Influence of visual constraints in the trajectory formation of grasping movements.

The main objective of the present study is to show that the visual context can influence the trajectory formation of grasping movements. We asked participants to reach and grasp a cylinder disposed at three different positions: -20 degrees , 0 degrees and 20 degrees of eccentricity with respect to the midsagittal axis. Grasping movements were performed in a direct and in an indirect visual feedback condition (i.e., controlled through a vertical video display). Results revealed that for grasping movements directed toward objects located at -20 degrees and 0 degrees , path curvatures of the wrist, the thumb and the index finger were significantly straighter in the indirect visual feedback condition. However, no significant difference concerning hand path curvature was observed when the movement was directed toward the object located at 20 degrees . This suggests that grasping movements controlled through a remote visual feedback tend to be planned in extrinsic space and that the effect of the visual context on movement planning appears to be not isotropic over the workspace.