Enhanced integration of multisensory body information by proximity to "habitual action space".

Previous research suggests integration of visual and somatosensory inputs is enhanced within reaching (peripersonal) space. In such experiments, somatosensory inputs are presented on the body while visual inputs are moved relatively closer to or further from the body. It is unclear, therefore, whether enhanced integration in ‘peripersonal space’ is truly due to proximity of visual inputs to the body space, or, simply the distance between the inputs (which also affects integration). Using a modified induction of the rubber hand illusion, here we measured proprioceptive drift as an index of visuo-somatosensory integration when distance between the two inputs was constrained, and absolute distance from the body was varied. Further, we investigated whether integration varies with proximity of inputs to the habitual action space of the arm – rather than the actual arm itself. In Experiment One, integration was enhanced with inputs proximal to habitual action space, and reduced with lateral distance from this space. This was not attributable to an attentional or perceptual bias of external space because the pattern of proprioceptive drift was opposite for left and right hand illusions i.e. consistently maximal at the shoulder of origin (Experiment Two). We conclude that habitual patterns of action modulate visuo-somatosensory integration. It appears multisensory integration is modulated in locations of space that are functionally relevant for behaviour, whether an actual body part resides within that space or not.