HAND AND JOINT PATHS DURING REACHING MOVEMENTS WITH AND WITHOUT VISION

This study examines whether the kinematics of pointing movements are a ltered by the sensory systems used to select spatial targets and to gu ide movement. Hand and joint paths of visually guided reaching movemen ts of human subjects were compared with two non-visual conditions wher e only proprioception was available: (1) movements of the same subject s with blindfolds, and (2) movements by congenitally blind subjects. W hile hand-path curvatures were overall quite small, sighted subjects w earing a blindfold showed a statistical increase in hand-path curvatur e compared with their visually guided movements. Blindfolded subjects also showed greater hand-path curvature than blind subjects. These inc reases in hand-path curvature for blindfolded subjects did not always lead to a decrease in joint-path curvature. While there were differenc es between blind subjects and sighted subjects using vision for some m ovement directions, there was no systematic difference between these t wo groups. The magnitude of joint-path curvature showed much greater v ariation than hand-path curvature across the movement directions. We f ound variation in joint-path curvature to be correlated to two factors , one spatial and one geometrical. For all subject groups, joint-path curvature tended to be smaller for sagittal-plane movements than for t ransverse or diagonal movements. As well, we found that the magnitude of joint-path curvature was also related to the relative motion at eac h joint. Joint-path curvature tended to increase when movements predom inantly involved changes in shoulder angle and was minimal when moveme nts predominantly involved elbow motion. The consistently small curvat ures of hand trajectory across blind and sighted subjects emphasize th e powerful tendency of the motor system to generate goal-directed reac hing movements with relatively straight hand trajectories, even when d eprived of visual feedback from very early in life.