Time course of auditory cortex activation during speech processing

The purpose of the studies summarized in this report was to determine the time course of auditory cortex involvement in speech and language processing in the context of auditory object formation. Forty-one subjects took part in the three studies summarized in this report. In all three studies, subjects performed a choice-reaction task that required their pressing an appropriate button in response to auditory stimuli (speech/non-speech, good/worse fused phonemes, first/second language words) presented through earphones. Event-related potentials (ERPs) were recorded during performance of the task from 21 scalp electrodes, in addition to peri-ocular electrodes for monitoring eye movements. Current densities within the gray matter of the brain were estimated using the LORETA (low resolution electromagnetic tomography) method. In general, except for some periods, processing phonetic and linguistic information was associated with elevated activity in the left auditory cortex. Peaks in auditory cortex activation corresponded in time to scalp recorded peaks in the latencies of P₁ and up to as late as P₃. The adjacent posterior temporal areas showed a similar temporal pattern of activation, but tended to be less lateralized to the left, or even biased toward right hemisphere predominance, depending on the stimulus, particularly in the later time frames. The results indicate that the auditory cortex is engaged in auditory processing from its early stages and as long as a few hundreds of msec, even after cessation of the stimulus, defining sounds as distinct auditory objects and differentiating speech from non-speech material, relying on acoustic cues. Hemispheric dominance fluctuates to include activity in the 'non-dominant' hemisphere depending on stimulus type and stage of processing.