Why don't we move faster? Parkinson's disease, movement vigor, and implicit motivation

People generally select a similar speed for a given motor task, such as reaching for a cup. One well established determinant of movement time is the speed-accuracy trade-off: movement time increases with the accuracy requirement. A second possible determinant is the energetic cost of making a movement. Parkinson's disease (PD), a condition characterized by generalized movement slowing (bradykinesia), provides the opportunity to directly explore this second possibility. We compared reaching movements of patients with PD with those of control subjects in a speed-accuracy trade-off task comprising conditions of increasing difficulty. Subjects completed as many trials as necessary to make 20 movements within a required speed range (trials to criterion, N_c). Difficulty was reflected in endpoint accuracy and N_c. Patients were as accurate as control subjects in all conditions (i.e., PD did not affect the speed-accuracy trade-off). However, N_c was consistently higher in patients, indicating reluctance to move fast although accuracy was not compromised. Specifically, the dependence of N_c on movement energy cost (slope S_N) was steeper in patients than in control subjects. This difference in S_N suggests that bradykinesia represents an implicit decision not to move fast because of a shift in the cost/benefit ratio of the energy expenditure needed to move at normal speed. S_N was less steep, but statistically significant, in control subjects, which demonstrates a role for energetic cost in the normal control of movement speed. Wepropose that, analogous to the established role of dopamine in explicit reward-seeking behavior, the dopaminergic projection to the striatum provides a signal for implicit "motor motivation.".