Choosing the fastest movement: perceiving speed-accuracy tradeoffs.

Several studies have shown that humans exhibit an intimate knowledge of prospective motor actions when imagining and planning movements. To probe this knowledge, we used a 2-alternative forced-choice task to determine whether people are consistent with Fitts's law when choosing the movement they perceive to require the least movement time. We hypothesized that participants would choose the target with the lower index of difficulty with a probability greater than 0.5 in all situations. Participants performed almost perfectly when one of the targets was closer, wider, or both. Contrary to expectations, however, participants showed biases for close targets when one of the targets was closer and narrower. We argue that this pattern of behavior may result from a subjective representation of movement time that is based on both Fitts's law and the distance to the target, suggesting a preference for movements that are less effortful.