Motor Illusions: What Do They Reveal About Proprioception?

Five illusions involving distortions in the perception of limb position, movement, and weight are described in the context of their contribution to understanding the sensory processes involved in proprioception. In particular, these illusions demonstrate that the position sense representation of the body and the awareness of limb movement result from the cross-calibration of visual and proprioceptive signals. Studies of the vibration illusion and phantom-limb phenomenon indicate that the perception of limb movement and position are encoded independently and can be dissociated. Postural aftereffects and the illusions of movement induced by vibration highlight the remarkable lability of this sense of limb position, which is a necessary feature for congruence between the spatial senses. Finally, I discuss the role of corollary discharges in the central processing of afferent information with respect to the size-weight and vibration illusions. The study of visual illusions has provided many valuable clues about the operation of the visual system (Coren & Girgus, 1978), to the extent that visual aftereffects have been described as the psychologist's microelectrode (Frisby, 1979). With the notable exception of the size-weight illusion, which was first described in the late nineteenth century (Charpentier, 1891), illusions involving the motor system have received much less attention. However, recent reports of illusory movements induced by mechanical vibration of the muscle tendon (Goodwin, McCloskey, & Matthews, 1972; Roll & Vedel, 1982; Vedel & Roll, 1983) have led to a revival of interest in kinesthetic illusions (Feldman & Latash, 1982a; Goodwin, 1976; Granit, 1972; Matthews, 1982). As with the classical visual illusions, these phenomena offer valuable insights into the perceptual mechanisms involved in proprioception and must be explained in terms of normal physiological mechanisms. I discuss disturbances in the perception of limb position and movement and of force and weight in this review in the context of their contribution to understanding perceptual processing in the sensorimotor system.