Body-centered visuomotor adaptation.

Previous research has shown that humans generalize distortions of visuomotor feedback in terms of egocentric rotations. We examined whether these rotations are linked to the orientation of the eyes or of the shoulder of the arm that was used. Subjects moved a hand-held cube between target locations in a sequence of adaptation and test phases. During adaptation phases, subjects received either veridical or distorted visual feedback about the location of the cube. The distortions were changes in azimuth either relative to the eyes or to the shoulder. During test phases subjects received no visual feedback. Test phases were performed either with the arm that was exposed to the distorted feedback or with the unexposed arm. We compared test movement endpoints after distorted feedback with ones after veridical feedback. For the exposed arm, the spatial layout of the changes in endpoints clearly reflected the small differences between a rotation around the shoulder and around the eyes. For the unexposed arm, the changes in endpoints were smaller for both types of distortions and were less consistent with the distortions. Thus although the adaptation closely matches the imposed distortion, it does not appear to be directly linked to the orientation of the eyes or of the exposed arm.