Visuomotor dependency on an initial fixation target involved in the disorder of visually-guided manual movement in Parkinson's disease.

Role of a central fixation target on the latencies of visually guided manual movement was analyzed on young healthy subjects, age-matched control subjects and patients with Parkinson's disease (Hoehn and Yahr stages II, III, and IV). Two paradigms were used: overlap paradigm where a central fixation target was lighted throughout the test, and gap paradigm where a central fixation target was turned off 200 ms before a peripheral target was lighted. The subject was first asked to fixate the central target then instructed to locate a peripheral target with a laser beam spot, operated with wrist flexion or extension as quickly as possible. Latencies of gap paradigm are always shorter than those of overlap task in all the groups. Latencies of both overlap and gap tasks prolonged from young to elder, from elder to PD II, from PD II to PD III and from PD III to PD IV. Also latencies were extremely prolonged in the overlap tasks and correlated with disease severity. Latencies in the gap tasks were less prolonged as compared with those in the overlap tasks. The visual fixation target prolonged the visuo-motor latency in association with severity of Parkinson's disease.