Effects of numerical magnitude on the online execution of grasping movements

Recent literature has established a directional influence of numerical magnitude on actions performed towards neutral objects. Such influence is illustrated by larger apertures during grasping following the presentation of large compared to small digits. The interaction between visuomotor control and magnitude has been commonly attributed to the planning stage of the action prior to motor execution. However, this assumption has not been directly empirically tested. In two experiments, we tested whether the effects of numerical magnitude on grasping derive from action planning or from action execution. Participants were asked to grasp an object following a short visually (Experiment 1) or auditory (Experiment 2) presentation of a small (1/2) or a large (8/9) digits. Grasping in both experiments was performed under either closed-loop (CL) or open-loop (OL) conditions, for which online vision was prevented during action execution. As expected, digit magnitude affected grip apertures in the CL condition, when online vision was allowed. However, magnitude had no effect on grip aperture in the OL condition. This pattern of results suggests that the effects of numerical magnitude on action originate from interactions between numerical magnitude and object size during the online execution of actions. Unlike previously assumed, the findings also suggest that the effect of magnitude on visuomotor does not result from a motor planning stage prior to action initiation. Meeting abstract presented at VSS 2017