Links between vision and somatosensation: Vision can improve the felt position of the unseen hand

During reaching movements, sensory signals must be transformed into appropr iate motor commands, Anatomical [1], electrophysiological [2-4], and neurop sychological [5, 6] evidence suggest that there is no single, supramodal ma p of space that is used to guide reaching, Instead, movements appear to be planned and controlled within multiple coordinate systems, each one attache d to a different body part. Recent neuropsychological investigations [6-11] demonstrating that somatosensory impairments can be ameliorated by visual cues, and visual impairments by proprioceptive cues, have been interpreted as evidence that arm-centered representations may exist in humans. A critic al difference between the findings obtained in the monkey and in humans, ho wever, is that in the latter case, vision of the limb appears be critical f or visual somatosensory binding [10]. Here, we report a case study of a pat ient (C.T.) recovering from unilateral somatosensory impairment, including tactile extinction, who executed reaches toward visually defined or proprio ceptively defined locations. We demonstrate that when the target location o f a reach was defined proprioceptively, by passively positioning our patien t's impaired hand beneath the table surface, vision of the workspace immedi ately adjacent to the unseen hand dramatically increased the endpoint accur acy of her reaching movements, even though such cues could not possibly sig nal the position of the target directly. (C) 2001 Elsevier Science Ltd. All rights reserved.