Decomposition of warning effects in Parkinson's disease

Neutral warning signals speed voluntary reactions by reducing temporal uncertainty and by triggering a brief burst of arousal. We attempted to isolate the phasic arousal mechanism in people with Parkinson's disease (PD) using a clock display to minimize temporal uncertainty. In this condition, the speeding of responses in a color-discrimination task by an accessory stimulus was the fully equivalent to the effect in age-matched control subjects. This indicates preserved phasic arousal in PD. Temporal preparation based on warning cues also appeared to be normal. By contrast, in a condition with high temporal uncertainty, the accessory stimulus (an air puff to the foot) impaired accuracy for the patients but not the neurologically normal subjects. The data are consistent with the view that PD disrupts internal but not external control of alertness.