Closed- and open-loop handwriting performance in patients with multiple sclerosis.

Normal subjects use an open-loop motor control strategy in handwriting, but they are able to switch to closed-loop motor control when the demands on accuracy increase. These closed-loop handwriting movements of normal subjects resemble the inefficient movements found in writing-impaired patients. The hypothesis that such movement deficits may in fact reflect the use of a closed-loop strategy was tested in a group of writing-impaired patients with multiple sclerosis (MS). The handwriting movements of 10 MS patients and 20 control subjects were examined with a digitizing tablet. Three conditions were used: a standard writing task (expt 1), a closed-loop condition (expt 2), and an open-loop condition (expt 3). Individual stroke movements were analysed. Stroke duration and segmentation were increased for MS patients in the standard writing task. The same was found for control subjects when they wrote under closed-loop conditions. However, under open-loop conditions, the handwriting movements of the MS patients were as fast and fluent as that of control subjects. The results support the hypothesis that the movement characteristics of the writing-impaired MS patients reflect an inadequate use of a closed-loop motor control strategy.