Dynamic size‐change of peri‐hand space through tool‐use: Spatial extension or shift of the multi‐sensory area

Several studies in humans and non‐human primates have shown that tool‐use can expand near peripersonal space ( Farnè & Làdavas, 2000 ; Iriki, Tanaka, & Iwamura, 1996 ). In humans, the extension of the near peripersonal space is revealed by an increase in the severity of cross‐modal extinction caused by visual stimulation at the distal edge of a rake after its use as a reaching tool. The crucial question addressed here concerns whether the dynamic re‐sizing of the peri‐hand space in humans constitutes a real spatial expansion of visual‐tactile peri‐hand area along the tool axis. Alternatively, it could constitute a shift of the integrative area from the hand towards the distal edge of the tool, or the formation of a novel visual‐tactile integrative area at the same distal location ( Holmes, Calvert, & Spence, 2004 ). We contrasted the alternative predictions made by these hypotheses in a group of RBD patients by probing, at different locations along the tool axis, the changes induced by tool‐use on cross‐modal extinction. By assessing the visual‐tactile extinction near the hand, midway along the tool, and at the distal edge of the tool we found an increase in visual‐tactile extinction after tool‐use both at the middle and the distal location along the tool axis. In contrast, no change intervened at the hand proximity. These findings support the view that the tool‐use dependent re‐mapping of peri‐hand space in humans consists of a continuous elongation of visual‐tactile peri‐hand area from the hand towards the tip of the tool.