SPEECH EVOKED ACTIVITY IN THE AUDITORY CORTEX OF THE MONKEY

Abstract

To determine whether phonetic features of human speech are reflected in activity patterns of the auditory cortex and its thalamic afferents and to differentiate between activity within cells of the auditory cortex and axons of thalamocortical afferents, concurrent recordings of auditory evoked potentials (AEPs) and multiple unit activity (MUA) to clicks and 3 synthetic syllables: /da/, /ba/ and /ta/ were performed in awake monkeys. The cortical AEP to clicks consist of 8 components. These components exhibit specific intracortical amplitude and latency changes and invert in polarity at defined depths. The current source density (CSD) is derived from the AEP and the response components are related to the laminar distribution of the CSD. Hypotheses concerning the distribution of transmembrane currents and the active neural elements generating the AEP are proposed. MUA from thalamocortical axons and cortical cells are differentiated on the basis of: (a) onset latency to click stimulation, (b) the distribution of MUA within and below the auditory cortex and (c) the spatial and temporal relationships of the MUA to the click evoked AEP.;Syllable fundamental frequency, voice onset time (VOT) and place of articulation are reflected in the temporal patterns of the MUA and AEP. These phonetic features are also reflected in the volume-conducted AEP recorded at the dorsal cortical surface. Duration of the voiced formant transitions is also reflected in the MUA. The relationship between the speech evoked responses and the cortical tonotopic organization is also determined. The responses from both low and high best frequency regions of auditory cortex reflect fundamental frequency and VOT. In contrast, place of articulation is reflected in the responses of loci that are differentially activated by the onset frequencies of the syllable formants. Responses to isolated formants sum in a complex but predictable manner to produce the response to the complete syllables. Oscillatory responses reflecting syllable fundamental frequency are more common in the activity of thalamocortical fibers while a sustained excitation to the syllables is more frequent in the responses of cortical cells.