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. 138This chapter will consider neurophysiological mechanisms underlying response alterations depending on the preparatory sets established by prior instructions. Such prior instructions may be thought of as “setting” the subject so that the same stimulus will trigger one of a number of different responses depending on the instruction that has preceded the stimulus.

No concept in experimental psychology has been more fully investigated than set, for under this heading are included an entire group of mediating processes whose existences have been inferred on the basis of changes in the response to a given stimulus as a function of the set of the subject at the time the stimulus is delivered. Within the present chapter, no attempt is made to review the history of the concept of set, since prior books on this topic confront us with an embarrassment of riches, both in the number of reviews that have dealt with these topics and in the antiquity of the concepts themselves. Boring (1957) and Herrnstein and Boring (1965) have described the development of the concepts, with accounts of the ideas of Plato, Aristotle, Descartes, Leibnitz, Hobbes, Locke, Berkeley, Hume, Mill and Spencer, and no attempt will be made to review the ideas of these philosophers in this chapter. The pioneering work on the experimental psychology of set is commonly attributed to Donders and to Kulpe. Donders carried out the reaction time experiments that led to much of the subsequent work on motor set while Kulpe performed the studies showing the effects of expectancy on perception, thereby laying the groundwork for subsequent work on selective attention.