Theory of mind deficits in Parkinson's disease: a product of executive dysfunction?

OBJECTIVE Patients with Parkinson's disease (PD) can perform poorly on tasks involving theory of mind (ToM): the ability to reason about mental states. We investigated whether patients' ToM deficits were independent of executive dysfunction. METHOD Experiment 1 aimed to establish that ToM deficits were present, and 2 following experiments manipulated the working memory (WM) demands of the ToM task. RESULTS In Experiment 1, 15 patients with PD performed significantly more poorly than controls on a false belief vignette task but not on a faux pas task. Errors were related to poor verbal fluency. In Experiment 2, 24 patients with PD made fewer errors on shorter false belief vignettes than the original FBT, and errors on the latter were related to WM impairment. In Experiment 3, the FBT was presented as a comic strip visible throughout questioning, reducing WM demands. Patients (n = 24) made memory errors but no false belief errors on the comic strip. They exhibited no verbal fluency or WM impairments, but did exhibit deficits on a black-and-white Stroop task. False belief errors were not correlated with executive performance. CONCLUSIONS PD patients made very few ToM errors that were independent of errors on memory questions, so in this sample, ToM deficits per se appear unlikely. However, patients still made errors on ToM tasks when associated incidental WM demands were considerably reduced, highlighting the need for future investigations of ToM in PD to account for the role of more general cognitive restrictions exhibited by even some medicated, early stage patients.