Abstract
The adaptation of reaching movements has typically been investigated by either distorting visual feedback of the reaching limb or by distorting the forces acting upon the reaching limb. Here, we investigate reach adaptation when error is created by systematically perturbing the target of the reach rather than the limb itself (Magescas and Prablanc in J Cogn Neurosci 18: 75–83, 2006). Specifically, we investigate how adaptation is affected by (1) the timing of the perturbation with respect to the movement of the eye and the hand and (2) participant awareness of the perturbation. In Experiment 1, participants looked and pointed to a target that disappeared either at the onset of their eye movement or shortly after their eye movement and then reappeared, displaced to the right, at the completion of the reach. In Experiment 2, we made the target displacement more explicit by leaving the target at its initial location until the end of the reach, at which point it was displaced to the right. In Experiment 3, we extinguished the target at the onset of the eye movement but also informed participants about the presence and magnitude of the perturbation. In the no-feedback post-test phase, participants for whom the target disappeared during the reach demonstrated much stronger aftereffects of the perturbation, misreaching to the right, whereas participants for whom the target stayed on until reach completion demonstrated rapid extinction of rightward misreaching. Furthermore, participants who were informed about the target perturbation exhibited faster de-adaptation than those who were not. Our results suggest that adaptation to a target displacement is contingent on the explicitness of the target perturbation, whether this is achieved by manipulating stimulus timing or instruction.
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Notes
However, also see Bekkering et al. (1995). These authors show reach adaptation when pointing to an unseen target jump that occurs online, but they do not control for the effects of saccadic adaptation and they argue, in fact, that their results indicate transfer of saccadic adaptation to the hand. The Magescas et al. (2009) study, which deliberately prevented saccadic adaptation, can, therefore, be viewed as a purer test of reach adaptation to an unseen online target perturbation.
Violation of the assumption of homogeneity of variance (HOV) among groups on the pretest (Bartlett, P = .0075), along with unequal ns among the groups, dictates that a non-parametric test should be applied. When a Kruskal–Wallis test is applied to the groups, the result is not significant, H(3, N = 32) = 6.49, P = .09, However, if one overlooks violation of HOV and applies a one-way ANOVA instead, a significant difference does emerge, F(3,28) = 2.96, P = .049. Subsequent post-hoc testing with Newman-Keuls reveals that the SacEnd and Informed groups do differ significantly in the pretest, P = .04. All other pairwise contrasts are not significant (P > .10).
Based on the finding that peak velocity linearly increased as the perturbation size increased across acquisition trials (r = .86), F(1,48) = 133.62, P < .001.
This ‘bottom-up’ explanation does not preclude the possibility that top–down factors (such as instruction in the Informed group) may initially induce visibility of a target displacement. If visibility were being induced in this way, we would still consider the bottom-up explanation to account for the reduced adaptation.
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This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant awarded to R. Chua. We would also like to thank two anonymous reviewers for their helpful comments on the manuscript.
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Cameron, B.D., Franks, I.M., Timothy Inglis, J. et al. Reach adaptation to explicit vs. implicit target error. Exp Brain Res 203, 367–380 (2010). https://doi.org/10.1007/s00221-010-2239-x
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DOI: https://doi.org/10.1007/s00221-010-2239-x