Evidence for slowing as a function of index of difficulty in young adults with Down syndrome

Speed-accuracy trade-offs in persons with Down syndrome and typically developing controls were tested with a Fitts' task. Movement time scaled linearly with index of difficulty in both groups, and there were no accuracy differences. Persons with Down syndrome were slower than typically developing individuals. Regression analysis on movement time and index of difficulty showed a nearly two-fold higher regression coefficient and a nearly three-fold larger intercept value in the Down syndrome group. The dwell time on a target was much longer for Down syndrome persons but scaled with index of difficulty in about the same percentage for participants in both groups. Because of differences primarily related to scaling, we conclude that mechanisms of motor control are similar in Down syndrome and typically developing groups.