POINTING ERRORS REFLECT BIASES IN THE PERCEPTION OF THE INITIAL HAND POSITION

By comparing the visuomotor performance of 10 adult, normal subjects i n three tasks, we investigated whether errors in pointing movements re flect biased estimations of the hand starting position. In a manual po inting task with no visual feedback, subjects aimed at 48 targets spac ed regularly around two starting positions. Nine subjects exhibited a similar pattern of systematic errors across targets, i.e., a parallel shift of the end points that accounted, on average, for 49% of the tot al variability. The direction of the shift depended on the starting lo cation. Systematic errors decreased dramatically in the second conditi on where subjects were allowed to see their hand before movement onset . The third task was to use a joystick held by the left hand to estima te the location of their (unseen) right hand. The systematic perceptua l errors in this condition were found to be highly correlated with the motor errors in the first condition. The results support the followin g conclusions. I) Kinesthetic estimation of hand position may be consi stently biased. Some of the mechanisms responsible for these biases ar e always active, irrespective of whether position is estimated overtly (e.g., with a matching paradigm), or covertly as part of the motor pl anning for aimed movements. 2) Pointing errors reflect to a significan t extent the erroneous estimation of initial hand position. This sugge sts that aimed hand movements are planned vectorially, i.e., in terms of distance and direction, rather than in terms of absolute position i n space.