Tonotopic features of speech-evoked activity in primate auditory cortex.

To further clarify the neural mechanisms underlying the cortical encoding of speech sounds, we have recorded multiple unit activity (MUA) in the primary auditory cortex (A1) and thalamocortical (TC) radiations of an awake monkey to 3 consonant-vowel syllables, /da/, /ba/ and /ta/, that vary in their consonant place of articulation and voice onset time (VOT). In addition, we have examined the responses to the syllables' isolated formants and formant pairs. Response features are related to the cortical tonotopic organization, as determined by examining the responses to selected pure tones. MUA patterns that differentially reflect the spectral characteristics of the steady-state formant frequencies and formant transition onset frequencies underlying consonant place of articulation occur at sites with similarly differentiated tone responses. Whereas the detailed spectral characteristics of the speech sounds are reflected in low frequency cortical regions, both low and high frequency areas generate responses that reflect their temporal characteristics of fundamental frequency and VOT. Formant interactions modulate the responses to the whole syllables. These interactions may sharpen response differences that reflect consonant place of articulation. Response features noted in A1 also occur in TC fibers. Thus, differences in the encoding of speech sounds between the thalamic and cortical levels may include further opportunities for formant interactions within auditory cortex. One effect could be to heighten response contrast between complex stimuli with subtle acoustical differences.