Abstract
If an object toward which you are moving your hand is suddenly displaced, you will make fast, unconscious corrections to your hand's movement. When reaching out to touch an object you will even do so if you are unable to see your moving hand or to detect the displacement (Goodale, Pélisson & Prablanc, Nature 320, 748–750, 1986.), suggesting that visual information about the intended endpoint of the movement is sufficient. Sometimes, however, visual information about the endpoint cannot be sufficient. For instance, people also make fast corrections when moving a cursor to a target with a computer mouse (Brenner & Smeets, Perception 30S, 97, 2001). They respond just as readily when the cursor is displaced as when the target is displaced, suggesting that they are relying on relative positions. To test this suggestion we placed subjects in front of a computer screen and asked them to quickly bring a cursor to a target by moving a mouse. Once they reached the target it disappeared and a new one appeared elsewhere. On some trials the target, the cursor, or both were displaced when the cursor was half way to the target. When the target and the cursor jumped in the same direction subjects reacted much less vigorously than if either jumped on its own, as would be expected for a reaction based on relative positions. On other trials, however, the cursor subtly changed its direction of motion with respect to the direction in which the mouse was moving. Although relative positions change very gradually when the cursor changes direction, subjects reacted strongly to such a perturbation. Moreover they reacted far less distinctly to a change in target position if the cursor changed direction at the same time (so that the cursor would reach the target if the subject continued moving as usual). We conclude that the fast corrections in the cursor task are based on visual information about the direction of cursor movement as well as the position of the target.