Dopaminergic contributions to the visual categorization of natural scenes: evidence from Parkinson's disease

Recent evidence raised the possibility that dopaminergic mechanisms in the neostriatum play an important role in categorization. In this study, we examined the electrophysiological correlates of natural scene categorization in Parkinson's disease (PD), which is characterized by the depletion of dopamine in the neostriatum. Event-related potentials (ERPs) were recorded in PD patients and age-matched control subjects using a natural scene categorization task. Subjects had to decide whether a briefly presented image contained animals or non-animals. In the control group, the mean amplitudes of N1 (150-250 ms) and N2 components (400-600 ms) were more negative for non-animal scenes as compared with stimuli containing animals, whereas P2 (250-350 ms) was more positive for animals. In contrast, in the PD group the mean amplitudes of N1 and N2 components were similar for both animal and non-animal stimuli, and the P2 amplitudes were reduced. In the case of N1 peak latency, there was no between-group difference, whereas the N2 and P2 components were significantly delayed in the PD group. These results suggest that both perceptual (N1) and semantic (N2) processes related to the categorization of natural scenes are specifically impaired in PD. In the temporal domain, the slowed semantic processing is preceded by a relatively normally paced perceptual analysis. These findings are in agreement with the hypothesis emphasizing the importance of striatal dopaminergic mechanisms in classification functions.