Manual actions cover symbolic distances at different speed

A privileged way of representing numbers in the human mind is along an oriented mental number line. Activation of this representation has been proposed to account for the impact of numbers on motor tasks, such as on grasping, pointing, and eye movements. Here we evaluated the impact of numbers on motor control, by exploiting the evidence that the speed reached by the manual connection of two points is correlated with their physical distance. We reasoned that, if irrelevant numbers induce a mis-perception of the distance between two points, this should be reflected in the movement speed. Results showed a speed difference in the manual connection of two numerically close numbers (i.e., connected slower) and two numerically distant numbers (i.e., connected faster), placed at equal physical distance. This representational length effect indicates not only that symbolic distance modulates speed movement as physical distance does, but suggests that the impact of numbers on action planning does not only involve action initiation but it extends to the definition of kinematic parameters. More generally, the reported findings show that the representation of numbers along a mental space affects our behaviour in the physical space.