Egocentric metric representations in peripersonal space: A bridge between motor resources and spatial memory

Research on visuo-spatial memory has shown that egocentric (subject-to-object) and allocentric (object-to-object) reference frames are connected to categorical (nonmetric) and coordinate (metric) spatial relations, and that motor resources are recruited especially when processing spatial information in peripersonal (within arm reaching) than extrapersonal (outside arm reaching) spaces. In order to perform our daily-life activities, these spatial components cooperate along a continuum from recognitionrelated (e.g., recognizing stimuli) to action-related (reaching stimuli) purposes. Therefore, it is possible that some types of spatial representations rely more on action/motor processes than others. Here we explored the role of motor resources on the combinations of these visuo-spatial memory components. A motor interference paradigm was adopted in which participants had their arms bent behind their back or free during a spatial memory task. This task consisted in memorizing triads of objects and then verbally judging what was the object: 1) closest to/farthest from you (egocentric coordinate); 2) on your right/left (egocentric categorical); 3) closest to/farthest from an object (allocentric coordinate); 4) on the right/left of an object (allocentric categorical). The triads appeared in participants’ peripersonal (Experiment 1) or extrapersonal (Experiment 2) spaces. The results of Experiment 1 showed that motor interference selectively damaged egocentric coordinate judgments but not the other spatial combinations. The results of Experiment 2 showed that the interference effect disappeared when the objects were in the extrapersonal space. A third follow-up study using a within-subject design confirmed the overall pattern of results. Our findings provide evidence that motor resources play an important role in the combination of coordinate spatial relations and egocentric representations in peripersonal space.