Dissociating visual and kinesthetic coordinates during pointing movements

Goal-directed movements imply that the visual coordinates in which the localisation of the goal is coded are transformed into proprioceptive coordinates in which the arm movement is coded. The two systems of coordinates are normally superimposed. Using a virtual reality device attached to the subject's head, we have created a situation where these systems were dissociated from each other. The virtual environment involved virtual visual targets and an image of the subject's hand reconstructed from the output of a data glove wore by the subject's right hand. When the subject's head was rotated, the visual targets and the image of the hand rotated by the same amount. Movements of the real hand were thus in conflict with those of the reconstructed hand, which appeared to err in the direction of head rotation. Pointing movements directed at five targets (0°, 26° and 52° on each side) were studied for five different head positions (0°, 45° and 80° to the right and to the left). The results showed a significant pointing bias towards head position, except for the left-most targets in the right head rotations. Constant errors in azimuth were proportional to the amount of head rotation. When the head was rotated to the right, constant errors in azimuth were greater during pointing towards right than left targets. Similarly, they were greater for left than for right stimuli when the head was rotated to the left. Errors in amplitude were not influenced by the direction nor the amount of head rotation. Finally, a decrease in the directional bias took place within blocks of trials. These results indicate that head position signals are used during the process of transforming motor coordinates from the visual to the pro-prioceptive system of reference.