DISCRIMINATION OF SPEECH-LIKE CONTRASTS IN THE AUDITORY THALAMUS AND CORTEX

The neurophysiologic discrimination of acoustic contrasts was investig ated as reflected by the mismatch negativity (MMN) response. Evoked re sponses were recorded from guinea pig thalamus (medial geniculate nucl eus) and epidural surface in response to synthesized speech contrasts /ga/-/da/ and /ba/-/wa/. From the caudomedial portion of the medial ge niculate nucleus, /ba/-/wa/ elicited a strong mismatch response, where as /ga/-/da/ did not. Neither stimulus contrast elicited an MMN from t he ventral, or primary, portion of medial geniculate. Both stimulus co ntrasts elicited an MMN from the midline surface. Neither contrast eli cited an MMN from the surface over the temporal lobe. Results indicate a hierarchy of processing of the spectrotemporal changes which charac terize formant transitions. Also, results indicate that the nonprimary portions of the auditory pathway contribute substantially to the MMN.