Neurophysiological Approaches to Brain Mechanisms for Preparatory Set
Edward V. Evarts Laboratory of Neurophysiology National Institute of Mental Health Bethesda, Maryland
This article considers neurophysiological studies of mechanisms underlying response alterations depending on preparatory sets established by instruction stimuli (ISs) that may be thought of a "setting" the subject so that the same trigger stimulus (TS) will elicit one of a number of different responses depending on the prior IS. These neurophysiological studies seek to identify the classes of neurons that enable the same TS to elicit one response when one preparatory set exists and a different response when a different preparatory set exists. For example, such studies have shown that the same TS elicits one sort of motor cortex response when a monkey is set to maintain stability of arm position and a different sort of response when the monkey is set to make a rapid arm movement. It is suggested that this altered motor cortex responsiveness to sensory inputs depends on an intracortical gating process associated with the shift from the '"open-loop" preparatory set for rapid movement to the "closed-loop" preparatory set for postural stability and that (as proposed by Allen and Tsukahara) the shift from postural stability to rapid movement involves a shift of dominant control of motor cortex output from one cerebellar output nucleus (the interpositus) providing kinesthetic feedback to a different cerebellar nucleus (the dentate) providing central commands. The experimental observations that are reviewed demonstrate that preparatory sets involve clearly recognizable changes in activity of neurons in cerebral cortex as well as cerebellum and it is concluded that the problem facing neurophysiologists is no longer to discover set-related brain activity, but rather to discover the role of such activity in changed input-output relations associated with different preparatory sets.