Verb specificity effects on semantic processing in Parkinson's disease.

Verb specificity is a sensorimotor semantic property reflecting the distinctiveness of motor patterns evoked by action verbs. While action-verb semantics are affected in Parkinson's disease (PD), the role of specificity is underexplored, particularly under different task demands. We investigated action-verb processing in PD, examining whether verb specificity and psycholinguistic properties (word length, frequency, polysemy) modulate performance under relatively shallow versus deeper semantic processing. Twenty PD patients and twenty-nine healthy controls (Brazilian Portuguese speakers) completed a lexical decision task (LDT) and semantic judgment task (SJT). Verbs were classified as specific (e.g., to sign, denoting a narrowly constrained motor pattern) or generic (e.g., to clean, applicable to a broader range of actions/objects). Generalized Linear Mixed Models showed that, in the LDT, PD patients showed lower accuracy than controls with a significant Group × Specificity interaction (β = .691, p = .012), indicating that specificity improved accuracy more strongly in the PD group. For the SJT, a Group × Specificity interaction (β = -.055, p = .010) revealed a stronger facilitatory effect of specificity on reaction times in the PD group. Simulation-based power analyses showed a sensitivity of .71 and .75 in the LDT and SJT interactions. The findings highlight the role of verb specificity in action-verb processing, dissociating effects across shallower (LDT) versus deeper semantic processing demands (SJT). In PD, this pattern may reflect altered motor representations or executive difficulties in inhibiting competing semantic activations for generic verbs.