Verb Inflection Performance in Parkinson’s Disease: Evidence from Error and Response Time Analyses

Abstract

As one of the core language abilities, inflection is essential for efficient communication. Theories explaining the cognitive underpinnings of verb inflection include three approaches: single-route, dual-route, and differential common encoding. The dual-route account, but none of the other approaches, assumes regular inflection to rely on rule application supported by basal ganglia circuits. Parkinson’s disease (PD), caused by degeneration of dopaminergic basal ganglia neurons, is known to affect not only motor control but also cognitive functions, including language. However, the interplay between basal ganglia circuits and inflection remains a subject of discussion. The present study examined verb inflection in patients with PD and neurologically healthy adults to reveal the effect of basal ganglia dysfunction on inflectional abilities. We tested 67 native Dutch speakers (39 individuals with idiopathic PD and 28 age-matched controls) on the ability to produce past tense verb forms from infinitives. In addition to accuracy, investigated in previous studies, we analysed response time (RT) via Bayesian estimation to keep control over a speed-accuracy trade-off. We also explored error types. Overall, our study did not reveal disease-related deficits in verb inflection performance in the PD group. Patients with PD experienced more difficulties with irregular verbs than regular ones and were prone to overregularization, but so did matched controls. The observed errors do not support the association between rule application deficits and PD predicted by the dual-route model. Observed differences in accuracy and RT between regulars and irregulars are consistent with the differential common encoding rather than the single-route account.