Proprioceptive alignment of visual and somatosensory maps in the posterior parietal cortex

A touch on one hand can enhance the response to a visual stimulus delivered at a nearby location [1, 2], improving our interactions with the external world. In order to keep such visual-tactile spatial interactions effective, the brain updates the continuous postural changes, like those typically accompanying hand actions, through proprioception, thus maintaining the somatosensory and visual maps in spatial register [2, 3]. The posterior parietal cortex (PPC) might be critical for such a spatial remapping [4]; nevertheless, a direct causal demonstration of its involvement is lacking. Here, we found that unattended touches to one hand enhanced visual sensitivity for phosphenes induced by occipital trancranial magnetic stimulation (TMS) [5] when the touched hand was spatially coincident to the reported location of the phosphenes in external space. Notably, this spatially specific cross-modal facilitation was maintained after hand crossing, suggesting an efficient visual-tactile remapping. Critically, after 1 Hz repetitive TMS interference [6] over the PPC, but not over the primary somatosensory cortex, phosphene detection was still enhanced by spatially coincident touches with uncrossed hands, but it was enhanced by spatially noncoincident touches after hand crossing. This is the first causal evidence in humans that the PPC constantly updates the representation of the body in space in order to facilitate cross-modal interactions.