Electroencephalography and clinical Neurophysiology, 1990, 75:419-432 419 Elsevier Scientific Publishers Ireland, Ltd. EEG 03727 Event-related brain potentials as indices of information extraction and response priming Gabriele Gratton, Carla M. Bosco, Arthur F. Kramer, Michael G.H. Coles, Christopher D. Wickens and Emanuel Donchin 1 Cognitive Psychophysiology Laboratory, Department of Psychology, University of Illinois at Urbana-Champaign, Champaign, IL (U.S.A.) (Accepted for publication: 26 July 1989) Summary Measures of overt response and of the event-related brain potential (ERP) were used to investigate the processing of a priming stimulus varying in its information content. Subjects were shown sequences of 2 letters that served as a priming and an imperative stimulus. In 3 randomly interspersed conditions the imperative stimulus had a 0.80, 0.50, or 0.20 probability of physically matching the priming letter. The different probability conditions were signaled by the position of a dot flanking the priming letter. Reaction time and accuracy data indicated that the subjects primed their responses as a function of the information conveyed by the priming stimulus. The amplitude and latency of the P300 to the priming stimulus were sensitive to the amount of information conveyed by the priming stimulus and the duration of the processing required. The readiness potential in the foreperiod was lateralized as a function of the priming stimulus. Furthermore, the larger the amplitude of the P300 to the priming stimulus, the larger the lateralization of the readiness potential, indicating that information extraction, indexed by the P300, was related to response priming, indexed by the readiness potential. The results indicate that ERP measures make manifest covert aspects of the priming process occurring in the foreperiod. Key words: P300; Lateralized readiness potential; Stimulus priming; Response priming; Information extraction Warning stimuli can markedly affect the speed and accuracy of responses to subsequent imper- ative stimuli (Woodworth 1938). Presumably, this is because of covert preparatory processes that occur during the foreperiod. In a particular type of paradigm (that may be referred to as the 'con- ditional probability paradigm') the warning stimulus provides information about the probabil- ity of a particular stimulus (and response). In this i Portions of this paper were presented at the 26th Annual Meeting of the Society for Psychophysiological Research, Montreal, Canada, 16-19 October 1986. Correspondence to: Gabriele Gratton, University of Illinois, Psychology Department, 603 E. Daniel, Champaign, IL 61820 (U.S.A.). paper we will use psychophysiological evidence to investigate how the information provided by the warning stimulus in this paradigm is used to pre- pare perceptual and response elements for the forthcoming imperative stimulus. When warning stimuli provide probability in- formation, it is assumed that they create 'expecta- tions' about the forthcoming stimulus (and re- sponse). There are attempts to account for the effects of expectancy on reaction time (RT) which emphasize enhanced processing of the stimuli. For example, Whitman and Geller (1972) attributed the speeded reaction to the creation of a 'stimulus set' which facilitates the encoding and recognition of the imperative stimulus. Other theorists, for example Rabbitt (1959) and Theios (1975), ascribe the facilitation of the reaction to the imperative 0013-4649/90/$03.50 © 1990 Elsevier Scientific Publishers Ireland, Ltd.