Explicit knowledge and real-time action control: anticipating a change does not make us respond more quickly.

When the target of a goal-directed reach changes location, people normally respond rapidly and automatically to the target shift. Here, we investigate whether explicit knowledge about a moving target (knowing whether a location change is likely/unlikely) improves responsiveness, with the goal of understanding top-down effects on real-time reaching. In Experiment 1, we presented participants with pre-cues that indicated a 20 or 80% likelihood of a target perturbation on that trial. When participants made pro-pointing responses to the target perturbations, their online response occurred later for 20% trials than for 80% trials, but this effect may have been due to suppression of the online response on 20% trials, rather than enhancement of the response on 80% trials. In Experiment 2, we presented participants with 50 and 100% likelihood pre-cues, and observed no shortening of the latency on 100% trials compared to 50% trials, which suggests that expectation does not enhance the automatic response to a perturbation. However, we did observe more vigorous responses to the perturbation on the 100% trials, and this contributed to shorter movement times relative to the 50% trials. We also examined, in Experiment 2, whether prior knowledge about the direction of the target perturbation would shorten the latency of the online response, but we did not observe any reduction in latency. In sum, the onset of the automatic response appears to be suppressible, but not augmentable by top-down input. The possibility that the forcefulness of the automatic response is modifiable by expectation is examined, but not resolved.