Reach adaptation to explicit vs. implicit target error.

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.