Dynamic size-change of pert-hand space following tool-use:: Determinants and spatial characteristics revealed through cross-modal extinction

In human and non human primates, evidence has been reported supporting the idea that near peripersonal space is represented through integrated multisensory processing. In humans, the interaction between near peripersonal space representation and action execution can be revealed in brain damaged patients through the use of tools that, by extending the reachable space, modify the strength of visual-tactile extinction, thus showing that tool-mediated actions modify the multisensory coding of near peripersonal space. For example, following the use of a rake to retrieve distant, otherwise non reachable objects, the peri-hand multisensory area has been documented to extend to include the distal part of a rake (Farne and Ladavas, 2000). The re-sizing of peri-hand space seems to be selective for tool-use, as directional motor activity alone (i.e., pointing without the tool) and visual/proprioceptive experience alone (protracted passive exposure to the tool) does not vary the extent of the visual-tactile peri-hand space (Farne et al., 2005a). Moreover, the amount of dynamic resizing varies with the length of the used tool, and is specifically centred on the functionally relevant part of the tool (Farne et al., 2005b). Here, besides reviewing and discussing these results, we report new evidence, based on a single-case study, supporting the idea that dynamic re-sizing of peri-hand space consists of a real spatial extension of the visual-tactile integrative area along the tool axis.