EFFECT OF BODY ORIENTATION WITH RESPECT TO GRAVITY ON DIRECTIONAL ACCURACY OF HUMAN POINTING MOVEMENTS

Errors in pointing to remembered target locations were measured in nor mal subjects in the upright posture and in a supine or prone lying pos ition, three conditions that differed in the orientation of the moving arm with respect to gravity. Vertical errors, or, more strictly, erro rs along the body axis, were significantly larger in both supine and p rone lying positions as compared with the vertical posture. Subjects' orientation with respect to gravity produced a consistent error patter n, i.e. subjects pointed below the remembered target locations in the vertical posture and above them under the two lying conditions, in the body-related co-ordinates. On the contrary, variable error (the confi dence ellipse) was differentially affected by the orientation of the m ovement trajectory with respect to gravity. In the supine body positio n the variable error was larger, and in the prone position it was smal ler than in the upright posture. The finding of gravity-related bias i n pointing errors suggests that the motor programme generated in the l ying position is not modified to adapt to an altered orientation with respect to gravity.