Impaired motor recycling during action selection in Parkinson's disease

Abstract

Behavioral studies have shown that the human motor system recycles motor parameters of previous actions, such as movement amplitude, when programming new actions. Shifting motor plans towards a new action forms a particularly severe problem for patients with Parkinson's disease, a disorder that, in its early stage, is dominated by basal ganglia dysfunction. Here, we test whether this action selection deficit in Parkinson patients arises from an impaired ability to recycle motor parameters shared across subsequent actions. Parkinson patients off dopaminergic medication (n=16) and matched healthy controls (n=16) performed a task that involved moving a handheld dowel over an obstacle in the context of a sequence of aiming movements. Consistent with previous research, healthy participants continued making unnecessarily large hand movements after clearing the obstacle (defined as "hand path priming effect"), even after switching movements between hands. In contrast, Parkinson patients showed a reduced hand path priming effect, i.e. they performed biomechanically more efficient movements than controls, but only when switching movements between hands. This effect correlated with disease severity, such that patients with more severe motor symptoms had a smaller hand path priming effect. We propose that the basal ganglia mediate recycling of movement parameters across subsequent actions. Significance statement The human motor system recycles motor parameters of previous actions when programming new actions, promoting efficient motor behavior. Here, we investigated the contribution of the basal ganglia to this transfer of motor parameters over subsequent actions. We assessed motor recycling by analyzing kinematic movement parameters during sequential hand movements that involved either a switch or no switch between hands. Compared to matched controls, Parkinson patients were impaired in transferring previously used motor parameters to new actions, but only when switching actions between hands. This suggest that the basal ganglia are important for motor recycling, and that the impaired ability of Parkinson patients to perform this computation may result in motor slowing.