Persistence in visual feedback control by the elderly.
Young and elderly subjects performed aiming movements to a visual target with a manipulandum to determine whether the elderly reduce their reliance on visual feedback after extended practice. Reliance on visual feedback was assessed by performance on trials in which the cursor displaying arm movement was unpredictably extinguished. Movements were divided into two subcomponents: a primary, ballistic submovement and a secondary, corrective submovement. For both age groups, removal of visual feedback prior to practice resulted in a decrease in the distance covered in the primary submovement, an increase in the distance of the secondary submovement, and a decrease in endpoint accuracy. After extensive practice with the cursor present, the proportion of distance traveled with the primary submovement was again assessed under trial conditions in which the cursor randomly disappeared. Following practice, the young demonstrated that they were capable of extending the primary submovement distance closer to the target. In addition, primary submovement distance was unaffected by the removal of vision following practice. After practice the elderly did not show evidence of lengthening the primary submovement, and submovement distance and endpoint accuracy continued to be altered by the removal of vision. This suggests that, unlike the young, the elderly do not benefit from practice so that they can place a greater proportion of the movement under program control. Thus, on a relative basis, a greater proportion of their overall movement requires corrective adjustments.