Overlap of movement planning and movement execution reduces reaction time by up to 100ms.

Abstract

Motor planning is the process of preparing the appropriate motor commands in order to achieve a goal. This process has been largely considered as occurring before movement onset and has been traditionally associated with reaction time. However, in a virtual line bisection task, we observed an overlap between movement planning and execution.\n\nIn this task performed with a robotic manipulandum, we observed that the participants (N=30) made straight movements when the line was in front of them (near target) but made often curved movements towards a farther target that was located sideways in such a way that they crossed the line perpendicular to it. Unexpectedly, movements to the far targets had shorter reaction times than movements to the near target (mean difference: 32ms, SE: 5ms, max: 104ms). In addition, the curvature of the movement modulated reaction time. A larger increase in movement curvature from the near to the far target was associated with a larger reduction in reaction time. These highly curved movements started with a transport phase during which accuracy demands were not taken into account.\n\nWe concluded that accuracy demand imposes a reaction time penalty if it is processed before movement onset. This penalty is reduced if the start of the movement can consist of a transport phase and if the movement plan can be refined in function of accuracy demands later in the movement, hence demonstrating an overlap between movement planning and execution.\n\nNew and NoteworthyIn the planning of a movement, the brain has the opportunity to delay the incorporation of accuracy requirements on the motor plan in order to reduce the reaction time by up to 100ms. Such shortening of reaction time is observed here when the first phase of the movement consists in a transport phase. This forces us to reconsider the idea that motor plans are fully characterized before movement onset.