A COMPARISON OF CURVATURES OF LEFT AND RIGHT-HAND MOVEMENTS IN A SIMPLE POINTING TASK

Human arm movements towards visual targets are remarkably reproducible in several tasks and conditions. Various authors have reported that t rajectories of unconstrained point-to-point movements are slightly cur ved, smooth and have bell-shaped velocity profiles. The hand paths of such movements show small - but significant - curvatures throughout th e workspace. The cause of these curvatures is still obscure. Tradition ally this curvature is explained as the result of an optimisation proc ess or is ascribed to mechanical or dynamic properties of the effector system. Recently, however, it has been suggested that these curvature s are due at least partly, to the visual misperception of straight lin es. To evaluate the latter hypothesis, we compared unconstrained, self -paced point-to-point movements that subjects made with their right an d left hand. We assume that the visual mis-perception may depend on th e position in the workspace, subject, etc. but not on the hand used to make the movement. Therefore we argue that if curvature is caused by a visual misperception of straight lines, curvatures should be the sam e for movements made with the left and right hand.Our experiments cast strong doubt on the hypothesis that curvatures are the result of a vi sual distortion, because curvatures of the left hand trajectories, mir rored in the mid-sagittal plane, are found to be accurately described by trajectories of the right hand. Estimates of the effect of visual d istortion on movement curvature show that, if present, this effect is very small compared with other sources that contribute to movement cur vature. We found that curvatures depend strongly on the subject and on the direction and distance of the movement. Curvatures do not seem to be caused purely by the dynamic properties of the arm, since curvatur es do not change significantly with increasing movement velocity. Ther efore, we conclude that curvatures reflect an inherent property of the control of multi-joint arm movements.