Hwang EJ, Smith MA, and Shadmehr R (2006) Dissociable effects of
the implicit and explicit memory systems on learning control of reaching, Experimental
Brain Research, 173:425-437.
Abstract: Adaptive control of reaching
depends on internal models that associate states in which the limb experienced
a force perturbation with motor commands that can compensate for it. Limb state
can be sensed via both vision and proprioception. However, adaptation of
reaching in novel dynamics results in generalization in the intrinsic
coordinates of the limb, suggesting that the proprioceptive states in which the
limb was perturbed dominate representation of limb state. To test this
hypothesis, we considered a task where position of the hand during a reach was
correlated with patterns of force perturbation. This correlation could be
sensed via vision, proprioception, or both. As predicted, when the correlations
could be sensed only via proprioception, learning was significantly better as
compared to when the correlations could only be sensed through vision. We found
that learning with visual correlations resulted in subjects who could verbally
describe the patterns of perturbations but this awareness was never observed in
subjects who learned the task with only proprioceptive correlations. We
manipulated the relative values of the visual and proprioceptive parameters and
found that the probability of becoming aware strongly depended on the
correlations that subjects could visually observe. In all conditions, aware
subjects demonstrated a small but significant advantage in their ability to
adapt their motor commands. Proprioceptive correlations produced an internal
model that strongly influenced reaching performance yet did not lead to
awareness. Visual correlations strongly increased the probability of becoming
aware, yet had a much smaller but still significant effect on reaching
performance. Therefore, practice resulted in acquisition of both implicit and
explicit internal models.
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