Cooperative tool-use reveals peripersonal and interpersonal spaces are dissociable.

The space surrounding people is often termed Interpersonal (IPS) in social psychology and Peripersonal (PPS) in neuroscience. In the current debate about their origin, the prevalent opinion is they share common functional characteristics. Bucking the trend, here we report a dissociation between PPS, operationalized as reachable space, and IPS, operationalized as comfort space. To probe their plasticity we introduced a novel type of cooperative long-tool-use that would modify both spaces. Results showed the estimated IPS referred to another individual was reduced, as expected following a positive social interaction. In sharp contrast, the estimated PPS toward the very same cooperative person was actually extended after use of the same long-tool. Control short-tool-use selectively reduced IPS, but not PPS, when performed in the same cooperative set or had no effect on either space estimation, when performed in a neutral set where the other person is not interacting cooperatively, but simply observing. The use of tools to perform actions in social settings allows us to report the first strong evidence that PPS and IPS underlie dissociable plastic representations: the former representation is sensitive to long-tool-dependent plasticity, whereas the latter representation, independently of use of a short or long tool, is sensitive to cooperation-dependent plasticity.