Parietal cortex and movement. II. Spatial representation.

Lesions in the two divisions of parietal cortex, 5/7b/MIP and 7a/LIP, produce dissociable reaching deficits. Monkeys with 5/7b/MIP removals were tested on reaching in the dark under two different conditions. All the reaches made on any day were from the same starting position to the same target position in the control condition. In the "transfer" condition, all the reaches were made to the same target position but consecutive reaches were made from different starting positions. The target could be represented as a constant pattern of joint and muscle positions in the control condition. The transfer condition required a representation of the starting position of the hand and/or a representation of the target in terms of its position in space. Removal of areas 5, 7b and MIP produced only a very mild impairment in the control condition and a severe impairment in the transfer condition. This suggests that 5/7b/MIP does not represent the limb in simple sensory or motor coordinates but in terms of its spatial position.