To be presented at the Society for Psychophysiological Research, Lisbon, 2005

Early visual event-related potential activity in extra-striate cortex

Julie Onton and Scott Makeig

In a modified version of the Sternberg memory task, series of eight black or green letters were presented to be memorized or ignored respectively. After a short delay, a probe letter prompted the subject to respond by button press response to indicate whether or not that letter had been memorized. Using independent component analysis (ICA), we separated the 71-channel EEG data into maximally independent component time courses, each with an associated scalp map of component projection strengths to the electrodes. We identified a class of components (one from each subject) that contributed to average visual event-related potential (ERP) features P1 and N1. These had remarkably similar scalp maps and bilateral equivalent dipole sources in extra-striate cortex along the ventral (color and form) visual stream. Log-spectral decomposition of the component activities in single trials revealed two distinct modes of event-related spectral modulation that contributed to the event-related potential (ERP) and desynchronization (ERD) respectively. A sub-class of these components showed lower-amplitude ERPs and more lateral equivalent dipole locations as well as differences in spectral mode frequencies, time courses, and relative intensities.