Introduction: Previous studies have demonstrated that non-demented Parkinson's disease (PD) patients have particular difficulty in producing verbs in a word generation situation. Two alternative explanations for this deficit have been proposed: the "motor" theory (verbs as action representawcttions) and the "grammatical" theory (verbs as lexical category). The aim of this study was to investigate the "motor" theory by exploring the neuronal substrates of action-related word production.
Methods: Event-related functional magnetic resonance with a 3T magnetic resonance imaging (MRI) was used to explore brain activity during two overt oral language tasks, i.e., object naming (ON) and generation of action-verbs (GenA), in 14 non-demented PD patients using a common set of object drawings. The objects used were either man-made objects (MMO) or manipulable biological objects (MBO). Stimuli and tasks were randomized across subjects. Neuroimaging data were analyzed using SPM2.
Results: Reaction times in GenA(MBO) were significantly longer than in the other three conditions (ON(MBO), ON(MMO), GenA(MMO)). The distribution of brain activities associated with each condition resembled that reported in previous studies on healthy subjects using similar tasks; a comparison between GenA and ON revealed slight differences located above all in the premotor and prefrontal cortices, particularly as regards GeneA(MBO)>ON(MMO). Furthermore, we found positive correlations between the severity of the motor deficit (as assessed by the Unified Parkinson's Disease Rating Scale - UPDRS score) and brain activity, particularly during GenA(MBO), in the pre- and post-central gyri bilaterally, left frontal operculum, left supplementary motor area (SMA) and right superior temporal cortex.
Conclusions: The direct comparison in our study of brain activity during the production of action-words and of object names did not reveal any major differences. However, our results point to a relationship between motor system dysfunction in PD and the extent of activation in verb generation, a task which implies in-depth processing of semantic representation of actions.