated by characteristic frequency (CF) but were dependent on the relationship between the masker level and the neurons' thresholds at the masker frequency. For the neurons that best preserved the maskers' envelope fluctuations, a neural release from masking was observed; rate-based neural masked thresholds were higher for the masker with the least fluctuating envelope. The results suggest that neural and psychophysical release from masking arises because the probe evokes larger rate changes, relative to the background response to the masker, during periods of low masker energy. Between two otherwise equivalent maskers, the one with the periods of lowest energy will produce the lower masked thresholds because rate changes are larger and more detectable.

Responses of chinchilla auditory-nerve fibers to synthesized stop consonants differing in voice onset time (VOT) were obtained. The syllables, heard as /ga/-/ka/ or /da/-/ta/, were similar to those previously used by others in psychophysical experiments with human and with chinchilla subjects. Average discharge rates of neurons tuned to the frequency region near the first formant generally increased at the onset of voicing, for VOTs longer than 20 ms. These rate increases were closely related to spectral amplitude changes associated with the onset of voicing and with the activation of the first formant; as a result, they provided accurate information about VOT. Neurons tuned to frequency regions near the second and third formants did not encode VOT in their average discharge rates. Modulations in the average rates of these neurons reflected spectral variations that were independent of VOT. The results are compared to other measurements of the peripheral encoding of speech sounds and to psychophysical observations suggesting that syllables with large variations in VOT are heard as belonging to one of only two phonemic categories.

Responses of chinchilla auditory nerve fibers to synthesized stop consonant syllables differing in voice-onset time (VOT) were obtained. The syllables, heard as /ga/-/ka/ or /da/-/ta/, were similar to those previously used by others in psychophysical experiments with human and chinchilla subjects. Synchronized discharge rates of neurons tuned to frequencies near the first formant increased at the onset of voicing for VOTs longer than 20 ms. Stimulus components near the formant or the neuron's characteristic frequency accounted for the increase. In these neurons, synchronized response changes were closely related to the same neuron's average discharge rates [D. G. Sinex and L. P. McDonald, J. Acoust. Soc. Am. 83, 1817-1827 (1988)]. Neurons tuned to frequency regions near the second and third formants usually responded to components near the second formant prior to the onset of voicing. These neurons' synchronized discharges could be captured by the first formant at the onset of voicing or with a latency of 50-60 ms, whichever was later. Since these neurons' average rate responses were unaffected by the onset of voicing, the latency of the synchronized response did provide as much additional neural cue to VOT. Overall, however, discharge synchrony did not provide as much information about VOT as was provided by the best average rate responses. The results are compared to other measurements of the peripheral encoding of speech sounds and to aspects of VOT perception.