Defects in accuracy of reaching after removal of posterior parietal cortex in monkeys.

We measured the capacity of monkeys to project the arm to visually located targets under conditions differing in the nature of visual control allowed during reaching. This capacity was impaired by a unilateral posterior parietal lesion which increased the magnitude and variability of errors in projecting the contralateral arm to targets located on either side of the midline. Furthermore, accurate projections of the arm were often accompanied by misorientations of the fingers of the hand. These errors occurred whether reaching with or without visual guidance of the limb. The presence or absence of vision influenced the direction of errors in arm projection. Under conditions permitting a view of the limb and sight of the target, errors occurred in a direction toward the side of the lesion. When all visual cues were eliminated during reaching, errors were directed away from on-target toward the midline as they are in normal monkeys, but were greater in magnitude following the lesion.