Syntax, action verbs, action semantics, and object semantics in Parkinson's disease: Dissociability, progression, and executive influences

Several studies have recently shown that basal ganglia (BG) deterioration leads to distinctive impairments in the domains of syntax, action verbs, and action semantics. In particular, such disruptions have been repeatedly observed in Parkinson's disease (PD) patients. However, it remains unclear whether these deficits are language-specific and whether they are equally dissociable from other reported disturbances -viz., processing of object semantics. To address these issues, we administered linguistic, semantic, and executive function (EFs) tasks to two groups of non-demented PD patients, with and without mild cognitive impairment (PD-MCI and PD-nMCI, respectively). We compared these two groups with each other and with matched samples of healthy controls. Our results showed that PD patients exhibited linguistic and semantic deficits even in the absence of MCI. However, not all domains were equally related to EFs and MCI across samples. Whereas EFs predicted disturbances of syntax and object semantics in both PD-nMCI and PD-MCI, they had no impact on action-verb and action-semantic impairments in either group. Critically, patients showed disruptions of action-verb production and action semantics in the absence of MCI and without any executive influence, suggesting a sui generis deficit present since early stages of the disease. These findings indicate that varied language domains are differentially related to the BG, contradicting popular approaches to neurolinguistics.