ACTIVE VERSUS PASSIVE TOUCH - FACTORS INFLUENCING THE TRANSMISSION OFSOMATOSENSORY SIGNALS TO PRIMARY SOMATOSENSORY CORTEX

Active and passive touch, respectively with and without voluntary move ment on the part of the subject, are frequently reported to be equival ent in terms of the resultant perceptual abilities. This review reexam ines the notion of perceptual equivalence in the light of growing evid ence that the transmission of tactile inputs is diminished, or ''gated ,'' during the course of active movement. It is concluded that there i s indeed gating of cutaneous inputs during active touch. In most exper iments, the paradoxical observation of perceptual equivalence between active and passive touch can partly be explained by the choice of task , namely, tactile discriminations that depend on relative, and not abs olute, differences in inputs. The surprising lack of evidence for any superiority of passive touch over active touch can likely be explained by several factors. First, performance with active touch may be enhan ced by the motor strategy, e.g., by reducing the speed of movement at critical points, and so reducing the degree of gating, and (or) by opt imally orienting the exploring digits so as to bring the most sensitiv e skin areas into contact with the object in question. Second, central influences, including attention and motor set, may be specifically ac tivated during voluntary movement and contribute to enhancing performa nce during active touch. Thus, the gating influences associated with a ctive touch may be offset, partly or wholly, by the combined influence of these factors to yield (near) perceptual equivalence for active an d passive touch.