Novelty . Task - Relevance and the Vep 133

A late positive waveform of the human evoked potential, the P3 or P3oo (latency 300500 msec), has been shown by numerous researchers to accompany the delivery of taskrelevant information which requires a decision or response from the subject (Sutton et al. 1965, 1967; Ritter and Vaughan 1969; Squires el al. 1973; Picton et al. 1974). The amplitude of this waveform is enhanced when such stimuli are delivered infrequently and unpredictably to the subject (Tueting et al. 1971 ; Squires el al. 1975). This wave can be elicited by task-relevant stimuli in any modality (or even by omitted stimuli) and is found to be widely distributed across the the central and parietal regions of the scalp (Hillyard et al. in press; Ritter et al. in press). Similar waveforms (also termed P3), however, have also been reported to follow unpredictable and infrequent stimuli when no explicit task assignment is given to the subject (Ritter et al. 1968; Vaughan and Ritter 1970; Roth 1973). This finding has prompted these authors to suggest that the P3 is one index of the orienting response to any rare, unpredictable event, even when it is not task-relevant. These two sets of data thus pose a paradox for solution :