Poster 13: Huntington's Disease Disrupts Motor Control Mechanisms That Rely on Internal Models

Abstract

Background Previous studies demonstrated that Huntington's disease (HD) impairs the ability to perform to perform online adjustments during reaching movements. It is not clear whether the difficulty is in responding to errors in observed or predicted trajectory. Do patients have trouble observing deviations of the hand from its goal, or does the difficulty lie in predicting where the hand should be based on internal models of the effects of motor commands? Methods We compared corrections made to two visuomotor perturbations that were equivalent in sensory space, but differed in the nature of the resulting error (goal-related vs effector-related). Subjects made planar reaching movements to guide a screen cursor to a target. Interspersed with baseline trials were perturbation trials, in which either the target jumped to a new location (target jump) or the direction of the cursor was rotated relative to the starting point (rotation). The target jump (to a new direction, ±30 degrees away) and the rotation (±30 degrees) were spatially matched so as to require the same change in hand movement direction in order to acquire the target. The target jump required a change in movement goal (external error), whereas rotation introduced a mismatch between actual hand position and hand position predicted by subject's visuomotor map (internal model error). Results and Discussion The corrections made by patients to target jumps were only slightly abnormal, with increased variability and slight undershoot. Their corrections to visuomotor rotation trials, on the other hand, were considerably abnormal. There was a systematic delay in the onset of the correction, and the amplitude of the correction was incomplete. These findings establish a selective impairment in monitoring errors in computation of predicted hand position. A function of the circuits affected by HD may be to monitor internal motor control models that are necessary to guide movements to their targets.