P300 and stimulus categorization: two plus one is not so different from one plus one.

Event related brain potentials (ERPs) were recorded from subjects who were instructed to count one of three, equally probable tones presented in a random sequence. In another condition, the subjects had to count one of two stimuli, one of which was presented with a probability of .33. The data support the view that the pattern of variation of P300 amplitude with the sequential structure of the series depends on the category to which events are assigned, rather than on the individual stimuli eliciting the P300. Furthermore, the data support the idea that the amplitude of P300 elicited by task-relevant stimuli is determined by the subjective probability associated with the eliciting event. DESCRIPTIONS: ERP, P300, Subjective probability. Stimulus categorization. The assessment of subjective probabilities is of 1979; K. Squires, Petuchowski, Wickens, & Donconsiderable importance in the study of the P300 chin, 1977; K. Squires, Wickens, N. Squires, & component ofthe human event-related brain poten- Donchin, 1976), it has been demonstrated that the tial (ERP). When subjects are presented with a P3(X) elicited by a sequence of Bernoulli events series of Bernoulli events, the magnitude of the varies from trial to trial despite constant prior probP300 elicited by each stimulus is inversely related to ability. These studies further showed that this variits prior probability (Duncan-Johnson & Donchin, ability can be attributed to the specific sequence of 1977; Roth, Ford, Lewis, & Kopell, 1976; Tueting, stimuli presented on the trials immediately precedSutton, & Zubin, 1970). In these studies, the vari- ing each stimulus. able controlling the magnitude of P300 amplitude To account for this variability in P300 amplitude, was assumed to be the prior probabilities of the K. Squires et al. (1976) proposed a model which stimuli. Yet, the prior probability of a stimulus assumes that the subjective probability (or "expecaccounts for only part of the variance of P300. In a tancy'') associated with the outeome of each event series of reports (Duncan-Johnson, 1978; Duncan- varies from trial to trial. According to this model, Johnson & Donchin, 1977; Johnson & Donchin, the effect of prior probability on P300 is modulated

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