Hand orientation for grasping depends on the direction of the reaching movement

The 3D orientation of the hand for grasping was studied while subjects reac hed for objects placed at several locations on a horizontal board, with mov ements starting from three initial hand positions. The hand movements were recorded with electromagnetic sensors giving 3D position and orientation in formation. The study focused on the azimuth, which is the projection of the hand orientation in a horizontal plane. The hand azimuth for grasping was linearly correlated with the direction of the reaching movement and not wit h the object direction in head- or shoulder-centered coordinates. This rela tionship was valid regardless of the initial hand position. A control exper iment with constant movement direction showed a weaker, probably postural, effect of object direction in shoulder-centered coordinates. We suggest tha t hand orientation for grasping is mainly controlled in relation to the rea ching movement direction. (C) 2000 Elsevier Science B.V. All rights reserve d.