FRAMES OF REFERENCE FOR HAND ORIENTATION

In reaching and grasping movements, information about object location and object orientation is used to specify the appropriate proximal arm posture and the appropriate positions for the wrist and fingers. Sinc e object orientation is ideally defined in a frame of reference fixed in space, this study tested whether the neural control of hand orienta tion is also best described as being in this spatial reference frame. With the proximal arm in various postures, human subjects used a hand- herd rod to approximate verbally defined spatial orientations. Subject s did quite well at indicating spatial vertical and spatial horizontal but made consistent errors in estimating 45 degrees spatial slants. T he errors were related to the proximal arm posture in a way that indic ated that oblique hand orientations may be specified as a compromise b etween a reference frame fixed in space and a reference frame fixed to the arm. In another experiment, where subjects were explicitly reques ted to use a reference frame fixed to the arm, the performance was con sistently biased toward a spatial reference frame. The results suggest that reaching and grasping movements map be implemented as an amalgam of two frames of reference, both neurally and behaviorally.