ABSTRACT
Voluntary of involuntary detection of an infrequent stimulus generates a large scalp P300 response. This P300 ERP (P for positive; 300 for the approximate peak latency poststimulation) has been widely used to study phasic attention and memory mechanisms. The P300 phenomenon, first reported in 1965 (Desmedt et al., 1965; Sutton et al., 1965) has been the subject of extensive research in normal, neurologic, and psychiatric populations. P300-like potentials have been described in rats(Ehlers et al., 1991; Yamaguchi et al., 1993), cats(Katayama et al., 1985; O'Connor and Starr, 1985; Wilder et al., 1981), and monkeys(Arthur and Starr, 1984; Neville and Foote, 1984; Paller et al., 1988; Pineda et al., 1989) supporting a broad ethologic significance of this electrophysiological marker of cognition(Fig. 1) (Swick et al., 1994).Theorists have focused on attention and memory formulations to account for the cognitive basis of the P300, although no clear consensus has emerged(Donchin and Coles, 1988; Verleger, 1988). Some of this disagreement results from the fact that the P300 does not represent a unitary brain potential arising from a discrete brain region or cognitive process as initially proposed. Instead, scalp positivities generated in the 300- to 700-ms poststimulus delivery measure activation of multiple neocortical and limbic regions dependent on the degree of voluntary and involuntary attention allocated to a stimulus. Support for this contention is provided by scalp topographic studies in normal subjects(Courchesne et al., 1975; Squires and Hillyard, 1975; Ruchkin et al., 1990a, 1992; Yamaguchi and Knight, 1991a; Bruyant et al., 1993), intracranial recording in epileptic patients (McCarthy et al., 1989; Puce et al., 1991; Paller et al., 1992; Baudena et al., 1995; Halgren et al., 1995a,b) and lesion studies in neurologic patients(Knight, 1984, 1997a; Knight et al., 1989; Yamaguchi and Knight, 1991b, 1992; Scabini, 1992).