Shaping multisensory action-space with tools: evidence from patients with cross-modal extinction

Recent findings from neurophysiology, neuropsychology and psychology have shown that peri-personal space is represented through an integrated multisensory processing. In humans, the interaction between peri-personal space representation and action execution can be revealed through the use of tools that, by extending the reachable space, modify the strength of visual-tactile extinction. We have previously shown that the peri-hand space whereby vision and touch are integrated can be expanded, and contracted, depending upon tool-use. Here, we show that these dynamic changes critically depend upon active tool-use, as they are not found after an equally long, but passive exposure to an elongated (hand+tool) body configuration. We also show that the extent of the peri-hand space elongation, as assessed at fixed far location (60 cm from the hand), varies according to the tool length such that a 30 cm long tool produced less elongation than a 60 cm long tool. This reveals for the first time that the distal border of elongated area is not sharply limited to the tool length, but extends beyond its physical size to include a peri-tool space whereby the strength of visual-tactile integration seems to fade. Remarkably, a similar amount of peri-hand space elongation was found when the effects of using a 30 cm long tool were compared with those produced by using a tool that was physically 60 cm long, but operationally 30 cm long. By dissociating with this 'hybrid' tool, the amount of space that is globally added to the hand (60 cm) from the one that is actually reachable (30 cm), we provide here the first evidence that the extent of peri-hand space elongation after tool use is tightly related to the functionally effective length of the tool, and not merely to its absolute length.