Reaching in Light and Dark After Unilateral Posterior Parietal Ablations in the Monkey

Reaching with the ipsilateral and contralateral hand in the light and dark was studied in four rhesus monkeys with unilateral posterior parietal ablations. It was found that reaching in the light was only impaired in the contralateral hand, with gross and predominantly medial misreaching. In the dark, reaching with the contralateral hand was even more impaired than in the light. Reaching with the ipsilateral hand in the dark was also impaired but to a much lesser degree and not as consistently as with the contralateral hand. No significant tendency towards medial misreaching with the contralateral hand in the dark could be found, and a predominance of lateral errors was observed for the ipsilateral hand in the dark. The results indicate that the disorder of reaching found after unilateral posterior parietal ablations in monkeys can not be explained as a disturbance of visual orientation. The findings suggest the presence of a disorder of visuomotor coordination in addition to a non-specific spatial disturbance in the dark and a contralateral somatosensory defect.