Coupling between reaching movement direction and hand orientation for grasping

In a previous work, we demonstrated that orientation of the hand in the horizontal plane (azimuth) at the time of grasping depends on the direction of the reaching movement in the horizontal plane. Here we report three experiments to further investigate the generality of this coupling. Azimuth of the hand for grasping was studied while subjects were reaching for objects placed at various locations on a horizontal board. Hand movements were recorded with an electromagnetic sensor giving information about hand 3D position and orientation. As expected, hand azimuth for grasping was coupled with movement direction in the central part of the workspace (but reached a limit for rightmost reaching directions). The coupling did not depend on the direction of where the object had to be put after grasping. Various initial positions and azimuths of the hand were compared to the most comfortable initial hand posture. The coupling between hand azimuth and movement direction remained whatever the initial hand azimuth. This demonstrates that reaching movement direction is coupled with azimuth at the time of grasping and not with a rotational hand movement. The coupling between hand azimuth and movement direction subsisted when the initial upper trunk orientation was changed. Thus our results cannot be explained by an invariance of the coupling coded in hand-centered or shoulder-centered coordinates. They rather suggest that the movement is produced in a frame of reference associated with the environment.