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Holmes NP, Calvert GA, Spence C (2004) Tools, brains, & bodies: Projecting sensations into outer space. Psychology Post Graduate Affairs Group (PsyPAG), Manchester Metropolitan University, 27th July    
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Abstract
It has often been claimed that tools are extensions of the body, and are ‘incorporated’ into the body schema. This claim has received little empirical attention, however, perhaps reflecting the abstract nature of the concept of the body schema. One recent alternative hypothesis is that tools extend peripersonal space – the multisensory region of space immediately surrounding our body parts. We examined this putative extension of peripersonal space using a behavioural measure of visuotactile integration in healthy human participants (crossmodal congruency task). Participants held a golf-club ‘tool’ containing two vibrotactile stimulators (targets) in each hand, one under each thumb (‘upper’), and one under each forefinger (‘lower’). Visual distractor stimuli (LEDs) were positioned in upper and lower locations at 0, 30, and 60 cm along the tool handles and shafts. The primary task was to discriminate the elevation (upper vs. lower) of targets, while trying to ignore the simultaneous, random, and non-informative distractors. For six different secondary ‘tool-use’ tasks, which were performed after every fourth trial of the crossmodal congruency task, we found that visual distractors presented on the same tool as the target interfered significantly more with vibrotactile discrimination performance than with distractors on the opposite tool. However, this same-tool versus opposite-tool effect was present only near the hands and at the tips of the tool, and only for active tool-use tasks performed beyond arm’s reach. These results suggest that tool-use does not simply ‘extend’ peripersonal space, but rather that only the space around the tips of tools is incorporated into peripersonal space