Physiologic correlates of the voice onset time boundary in primary auditory cortex (A1) of the awake monkey: temporal response patterns.

Behavioral studies in animals support the view that categorical, phonetic phenomena are based upon specific response properties of the auditory system. This study investigated physiologic responses reflecting the phonetic parameter of voice onset time (VOT). We examined multiunit activity (MUA) in the primary auditory cortex (A1) of awake monkeys elicited by the consonant-vowel syllables /da/ and /ta/ that varied in VOT from 0 to 60 msec. Two temporal response patterns encode VOT. The first pattern contains responses time-locked to stimulus onset and to the onset of voicing. In 10 of 17 electrode penetrations that display this pattern, MUA reflects the VOT perceptual boundary by containing a prominent response to voicing onset only for /ta/ stimuli. The second pattern contains responses phase-locked to the periodic portion of the syllables. MUA exhibiting this temporal pattern does not display categorical-like properties. We conclude that specific temporal response patterns in A1 reflect the perceptual boundary for VOT and may represent a physiologic correlate for categorical perception of this phonetic parameter.