Choosing between alternative wrist postures: action planning needs perception

When normal subjects grasp with their right hand a rectangular object placed at different orientations in the horizontal plane, they change from a 'thumb left' (clockwise) to a 'thumb right' (anti-clockwise) grasp when the orientation exceeds about 110 degrees , with respect to the mid-sagittal plane. This suggests planning of the final grip orientation at, or before the start of the prehension movement. The current study assessed performance of two visual agnosic patients (SB and DF) on a grasping task requiring the planning of final grip posture. Five healthy subjects were also tested. Subjects were required to grasp a triangular-section block, which was presented at one of seven different orientations (80-140 degrees). The healthy subjects showed a consistent relation between object orientation and hand orientation just before contact. In addition, they consistently used a clockwise grasp when object orientation was less than 100 degrees , and an anti-clockwise grasp when it was more than 110 degrees, with a sharply defined switch-point being identifiable for each subject. For both visual agnosic patients, hand orientation was also reliably related to object orientation. However, the selection of grasp posture was markedly abnormal: they did not consistently switch between clockwise and anti-clockwise grasps within the normal orientation range, and the switch, when it did occur, was not at all sharply defined. These results suggest that the planning of hand orientation during a grasp depends on a perceptually based judgement of the awkwardness of alternative movements. This would presumably involve ventral stream processing, which is disrupted in the visual agnosic patients.