Components of the event-related potential following degraded and undegraded visual stimuli.

Event-related potentials were recorded in response to visual stimuli in two different reaction tasks in which subjects were instructed to react immediately to the stimuli, or to delay their response for a 2 sec period, respectively. There were four types of stimuli: frequent-degraded, frequent-undegraded, infrequent-degraded and infrequent undegraded letters. In all conditions the stimuli evoked complex waveforms which comprised a frontal-negative wave (N400) and a late positive wave that reached a maximum amplitude on the parietal scalp site (P500). In addition, a slow positive wave with a central-parietal scalp distribution was found in the waveforms that were associated with the delayed reaction task. A principal components analysis of the waveforms yielded two major components: an early composite component that peaked around 400 msec, and a late component that became maximally active towards the end of the recording epoch. The scores of the earlier component were more negative (or less positive) and the scores of the late component were more positive when infrequent or degraded stimuli were presented, in comparison with frequent or undegraded stimuli.