The space of affordances: a TMS study

Previous studies have shown a motor recruitment during the observation of graspable objects. This recruitment has been considered crucial in encoding the observed objects in terms of one or more potential motor acts. However, an agent can actually act upon an object only when the latter is close enough to be reached. Thus, the question we deal with in this paper is whether the motor system is always activated whenever a graspable object comes into view or whether it requires the object to be located within the reachable space of the perceiver. The left primary motor cortex was magnetically stimulated and motor evoked potentials (MEPs) were recorded while participants observed graspable and non graspable objects located within or outside their own reachable space. We found higher MEPs during the observation of graspable objects falling within the reachable space compared to the observation of either a non graspable object or a graspable object falling outside the reachable space. Our results shed new light on the functional role of the motor system in encoding visually presented objects. Indeed, they clearly indicate that its recruitment is spatially constrained, as it depends on whether the object falls within the actual reaching space of the onlooker.