Adaptation to a direction-dependent visuomotor gain in the young and elderly.

Consistent with the widely accepted notion of separate specification of movement amplitude and direction, it has been argued that there is also a categorical difference between adaptation to novel visuomotor rotations and to novel visuomotor gains. In line with this view, ageing seems to affect rotation and gain adaptation differently in that age-related impairments are consistently found for the former, but not for the latter. In this study we ask whether the contrasting findings could also be ascribed to differences in the level of difficulty of gain and rotation adaptation tasks, respectively. In order to increase the difficulty of gain adaptation, younger and older participants had to adapt to a direction-dependent gain transformation. Results revealed direction-dependent adaptation in both groups. More importantly, we replicated the typical findings of age-related impairments of adaptation, but not of aftereffects, that were previously only reported for rotation adaptation. Younger participants also showed superior explicit knowledge regarding the novel visuomotor mapping as compared to the older participants. We show that this knowledge was used by younger participants to selectively augment adaptive shifts. Finally, our findings suggest that the difficulty of the novel visuomotor transformation and, related to this, the involvement of explicit knowledge in adaptation is critical for age-related changes to show up, but not the type of adaptation task, rotation and gain adaptation, respectively.