Distal and proximal prehension is differentially affected by Parkinson's disease. The effect of conscious and subconscious load cues.

Prehension movements consist of distal (grasp) and proximal (reach, lift) components. The proximal lifting movements (achieved at the wrist) of patients with Parkinson's disease (PD) are characterized by bradykinesia. With respect to the distal component, PD patients show pathologically high grip forces (generated by the fingers) and slowing of force development indicative of disturbed sensorimotor adjustments during prehension. Combining kinematic and force analyses of prehension movements, we investigated whether PD differentially affects the adjustments of the distal or proximal prehension components to current load conditions. First, PD patients (n = 12) and healthy, age-matched control subjects grasped and lifted light and heavy objects without any load cues. Then, they were presented with cues that indicated changes in object load. These load cues were either consciously perceived or rendered subconscious through use of the technique of metacontrast masking. Consistent with the functional organization of the basal ganglia, patients with PD could adapt distal prehension components (grip force) to current load conditions using both types of cues. However, they were impaired in adjusting proximal prehension components (lift velocity). While controls were able to normalize lift velocity with the help of both conscious and subconscious load cues, the PD patients could use neither form of cue, and retained a pathological overshoot in lift velocity. Our results demonstrate that visuomotor integration during prehension movements differs at distal and more proximal joints and that deficits in this integration are pronounced for the latter in Parkinson's disease.