The representation of space near the body through touch and vision

This review discusses how visual and the tactile signals are combined in the brain to ensure appropriate interactions with the space around the body. Visual and tactile signals converge in many regions of the brain (e.g. parietal and premotor cortices) where multisensory input can interact on the basis of specific spatial constraints. Crossmodal interactions can modulate also unisensory visual and somatosensory cortices, possibly via feed-back projections from fronto-parietal areas. These processes enable attentional selection of relevant locations in near body space, as demonstrated by studies of spatial attention in healthy volunteers and in neuropsychological patients with crossmodal extinction. These crossmodal spatial effects can be flexibly updated taking into account the position of the eyes and the limbs, thus reflecting the spatial alignment of visuo-tactile stimuli in external space. Further, studies that manipulated vision of body parts (alien, real or fake limbs) have demonstrated that passive viewing of the body can influence the perception of somatosensory stimuli, again involving areas in the premotor and parietal cortices. Finally, we discuss how tool-use can expand the region of visuo-tactile integration in near body space, emphasizing the flexibility of this system at the single-neuron level in the monkey's parietal cortex, with corresponding multisensory effects in normals and neuropsychological patients. We conclude that visuo-tactile crossmodal links dominate the representation of near body space and that this is implemented functionally in parietal and premotor brain regions. These integration processes mediate the orienting of spatial attention and generate an efficient and flexible representation the space around the body.