Ten patients with Parkinson's disease performed a simple reaction time task in which, on hearing a tone, they pressed a button with the left thumb. In the first experiment tones sometimes occurred unannounced and at other times were preceded (by between 0 and 3200 ms) by a warning signal. The second experiment was identical to the first except that the subject had simultaneously to perform a simple continuous task with his right hand. Patients had slower reaction times than controls under all circumstances. In general, however, the effect of a warning signal and the effect of a second task were the same for both groups. In the control group the effect of a warning signal depended on whether or not a second task was being performed. Specifically, the advantage of a warning signal for reaction time was lost after long intervals (greater than 200 ms) when a second task was being performed. Parkinson's disease patients lost this advantage even when they were not performing a second task. Animal studies have suggested that dopamine deficiency results in an increase in neural "noise" in the basal ganglia. The behavioural consequences of this may be that Parkinson's disease patients always perform as if they were carrying out another task at the same time. In contrast, their ability to benefit from a warning signal and to allocate attentional resources are unimpaired.
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