Normal motor adaptation in cervical dystonia: a fundamental cerebellar computation is intact

Abstract

The potential role of the cerebellum in the pathophysiology of dystonia has become a focus of recent research. However, direct evidence for a cerebellar contribution in humans with dystonia is difficult to obtain. We examined motor adaptation, a test of cerebellar function, in 20 subjects with primary cervical dystonia and an equal number of aged matched controls. Adaptation to both visuomotor (distorting visual feedback by 30°) and forcefield (applying a velocity-dependent force) conditions were tested. Our hypothesis was that cerebellar abnormalities observed in dystonia research would translate into deficits of cerebellar adaptation. We also examined the relationship between adaptation and dystonic head tremor as many primary tremor models implicate the cerebellothalamocortical network which is specifically tested by this motor paradigm. Rates of adaptation (learning) in cervical dystonia were identical to healthy controls in both visuomotor and forcefield tasks. Furthermore, the ability to adapt was not clearly related to clinical features of dystonic head tremor. We have shown that a key motor control function of the cerebellum is intact in the most common form of primary dystonia. These results have important implications for current anatomical models of the pathophysiology of dystonia. It is important to attempt to progress from general statements that implicate the cerebellum to a more specific evidence-based model. The role of the cerebellum in this enigmatic disease perhaps remains to be proven.

Fig. 1

a Overview of experimental design. Each epoch consisted of four trials. b Robotic apparatus and baseline task. Subjects were seated and held a manipulandum with their right hand (see text for full description). Upon appearance of the target they were trained to make a shooting movement through this as accurately as possible (the perfect path is indicated on the diagram by the dashed line). c Schematic drawing of the perturbation conditions. The visuomotor condition consisted of a distortion of visual feedback by 30° in the clockwise (positive) or anticlockwise (negative) direction. The forcefield condition consisted of a rightward (positive) or leftward (negative) velocity dependent force applied to the robotic arm during movement (3 N/(m/s))

Fig. 2

Group data for angular error across baseline, adaptation and washout conditions. All data sorted into the same order for figure (experimental design consisted of four possible order combinations.) Control data shown in red; cervical dystonia data shown in blue. The solid line indicates the mean and the shaded regions the standard error

Fig. 3

The performance of the model for an individual patient, and its ability to capture the rate of learning for each condition, is shown in the boxes either side of the bar chart. The central bar chart plots mean learning index for the two groups with the standard error of the mean indicated by the error bars. Red (left bar) control. Blue (right bar) cervical dystonia