Social cognition in basal ganglia pathologies: Theory of Mind in Huntington's and Parkinson's diseases.

Theory of Mind (ToM) is the ability to predict the behaviour of others by inferring their cognitive and affective states. The literature suggests that different neural substrates within the basal ganglia are involved in the affective (ventral striatum) and cognitive (dorsal striatum) components of ToM. We investigated ToM dysfunction in two different basal ganglia pathologies, Huntington's disease (HD) and Parkinson's disease (PD), in their early stages. Indeed, a different progression of neurodegeneration from the dorsal striatum to the ventral striatum is described in the two diseases. We also investigated whether there is a correlation between ToM and executive function. Twenty-one patients with HD, 21 with PD, and 22 healthy subjects (HS) were recruited. All participants completed a ToM assessment using the Yoni task, which assesses both cognitive and affective components at two levels of meta-representational difficulty (i.e. first-order items only require inferring the mental state of a person, while second-order items also require inferring the mental states of a person about others). The clinical groups also underwent a full neuropsychological assessment. In HD patients, both cognitive and affective ToM were equally impaired, whereas in PD patients, impairment of the cognitive component predominated. Specifically, compared to HS, HD patients scored lower on both inferential levels and on both cognitive and affective components, whereas PD patients scored lower than HS only on second-order and cognitive items. In the clinical groups, there was an imbalance between the cognitive and affective components, with higher accuracy on affective items. Performance on the Yoni task did not correlate with tests assessing executive functions. We suggest that the different pattern of ToM alteration in HD and PD may be a result of differential involvement of the ventral and dorsal striatum and that ToM abilities in these clinical populations are not directly supported by executive functioning.