REGIONAL MODULATION OF HIGH-RESOLUTION EVOKED-POTENTIALS DURING VERBAL AND NONVERBAL MATCHING TASKS

Nine subjects performed a cued S1-S2 matching task in which two sequen tially presented visual stimuli (either letter strings or non-verbal g raphical patterns) were compared according to verbal (phonemic, semant ic, syntactic) or non-verbal (graphic identity) criteria. The Laplacia n derivation was used to spatially enhance the topography of averaged evoked potentials (EPs) recorded from 59 scalp electrodes. Several eff ects distinguished the non-verbal from the verbal conditions. For exam ple, following S1 a P250 EP that reached maximum amplitude over the oc cipital area was larger far the non-verbal patterns, whereas word and word-like letter strings (but not unfamiliar characters) elicited an N 470 in the left temporal region. In anticipation of S2, a CNV-like slo w potential was enhanced over posterior regions for the non-verbal sti muli. During the matching interval following S2, a P475 peak was obser ved to be larger for non-verbal patterns than for letter strings over right frontal and temporal regions. Other effects distinguished the ve rbal conditions from one another. In particular, following S1 a left f rontal P445 potential was enhanced to closed class versus open class w ords, and following S2 a P620 potential in the left temporal region wa s enhanced for phonological matching relative to semantic matching. Th ese results suggest that processing of verbal and non-verbal stimuli d epends on a network of subprocessors that are regionalized to function ally specialized cortical areas and that operate both sequentially and in parallel in order to extract and synthesize multiple forms of attr ibute-specific information. In contrast to neuropsychological approach es to the study of pattern recognition and reading, the fine-grain tem poral resolution of EP measurements, in combination with the improved spatial resolution obtained through computation of Laplacian derivatio n wave forms from a large number of electrodes, permits characterizati on of both the regionalization of subprocesses and the subsecond dynam ics of their engagement.