P3 Waves to the Discrimination of Targets in Homogeneous and Heterogeneous Stimulus Sequences

Abstract

ABSTRACT Event‐related potentials (ERPs) were recorded from subjects who discriminated infrequent target slides (particular letters or numbers) within a sequence of non‐target letters. In the first experiment, subjects counted occurrences of a fixed target letter that were randomly intermixed within either a homogeneous sequence of As (deviating condition) or a heterogeneous sequence of all the letters of the alphabet (non‐deviating condition). The P3 components (300‐600 msec) of the ERPs to the deviating and non‐deviating targets were virtually identical in amplitude, waveshape, latency and scalp distribution. Thus, the deviation of a target from an on‐going sequence is not a prerequisite for the elicitation of high‐amplitude P3 waves. A significant correlation of RT with P3 latency was found across‐subjects, but not within‐subjects. In a second experiment, subjects were presented with randomized sequences consisting of 80% A s, 10% B s, and 10% of either letters (between C and Z ) or numbers (between 0 and 23 ). Although the a priori probability of B s was about 20 times greater than any one of the letter or number slides, the averaged P3 waves to each of these types of targets did not differ from one another in amplitude, waveshape or scalp distribution. Apparently, the P3 wave is determined more by the probability of a stimulus class and the associated psychological operation than to the a priori probability of an individual stimulus .