Submovements in visually-guided and memory-guided reaching tasks: changes in Parkinson's disease

Movements in people with Parkinson's disease are often hypometric, although we have found that this was not the case in an experimental visually-guided reaching task. We wished to explore our hypotheses that (1) people with Parkinson's disease produce hypometric primary submovements but (2) are able to use visual feedback to accurately reach the target in a single overall movement, and (3) this effect may be greater in memory-guided tasks in which an internal representation of the target location is used instead of a fixation-centered representation of the target. Visually- and memory-guided reaching movements were examined in 22 people with mild to moderate severity Parkinson's disease on medication, along with age-matched and sex-matched controls. Primary submovements were extracted from 5149 movements using a method based upon zero crossings of jerk (3rd derivative of position), with several additional criteria to minimize the detection of submovements due to noise or tremor. There was no difference in the end position of the overall reaching movement between the two groups, although the movement was smaller in the memory-guided task. In contrast, the gain of the primary submovement was smaller in the Parkinson's disease group than the control group, with this difference being greater on the memory-guided task. There was no task effect on the primary submovement gain in the control group. Our results show that the underlying primary submovement in visually-guided movements in people with Parkinson's disease is hypometric, and that the degree of hypometria is even greater in memory-guided movements.