Expectancy and response strategy to sensory stimuli.

We investigated the relationship between sensory discrimination and the selection of appropriate responses in subjects performing two different reaction-time tasks, in which three auditory stimuli were presented in random order and with a different likelihood of occurrence. Subjects anticipated the need to make different responses based on the likelihood that a particular stimulus would occur on a particular trial. This was determined by the occurrence and distribution of premovement potentials prior to the stimulus, which were consistent with preparation to respond to the most frequently occurring stimulus. These anticipatory cerebral events, however, could be altered after recognition that this frequent stimulus had not occurred. Thus, after the occurrence of a stimulus other than the anticipated frequent tone, the scalp distribution of cerebral potentials changed in a manner suggesting that the next most frequently occurring stimulus was anticipated. Nonetheless, subjects were able to respond to the least probable stimulus both accurately and rapidly despite a failure to anticipate it correctly, as judged by the cerebral "lateralized readiness potential." These results indicate that stimulus evaluation and response selection are integrated and dynamic cerebral processes, and raise doubt about the functional significance of the so-called premovement readiness potential.