Society for Psychophysiology Research Lisbon, Portugal Sept. 21-24, 2005 Peter Ullsperger-1, Hilit Serby-2, Scott Makeig-2 , 1-Federal Institute for Occupational Safety and Health, Berlin, Germany 2-Swartz Center, Institute for Neural Computation, University of California San Diego BEYOND AVERAGING: EXPLORING N400 BY INDEPENDENT COMPONENT ANALYSIS Event-related brain dynamics of 20 younger and 8 older participants (mean ages, 24 and 48 years) were recorded during a semantic decision task involving auditorily presented German noun pairs. Each trial began with a prime word, followed after 1100 ms by a target word. The inter- trial asynchrony was 2400 ms. Participants indicated by choice button press whether each target word was semantically related or unrelated to the previous prime word. A total of 240 word pairs (120 related and 120 unrelated) were presented to each subject twice. EEG was recorded from 60 scalp channels. N400 deflections in average event-related potentials (ERPs) time locked to target word onsets were larger following unrelated targets. ICA, performed on the continuous data after artifact rejection, produced several clusters of independent components (ICs). Most clusters had similar ERPs to prime and target word presentations. One IC cluster accounted for much of the N400 response difference. ERP-image plots of sorted single-trial epochs of channel data or IC activations, smoothed across neighboring trials, suggest several insights into the semantic processing of the target words that could not be gained from conventional averages. While further systematic investigations are needed to understand the interplay between independent brain activities during cognitive information processing, the results demonstrate the value for ERP analysis of disentangling the activities of spatiotemporally overlapping brain processes in single trials.

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