Late Positive Event Related Potentials in Schizophrenia

Abstract

As early as 1965 Sutton et al. described a late single positive component of the event related potentials (ERP) with a peak latency of about 300 ms which appeared to be related to stimulus uncertainty. This potential is ‘endogenous’ in that it does not reflect physical parameters of the eliciting stimuli and it can be elicited by stimuli of different sensory modalities or even by the absence of an expected stimulus. In this sense it can be said to reflect ‘active cognitive processing of stimulus information on the part of the subject’ (Pritchard, 1981). It is now clear that P3 is part of a late positive complex (LPC) rather than a unitary phenomenon. It consists of an early positive wave of 220 to 280 ms latency with fronto-central scalp distribution often termed P3a (Squires et al., 1975), a slightly later wave, called P3b, with a peak latency of 310-380 ms and maximal parietal distribution, and a slow wave (SW) overlapping with and following P3a and P3b and showing a parietal distribution similar to P3b but associated with increased negativity over the frontal leads. While the P3a is elicited by infrequent stimuli which were not attended to, a P3b occurs with the introduction of a discrimination task (e.g. counting target stimuli) involving infrequent stimuli (Squires et al., 1975). Apart from discriminative tasks, P3b can be elicited by stimuli which give feedback on the outcome of a prior task or stimuli which require a motor response. While SW and P3b covary in relation to many stimulus conditions, under certain conditions a dissociation can be observed (Ruchkin & Sutton, 1983).