Neural correlates of adaptation to gradual and to sudden visuomotor distortions in humans.

This study aimed at scrutinizing the neural correlates of sensorimotor adaptation. Subjects were exposed either to a gradually (group G) or to a suddenly introduced perturbation (group S) followed by a test of aftereffects. They were also exposed to a control condition equated for their movement errors during the adaptation condition. We registered subjects' brain activity by functional magnetic resonance imaging. Behavioral data revealed no difference between aftereffects in G and S, while imaging data suggest different neural correlates. Direct comparison between groups showed more adaptation-related activation in left cingulate and inferior frontal as well as right caudate and temporal areas in S than in G. In contrast, no neural activity was related more to G than to S and no common activations were found for both groups. Within-group analyses further revealed right inferior parietal lobe, cerebellar and cingulate cortex activity in group S and activation of frontal lobe and left cerebellum in group G for a contrast between adaptation condition and baseline. Less brain activity was observed when controlled for movement errors: the contrast between adaptation and control condition yielded left occipital lobe activity in group S, and left posterior dentate nucleus and brainstem activity in group G. The present data confirm an involvement of the cerebellar cortex in error processing during sudden adaptation, since this activation was found for the contrast 'adaptation-baseline' but not for 'adaptation-control.' In addition, our data suggest an involvement of deep cerebellar nuclei in the adaptation to gradually introduced distortions.