RESEARCH ARTICLE Hedging Your Bets: Intermediate Movements as Optimal Behavior in the Context of an Incomplete Decision

Existing theories of movement planning suggest that it takes time to select and prepare the actions required to achieve a given goal. These theories often appeal to circumstances where planning apparently goes awry. For instance, if reaction times are forced to be very low, movement trajectories are often directed between two potential targets. These interme-diate movements are generally interpreted as errors of movement planning, arising either from planning being incomplete or from parallel movement plans interfering with one anoth-er. Here we present an alternative view: that intermediate movements reflect uncertainty about movement goals. We show how intermediate movements are predicted by an optimal feedback control model that incorporates an ongoing decision about movement goals. According to this view, intermediate movements reflect an exploitation of compatibility be-tween goals. Consequently, reducing the compatibility between goals should reduce the incidence of intermediate movements. In human subjects, we varied the compatibility be-tween potential movement goals in two distinct ways: by varying the spatial separation be-tween targets and by introducing a virtual barrier constraining trajectories to the target an