Event-related potentials associated with the discrimination of acoustic and semantic aspects of speech.

Analysis of a word's acoustic structure must precede identification of its meaning. Therefore, these aspects of speech processing could be associated with event-related potential (ERP) components that differed in their timing. To identify electrophysiologic indices of the cortical processing of acoustic and semantic features of speech, we recorded ERPs to the random presentation of nonsense or real words in four conditions designed to manipulate the extent to which the speech sounds were processed. In one condition subjects responded to all stimuli; in a second and third, to a designated nonsense or real word; and in the final condition to words within a specified semantic category. To define the cortical activity associated with acoustic processing, ERPs obtained when no discrimination was required were subtracted from those recorded during the identification of a specified speech target. The difference waveforms exhibited a negative potential that began about 50 msec after stimulus onset and lasted about 200 msec. Difference waveforms obtained by subtracting the non-discrimination ERP from those obtained during semantic discrimination exhibited a negative potential with similar onset timing. We concluded that the early negative potential indexed acoustic processes necessary for stimulus identification. To identify potentials associated with determination of a word's meaning, we subtracted the verbal discrimination from the semantic discrimination ERPs. This difference waveform exhibited a later negativity beginning at 150 msec and lasting about 250 msec. This potential may be related to the semantic processing of speech.