Comparison of visually guided reaching in normal and deprived infant monkeys

A stump-tailed macaque, deprived of sight of its hands and body since shortly after birth, appears to be deficient in the accuracy and precision of its visually guided reaching. These deprived monkeys quickly develop adequate reaches if they are allowed to view their hands. To assess the deficit, we developed a method of open-loop testing (no sight of the reaching limb) for use on both an experimental group deprived of sight of limbs and a control group raised under identical restraints but allowed sight of limbs. The results show that the deprived monkeys can learn to reach a given visible target, but the learned reach is not as precise in the training condition or as precise to new target directions as it is for controls. Furthermore, there is little intermanual transfer of reaching skill for the experimentals but nearly 100% transfer for the controls. Finally, experimentals show a loss of precision in retention testing following a lack of practice, but controls do not. We conclude that the differences in visually guided reaching behavior of the two groups is evidence that the normal accuracy results from unconstrained vision of the hands which produces a mapping of the coordinates of motor response onto the space of vision.