Electrophysiological analysis of expectancy: P3 in informed guessing.

18 healthy subjects had to guess, which of two nonequiprobable events would occur next. Each trial was preceded by a cue which increased the probability of the corresponding event as compared to its global probability. Event-related potentials (ERPs) were recorded and then classified according to global probability of events, their relation to the preceeding cue (valid versus invalid cues) and to subject's prediction (predicted versus nonpredicted). Two late positive waves (P350 and P550) with parietal maxima were distinguished. Both waves had larger amplitudes in response to improbable events than to highly-probably events. Similarly, both had larger amplitudes following invalid cues than following valid cues, and this difference was larger in those subjects who tended to follow the cue than in those who tended to reject it. No difference in terms of ERP component amplitudes was found between predicted and unpredicted events; however, the latency of the P350 peak was longer following unpredicted events. Taken together with data of the literature, the present results indicate that ERP allow us to distinguish between two meanings of the word "expectancy": (1) the rule-related expectancy as cognitive estimation of the likelihood or "representativeness" of an event (based on grasping event contingencies), and (2) the goal-related expectancy manifested in the subject's overt behavior. Only the former "expectancy" affects the amplitude of the late positive wave ("P3"), while the latter does not.