Dealing with individual variability: when telling what is real depends on telling who is acting

You are queuing at the bus stop, and notice that someone suddenly turns her walk into a run: typically, you assume that she wants to catch the bus and may want to tell the driver to wait. Faced with a sudden speed change, rather than considering it bizarre or unnatural, observers attach a meaning to it, and act consequently. In a social context, speed of a movement often bears as much significance as its form, and can be adapted to vehicle precise meanings. This pragmatic rule facilitates decoding of non-verbal messages from other individuals, but may not necessarily apply when observing one's own movements, for which intentions should be informative enough. Hence, the range of motion speeds labeled as 'natural' could be broader for other people's actions compared to one's own. We explored this possibility through a task in which human observers decided whether speed of a gesture had been artificially modified. A virtual hand was presented, which - unbeknownst to participants - moved according to the kinematics of either the observer, or another individual. Although a self/other distinction was never required, participants applied different criteria when dealing with self compared to other people's gestures, suggesting that the brain implicitly extracts identity information before any overt judgment is produced. Interestingly, observers were reluctant to labeling movements of another individual as artificial, in keeping with the hypothesis that large variations in movements' speed can vehicle social messages, and therefore are not regarded a priori as unnatural.