Modulating Fitts's Law: the effect of disappearing allocentric information.

A modulation of Fitts's Law occurs when rapid pointing movements are made to targets within a placeholder array. Although Fitts's Law predicts increasing movement times with larger movement amplitudes, with the placeholder array the furthest target is reached faster than the second furthest. This study analyzed the timing of this effect by removing placeholders at either target onset or movement onset. Placeholders shown during movement planning were sufficient to cause the violation of Fitts's Law. Placeholders removed at the target onset did not produce the last position advantage, a result which was confirmed in an independent data set. Together, these results converge with recent proposals that pinpoint the contribution of allocentric information to visuo-motor control largely to the movement planning stage.