Basal ganglia and kinematics modulation: insights from Parkinson's and Huntington's diseases.

Movement kinematic variables related to force production can be modulated to respond appropriately to different contexts. We previously showed that in a choice-reaction time and a predictable timed-response task, normal subjects perform reaching movements to the same targets with two different kinematic patterns, a marker of flexibility. Here, we used the two tasks to determine whether basal ganglia are involved in the selection and modulation of movement kinematics and therefore in flexible force production. We tested seventeen patients in the early stages of Parkinson's disease, eleven pre-symptomatic Huntington's disease carriers and sixteen age-matched normal controls with the above-mentioned motor tasks. In both patient groups, the difference in kinematics (movement duration, peak velocity and acceleration) between the two tasks was significantly reduced compared to controls, indicating a limited range of choices or flexibility. However, this reduction was skewed in opposite directions in the two disorders, with force production being generally higher in Huntington's carriers and lower in Parkinson's patients compared to controls. We conclude that basal ganglia are involved in adapting movement to different contexts and selecting the appropriate movement force. The opposite trends in Parkinson's and Huntington's disease suggest that such regulation might depend on the balance between the outputs of direct and indirect pathways.