Stimulus novelty, task relevance and the visual evoked potential in man

Abstract

Visual evoked potentials (VEPs) were recorded from normal adult subjects performing in a visual discrimination task. Subjects counted the number of presentations of the numeral 4 which was interposed rarely and randomly within a sequence of tachistoscopically flashed background stimuli (numeral 2s). Intrusive, task-irrelevant (not counted) stimuli were also interspersed rarely and randomly in the sequence of 2s; these stimuli were of two types: simples, which were easily recognizable ( e.g., geometric figures), and novels, which were completely unrecognizable ( i.e., complex, colorful patterns). It was found that the simples and the counted 4s evoked posteriorly distributed P 3 waves (latency 380–430 msec) while the irrelevant novels evoked large, frontally distributed P 3 waves (latency 360–380 msec). These large, frontal P 3 waves to novels were also found to be preceded by large N 2 waves (latency 278 msec). These findings indicate that “the P 3” wave is not a unitary phenomenon but should be considered in terms of a family of waves, differing in their brain generators and in their psychological correlates. These late positive components are discussed in terms of task-relevance, recognition and Pavlov's “what is it” response.