Pointing Errors Reflect Biases in the Perception of the InitialHand Position

Vindras, Philippe, Michel Desmurget, Claude Prablanc, and Paolo Viviani. Pointing errors reflect biases in the perception of the initial hand position. J. Neurophysiol. 79: 3290–3294, 1998. By comparing the visuomotor performance of 10 adult, normal subjects in three tasks, we investigated whether errors in pointing movements reflect biased estimations of the hand starting position. In a manual pointing task with no visual feedback, subjects aimed at 48 targets spaced 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 total variability. The direction of the shift depended on the starting location. Systematic errors decreased dramatically in the second condition 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 estimate the location of their (unseen) right hand. The systematic...