Task switching deficits associated with Parkinson's disease reflect depleted attentional resources

Using a Stroop task switching paradigm, Brown and Marsden [Brain 111 (1988) 323; Brain 114 (1991) 215] proposed that set shifting deficits in Parkinson's disease (PD) reflect limited attentional resources rather than deficits in internal control, as was previously supposed. In the present study, we tested this claim using a more recently developed Stroop task switching paradigm for which the internal control and attentional resources accounts made contrasting predictions. A PD group (N=30) was compared with an age-matched control group (N=34) on vocal response time (RT) for color naming and word reading in response to neutral and incongruent Stroop stimuli. Participants carried out four blocks of task repetition trials, and eight blocks of task switching trials. The results revealed that a deficit due to PD was absent for two conditions necessitating internal control, but was present in the condition which placed the highest demand on attentional resources. This selective deficit is congruent with Brown and Marsden's conclusions that depleted attentional resources, not an impairment in internal control per se, is the basis of the set shifting deficits associated with PD.