Difficulty modifying a sustained motor response in prodromal Huntington's disease.

We investigated the nature of motor symptoms in the preclinical stage of Huntington's disease. Individuals with the CAG expanded repeat of Huntington's disease (prHD) and two control groups were tested on a task requiring a releasing movement (releasing a depressed button) followed by a ballistic movement (pressing a different button). Movement times were measured separately for releasing and ballistic movements. The mean reaction time of the prHD group was significantly longer when releasing a movement than that of the other groups. The groups, however, did not differ significantly on movement time for ballistic movements. Our results show that motor slowing is evident prior to the clinical diagnosis of Huntington's disease and may reflect difficulty in modifying a sustained motor program.