Dynamic size-change of hand peripersonal space following tool use.

Humans and monkeys share similar sensory integrated processing of tactile and peri-hand visual inputs for coding peripersonal space surrounding the hand. In monkeys, tool use is known to induce a transient elongation of hand-centred peripersonal space along the tool axis. Here we report evidence that, also in humans, the use of a tool can increase the spatial extent of the representation of peri-hand visual space to incorporate the tool. We investigated this phenomenon in patients with tactile extinction, by using a cross-modal paradigm well suited to reveal visual-tactile integration near patients' hand. In the present study cross-modal extinction was assessed far from patients' ipsilesional hand, at the distal edge of a hand-held rake. We found that cross-modal extinction was more severe after patients used the rake to retrieve distant objects with respect to a condition in which the rake was not used. This evidence of an expansion of peri-hand space lasted only a few minutes after tool use. By contrast, peri-hand space expansion was not observed when motor actions towards distant objects did not involve the tool. These findings show that visual peri-hand space has important dynamic properties in humans; it can be expanded and contracted depending upon tool use.