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Parkinson's disease (PD) patients exhibit cognitive deficits, including reinforcement learning, working memory (WM) and set shifting. Computational models of the basal ganglia–frontal system posit similar mechanisms for these deficits in terms of reduced dynamic range of striatal dopamine (DA) signals in both medicated and non-medicated states. Here, we report results from the first study that tests PD patients on and off dopaminergic medications in a modified version of the AX continuous performance (AX-CPT) working memory task, consisting of three performance phases and one phase requiring WM associations to be learned via reinforcement feedback. Patients generally showed impairments relative to controls. Medicated patients showed deficits specifically when having to ignore distracting stimuli during the delay. Our models suggest that this impairment is due to medication causing excessive WM …