Abstract
The processes underlying short-term plasticity induced by visuomotor adaptation to a shifted visual field are still debated. Two main sources of error can induce motor adaptation: reaching feedback errors, which correspond to visually perceived discrepancies between hand and target positions, and errors between predicted and actual visual reafferences of the moving hand. These two sources of error are closely intertwined and difficult to disentangle, as both the target and the reaching limb are simultaneously visible. Accordingly, the goal of the present study was to clarify the relative contributions of these two types of errors during a pointing task under prism-displaced vision. In “terminal feedback error” condition, viewing of their hand by subjects was allowed only at movement end, simultaneously with viewing of the target. In “movement prediction error” condition, viewing of the hand was limited to movement duration, in the absence of any visual target, and error signals arose solely from comparisons between predicted and actual reafferences of the hand. In order to prevent intentional corrections of errors, a subthreshold, progressive stepwise increase in prism deviation was used, so that subjects remained unaware of the visual deviation applied in both conditions. An adaptive aftereffect was observed in the “terminal feedback error” condition only. As far as subjects remained unaware of the optical deviation and self-assigned pointing errors, prediction error alone was insufficient to induce adaptation. These results indicate a critical role of hand-to-target feedback error signals in visuomotor adaptation; consistent with recent neurophysiological findings, they suggest that a combination of feedback and prediction error signals is necessary for eliciting aftereffects. They also suggest that feedback error updates the prediction of reafferences when a visual perturbation is introduced gradually and cognitive factors are eliminated or strongly attenuated.
Highlights
That sensorimotor adaptation can be induced by wearing prisms, which shift the visual field laterally, has been known since at least von Helmholtz (1910)
These results indicate a critical role of hand-to-target feedback error signals in visuomotor adaptation; consistent with recent neurophysiological findings, they suggest that a combination of feedback and prediction error signals is necessary for eliciting aftereffects
This study investigated adaptation induced by prism exposure in two conditions: a “terminal feedback error” condition and a “movement prediction error” condition
Summary
That sensorimotor adaptation can be induced by wearing prisms, which shift the visual field laterally, has been known since at least von Helmholtz (1910). The subject unexpectedly experiences a pointing error in the opposite direction to that induced by the prisms. This negative aftereffect persists after a few trials. This simple experiment provides one of the simplest illustrations of the short-term plasticity of the central nervous system (CNS), which allows it to adapt to changes in the relationships between visual inputs and corresponding motor outputs (for a review, see Redding et al, 2005)
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