Awareness of sensorimotor adaptation to visual rotations of different size

Previous studies on sensorimotor adaptation revealed no awareness
of the nature of the perturbation after adaptation to a 30° rotation of
visual feedback or after adaptation to gradually introduced
perturbations. However, assessment of awareness by means of
questionnaires, as done in those studies, can be problematic. Thus,
the present study used the process dissociation procedure, known
in the field of cognitive psychology, to measure awareness and
unawareness indices. We were interested in determining whether
degree of awareness depends on the magnitude of the visual
rotation. We further wished to determine if it can be manipulated by
providing the subjects with an explicit strategy before adaptation. To
this end, two groups of subjects (explicit and implicit) performed
alternating blocks of center-out reaching movements under null and
adaptation conditions. Adaptation was to 20°, 40° and 60° cursor
rotations in different adaptation blocks. The order of blocks was
randomized between subjects. After each adaptation block the
process dissociation procedure consisting of an inclusion and an
exclusion condition was conducted. During inclusion condition
subjects were asked to perform the task the same way as during the
adaptation block and during exclusion they were asked to refrain
from using what was learned and instead perform movements as
during the baseline block. We determined an awareness index by
subtraction of performance during the exclusion from performance
during the inclusion condition and an unawareness index from the
performance during the exclusion condition. Analysis of variance
(ANOVA) of the adaptation performance revealed no significant
differences between groups for the last third of each adaptation
block. A further ANOVA of the awareness index with the factor
Rotation Size (20°, 40°, 60°) yielded significant effects of Group and
of Rotation Size. Post hoc analysis showed a significant increase of
awareness from 20° to 60°, but no difference between 40° and either
20° or 60°. For the unaware index statistical analysis revealed a
significant effect of Rotation Size only. Contrary to the awareness
index, smaller rotations produced more unaware adaptation as
confirmed by post hoc analysis. Our results suggest that aware and
unaware states are differentially involved during adaptation to
different perturbation sizes and awareness can be manipulated by
instructions. Since it has been shown that gradual, i.e. unaware, and
sudden adaptation are based on different neural correlates, it is
possible that distinguished neural networks are differentially
involved in adaptation to varying perturbation sizes.