SPEECH-EVOKED ACTIVITY IN PRIMARY AUDITORY-CORTEX - EFFECTS OF VOICE ONSET TIME

Neural encoding of temporal speech features is a key component of acou stic and phonetic analyses. We examined the temporal encoding of the s yllables /da/ and /ta/, which differ along the temporally based, phone tic parameter of voice onset time (VOT), in primary auditory cortex (A 1) of awake monkeys using concurrent multilaminar recordings of audito ry evoked potentials (AEP), the derived current source density, and mu ltiunit activity. A general sequence of A1 activation consisting of a lamina-specific profile of parallel and sequential excitatory and inhi bitory processes is described. VOT is encoded in the temporal response patterns of phase-locked activity to the periodic speech segments and by ''on'' responses to stimulus and voicing onset. A transformation o ccurs between responses in the thalamocortical (TC) fiber input and A1 cells. TC fibers are more likely to encode VOT With ''on'' responses to stimulus onset followed by phase-locked responses during the voiced segment, whereas A1 responses are more likely to exhibit transient re sponses both to stimulus and voicing onset. Relevance to subcortical s peech processing, the human AEP and speech psychoacoustics are discuss ed. A mechanism for categorical differentiation of voiced and unvoiced consonants is proposed.