Abstract
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 intermediate movements are generally interpreted as errors of movement planning, arising either from planning being incomplete or from parallel movement plans interfering with one another. 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 between goals. Consequently, reducing the compatibility between goals should reduce the incidence of intermediate movements. In human subjects, we varied the compatibility between potential movement goals in two distinct ways: by varying the spatial separation between targets and by introducing a virtual barrier constraining trajectories to the target and penalizing intermediate movements. In both cases we found that decreasing goal compatibility led to a decreasing incidence of intermediate movements. Our results and theory suggest a more integrated view of decision-making and movement planning in which the primary bottleneck to generating a movement is deciding upon task goals. Determining how to move to achieve a given goal is rapid and automatic.
Highlights
In the reaction time before a movement is initiated, two distinct processes are thought to occur: first, the exact goals of the movement must be decided upon and, second, the actions that will achieve the chosen goal must be selected and/or prepared [1]
Two critical processes need to occur before a movement can be made: identification of the goal of the movement and selection and preparation of the motor commands that will be sent to muscles to generate the movement—in other words, what movement to make, and how to make it
Intermediate Movements as Optimal Behavior consuming process, and theories advocating this view have pointed to instances where apparently the wrong motor commands are issued if insufficient time is available to prepare them
Summary
In the reaction time before a movement is initiated, two distinct processes are thought to occur: first, the exact goals of the movement must be decided upon and, second, the actions that will achieve the chosen goal must be selected and/or prepared [1]. The process of selecting and/or preparing the actions to achieve a chosen goal, which we refer to here as movement planning, is classically thought to require further time-consuming computations [4,5,6]. Similar intermediate movements are observed if a target jumps shortly before movement onset [11,12,13,14,15] or in tasks that either deliberately or inadvertently create ambiguity about task goals [16,17,18,19]. Either interpretation suggests that intermediate movements occur as an unintentional artifact of stressing an underlying planning mechanism
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