Misreaching in monkeys after combined unilateral occipital lobectomy and splenial transection.

Studies of the effects of lesions of the parietal cortex suggest that this region plays a critical role in visuomotor guidance, whereas experiments on the effects of disconnecting this region from visual input have been inconsistent in supporting this hypothesis. In the present experiment, monkeys with a unilateral occipital lobectomy and splenial transection initially showed marked misreaching by the hand contralateral to the lobectomy when tested informally with peanuts presented by hand. Although this severe misreaching recovered within a few weeks, there was a persistent deficit in the contralateral hand on two, more difficult, reaching tasks. One task allowed full vision of the reaching hand; the other limited vision by requiring the monkeys to reach under an opaque shelf. Misreaching persisted on these tasks for at least 200 days after surgery. Monkeys with only an occipital lobectomy did not show contralateral-hand misreaching on informal testing or on the limited-vision task, and only very minor misreaching in two of three animals on the full-vision task. Results indicate that geniculostriate visual information must reach posterior cortical regions for visually guided reaching by the contralateral limb to be fully accurate.