Unspeakable motion: Selective action-verb impairments in Parkinson’s disease patients without mild cognitive impairment

•We examined processing of action verbs and nouns in Parkinson’s disease (PD).•We assessed patients with and without mild cognitive impairment (PD-MCI and PD-nMCI).•In PD-MCI, deficits emerged for both action verbs and manipulable nouns.•In PD-nMCI, only action verbs with high motion content were disrupted.•Action-verb deficits may represent an early, sui generis alteration in PD. Parkinson’s disease (PD) patients show marked impairments in processing action verbs, and to a lesser extent, concrete (specially, manipulable) nouns. However, it is still unclear to what extent deficits in each of these categories are influenced by more general cognitive dysfunctions, and whether they are modulated by the words’ implied motility. To examine these issues, we evaluated 49 non-demented PD patients and 49 healthy volunteers in an oral production task. The patients were divided into two groups depending on the presence or absence of mild cognitive impairment (PD-MCI and PD-nMCI, respectively). Participants named pictures of actions varying in motion content (low and high) and of objects varying in manipulability (low and high). The PD-MCI group showed deficits across all four categories. However, PD-nMCI patients exhibited a selective difficulty for high-motion action verbs. This finding corroborates and refines previous results suggesting that disturbances of action-related lexico-semantic information in PD constitute a sui generis alteration manifested early in the course of the disease’s physiopathology. Moreover, it suggests that the grounding of action verbs on motor circuits could depend on fine-grained intracategorical semantic distinctions.