Dual task performance and processing resources in normal subjects and patients with Parkinson's disease. In recent years, there has been a growing consensus among investigators that the presence or absence of external cues guiding behaviour and attention is an important factor in determining whether or not deficits are found in patients with Parkinson's disease (PD). In an earlier study, the authors suggested that the pattern of impaired and intact performance could be explained in terms of differential resource demands of the tasks, combined with depleted levels of central processing resources in PD patients. Two experiments are reported, both employing dual-task paradigms. The first assessed, in normal subjects, the relative processing demands of a cued and an uncued version of the Stroop task. The results supported the proposal that the noncued task made greater demands on the subject's limited processing resources. Further, performing a resource demanding secondary task concurrently with the Stroop test produced, in normal subjects, the same pattern of impaired performance as that reported previously in PD patients. In the second experiment the same dual-task paradigm was employed with a group of PD patients and normal aged-matched controls. Only the patients showed an increase in reaction time on the Stroop task when performing a resource demanding secondary task. The patients also showed an interfering effect with concurrent foot tapping but not with an articulatory suppression task. The results were taken to support the hypothesis that PD patients have depleted central processing resources. In considering the present data, alternative explanations for the results are considered, in particular the possibility that they represent a deficit in switching processing resources between two tasks as the combined demands outweigh available resources. 