Irrelevant peripheral visual stimuli impair manual reaction times in Parkinson's disease

In three experiments, the effects of irrelevant visual information on the time to initiate and to complete a simple movement of the hand in response to a visual signal were studied in patients with a diagnosis of Parkinson's disease, and in age-matched normal controls. Subjects gazed at the centre of a TV monitor and were instructed to move their preferred hand from one metal plate to another as soon as a blue disc appeared in the centre of the screen. This control condition was compared with other conditions in which the surrounding area of the screen was simultaneously filled with fields of irrelevant discs, which in Experiment 1 were either stationary, or streamed out from or in towards the centre of the screen. Reaction times, but not movement times, were significantly slowed in the patients (but not the controls) by the irrelevant disc fields. When the irrelevant dots were continuously present (between as well as within trials--Experiment 2), they had no effect on RT. When they were present between trials, but turned off as the movement signal was turned on, RT was again slowed in patients. The results are discussed in relation to the akinesia ("freezing") experienced by some patients in confined spaces (such as doorways), and to possible abnormalities of visual cortical and striato-nigro-collicular activity.