A triarchic model of P300 amplitude.

A model of P300 amplitude is proposed that reduces the many hypothetical constructs invoked to explain variations in P300 amplitude to three dimensions: 1) Subjective Probahility, 2) Stimulus Meaning, and 3) Information Transmission. Evidence is presented to support the assertion that variables on the subjective prwbability and stimulus meaning dimensions have independent and additive contributions to overall P300 amplitude. The amplitude contributions of both of these dimensions, however, are modulated by a multiplicative relation with the proportion of transmitted stimulus information. Within each dimension, the fundamental experimental variables and their interrelations are specified. An example is presented to show how, by using an additive factors method, the respective amplitude effects of the probability and stimulus meaning dimensions can be separated. Supporting data are presented to show that the proposed model provides a reasonable and testable framework in which to conceptualize P300 results. DESCRIPTORS: Event-related potentials, P300. The prospect of having an electrophysiological index of cognitive operations has led many researchers to explore the nature of the P300 component of the event-related brain potential (ERP). Since its discovery by Sutton and his colleagues (Sutton, Braren, Zubin, & John, 1965; Sutton, Tueting, Zubin, & John, 1967), studies have demonstrated that P300 amplitude and latency can be used as indices of the nature and timing of a subject's cognitive response to a stimulus. ' As a result of 'Although there now appears to be a variety of late positive waves that are elicited under different circumstances, the model proposed here applies only to the best understood of these positivities, the P300 or P3b. This paper is based on an address given upon receipt ofthe Early Career Contribution Award, Society for Psychophysiological Research, Houston, Texas, October, 1985, and represents an elaboration of ideas first presented as part of a doctoral dissertation in 1979. I am grateful for the numerous helpful comments given by John Rohrbaugh, Connie Duncan, Dan Ruchkin, Sam Sutton, Walter Ritter, and Dave Friedman. Address requests for reprints to: Ray Johnson, Jr., National Institutes of Health, NINCDS, MNB, Building 10, Room 4N246, Bethesda, Maryland 20892. Studying the P300 in a wide variety of behavioral paradigms, a large number of hypothetical constructs have been suggested to account for the observed variations in P300 amplitude (e.g., attentiveness, orienting, decision making, uncertainty reduction, intentional engagement, processing demand, salience, task relevance, equivocation, and value). Although there are many similarities among the proposed constructs, little effort has been made to sort out the nature and interactions of the basic factors that are reflected in P300 amplitude. The desirabihty of developing such a taxonomy is threefold: First, a viable schema would be heuristically useful and provide conceptual guidance for making accurate a priori predictions about the results of proposed experiments; second, it would systematize the existing data base and thereby aid the development of theories about the functional significance of P300; and third, it would provide a theoretical framework for interpreting amplitude variations such as those in P300 studies of patient groups. Any theoretical account must begin with the recognition that many ofthe constructs used to explain P300 amplitude are related to the delivery and subsequent processing of relevant information. For ex-