Speech Evoked Activity in the Auditory Radiations and Cortex of the Awake Monkey

To determine whether phonetic features of human speech are reflected in activity patterns of the auditory cortex and its thalamic afferents, concurrent recordings of multiple unit activity (MUA) and averaged evoked potentials (AEP) to 3 synthetic syllables: /da/,/ba/ and /ta/, were performed in awake monkeys. Using clicks, responses from thalamocortical axons and cortical cells were differentiated on the basis of their response latency, spatial distribution, and relationships to AEP components. Voice onset time was reflected in MUA time-locked to consonant release and voicing onset, and phase-locked to the syllables' fundamental frequency. Place of articulation was reflected in discriminative 'on' and phase-locked responses occurring to the formant transitions of the syllables. Duration of the voiced formant transitions was represented by an accentuation of the phase-locked responses occurring to this period. Activity of thalamocortical fibers and cortical cells differed. Thalamocortical fibers were more responsive to speech sounds, as well as responding more frequently with a phase-locked response pattern. Cortical cells responded with sustained activity to a greater degree. Responses to identical portions of the vowels were biased by the preceding consonant. The spatial extent and timing of the responses demonstrate that speech sounds are processed along parallel, but not synchronous, channels. Relevance to human psychoacoustical phenomena is discussed.