Sensory timing cues improve bradykinesia of grasping movements in Parkinson's disease: a comparison to the effects of STN stimulation

Here we study the effects of subthalamic nucleus stimulation and external timing cues on the bradykinesia of finger force development and arm movements when grasping to lift an instrumented object in Parkinson's disease. Six Parkinsonian subjects with chronic stimulation of the subthalamic nucleus and six sex- and age-matched healthy controls grasped, lifted and held an object between the tips of the thumb and index finger. The grip-lift task was either performed at self-determined speed or in response to an auditory cueing signal. Parkinsonian subjects performed the task with subthalamic nucleus stimulation switched on and off. We found that in Parkinson's disease stimulation of the subthalamic nucleus and the presentation of auditory timing cues improved bradykinesia of both the grasp and lift components of the task. The finding that auditory timing signals improved bradykinesia in the absence of subthalamic nucleus stimulation suggests that the basal ganglia are less involved in the control of movements made in response to environmental cues. However, subthalamic nucleus stimulation caused Parkinsonian subjects also to apply excessive grip forces, regardless of whether the movement was made under self-determined or externally guided speed conditions. This observation shows that although subthalamic nucleus stimulation increases the speed and quantity of voluntary movement, it also hampers the efficiency of grip force scaling.