Active versus passive touch: factors influencing the transmission of somatosensory signals to primary somatosensory cortex.
Active and passive touch, respectively with and without voluntary movement on the part of the subject, are frequently reported to be equivalent in terms of the resultant perceptual abilities. This review reexamines the notion of perceptual equivalence in the light of growing evidence that the transmission of tactile inputs is diminished, or "gated," during the course of active movement. It is concluded that there is indeed gating of cutaneous inputs during active touch. In most experiments, 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 absolute, 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 enhanced 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 optimally orienting the exploring digits so as to bring the most sensitive skin areas into contact with the object in question. Second, central influences, including attention and motor set, may be specifically activated during voluntary movement and contribute to enhancing performance during active touch. Thus, the gating influences associated with active touch may be offset, partly or wholly, by the combined influence of these factors to yield (near) perceptual equivalence for active and passive touch.