Intermanual transfer of visuomotor adaptation depends on awareness

In previous studies, intermanual transfer after gradual and sudden sensorimotor adaptation were compared to find out whether transfer depends on awareness of the nature of the perturbation. The results of these experiments, however, were contradictory. Furthermore, the results of our own recent study suggest that awareness depends on perturbation size. Thus, we hypothesize that if intermanual transfer is indeed related to awareness, we should find it only after sudden adaptation to a large perturbation but not after sudden adaptation to a small perturbation or after gradual adaptation.

To confirm this idea, four groups (S30, G30, S75, G75) of subjects performed centre-out-and-back pointing movements with their right arm. In a baseline block, they received veridical visual feedback about their movement. In the subsequent adaptation block, feedback was rotated by 30° (S30, G30) or 75° (S75, G75). This rotation was either introduced suddenly (S30, S75) or gradually in steps of 3° (G30, G75). After the adaptation block an awareness test consisting of an exclusion and an inclusion condition was conducted. The experiment concluded with an intermanual transfer block, in which movements were performed with the left arm under rotated feedback, and a washout block again under veridical feedback. We determined adaptation, awareness, unawareness, transfer and washout indices from the subjects pointing directions.

Analysis of variance (ANOVA) of the adaptation indices revealed no significant difference between groups for the end of adaptation block. Further ANOVAs of the awareness, unawareness, transfer and washout indices each yielded significant effects of Group. Post hoc analyses revealed the same significant pattern for awareness and transfer index and for unawareness and washout index, respectively. While awareness and transfer was larger in S75 than in all other groups, unawareness and washout was smaller in G75 than in S30 and S75, and was even smaller in S75 than in G75. Our results suggest that intermanual transfer does depend on the awareness of the visuomotor distortion and, thus, on the explicit process of motor adaptation.