Perceptual factors contribute to akinesia in Parkinson's disease.

Parkinson's disease (PD) patients have longer reaction time (RT) than age-matched control subjects. During the last decades, conflicting results have been reported regarding the source of this deficit. Here, we addressed the possibility that experimental inconsistencies originated in the composite nature of RT responses. To investigate this idea, we examined the effect of PD on different processes that compose RT responses. Three variables were manipulated: the signal quality, the stimulus-response compatibility and the foreperiod duration. These variables have been shown to affect, respectively, the ability to extract the relevant features of the stimulus (perceptual stage), the intentional selection of the motor response (cognitive stage) and the implementation of the muscle command (motor stage). Sixteen PD patients were tested on and off-medication and compared with an age and gender-matched control group. Results indicated that degrading the legibility of the response stimulus affected the latency of simple key-press movements more dramatically in the off-medication PD group than in the control population. The stimulus-response compatibility and the foreperiod duration had similar effects in the two groups. Interestingly, the response slowing associated with the degradation of the stimulus was the same whether the patients were on or off dopaminergic medication. This suggests that the high-level perceptual deficits observed in the present study do not have a dopaminergic origin.