Visuo-tactile integration in action and social spaces.

The space immediately surrounding the body is of foremost importance for any living being as it is the space in which interactions with the environment take place. Several spaces around the body have been described, contributing to interactions with objects (peripersonal space) or people (interpersonal and personal spaces) and presumably implying multisensory integration. We investigated the relationship between these different spaces by requiring participants to indicate when an approaching stimulus (human, robot or lamp) was reachable (peripersonal space), at the most comfortable distance to interact with (interpersonal space) or causes discomfort (personal space). Results showed that participants are comfortable when stimuli are further away than peripersonal space, and felt uncomfortable when the stimuli were well inside the peripersonal space. In addition, all three spaces were positively correlated, supporting the view that peripersonal action space contributes to the regulation of social distances. We concurrently investigated trunk-centred visuotactile multisensory integration by requiring participants to detect a tactile stimulation provided on the sternum while the visual stimuli were at different distances from the participant. We found that multisensory integration extended beyond all spaces and correlated to personal space when facing a robot or human, suggesting it is involved in interactions both with objects and people but is fundamentally associated with a defensive purpose.