Reach adaptation to online target error

Magescas et al. (Exp Brain Res 193:337-350, 2009) recently suggested that online error, unlike terminal error, does not lead to reach adaptation. The present study re-examines adaptation to online target error, but uses a small target perturbation and eliminates online vision of the limb, factors that may affect adaptation. We compared 3 groups: terminal error, online error, and control. All groups completed a pretest, exposure, and posttest phase. Participants made look-and-point movements to a target, and we examined how repeated rightward target perturbations during the exposure phases of the experimental groups influenced reaches to a stationary target in the posttest. Exposure phases of each group contained an equal number of interleaved look-and-point and look-only trials, the latter of which were designed to inhibit build-up of saccadic adaptation in the online error group. On look-and-point trials the target either disappeared at saccade onset and then re-appeared 3.75 cm to the right when the hand landed (terminal error group), immediately jumped right by 3.75 cm at saccade onset and remained lit throughout the saccade and reach (online error group), or remained lit but stationary throughout the saccade and reach (control group). In all groups, vision of the limb was only provided at the start and end of the reach. Our results show that both the terminal error and the online error groups developed significant aftereffects. It appears, therefore, that online error can produce reach adaptation.