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Holmes NP, Calvert GA, Spence C (2005) Where does the body end & the world begin? Multisensory interactions in peripersonal space. Wellcome Trust Final Year Doctoral Students Meeting, London, UK, 27th January    
‘Where does the body end and the world begin?’ The obvious answer for a neuroscientist might be: ‘the somatosensory nerve endings’. But this will not do. Our fingernails, hair, and teeth and the horny appendages of other animals are very much a part of the body, and we feel as if tactile perceptions arise from the distant ends of such protuberances1,2, yet such body parts contain no somatosensory receptors. Similarly, a number of animals, including humans, use inanimate objects as tools to peruse, pick, and probe the world in ways that both resemble and extend the capabilities of the body itself3. To what extent are such tools ‘incorporated’ into the bodies of the tool-users? To what extent is it true that ‘tools extend the body?’ My research examines a number of experimental accounts for the ability of humans and animals to use tools as extensions of their bodies. Tools have long been thought to act as, or to become, extensions of the body, in fields as diverse as literature4, philosophy5,6, psychology1,2,7, and neurology8. Unfortunately, this intriguing idea has received little experimental attention, and has suffered from a somewhat vague and hyperbolic expression not conducive to careful empirical study. In short, this idea needs a little more ‘fleshing-out’! A recent renaissance of this idea in the neurosciences has generated the claim that tools extend ‘peripersonal space’9, the visual space immediately surrounding our body and its members10. Neurons involved in representing this space typically respond both when visual stimuli are presented near visible body parts, and also when that body part is touched. Intriguingly, some of these neurons appeared to ‘stretch’ their visual response fields when a hand-held tool was used actively during such sensory testing9. Similar experiments with human participants and brain-damaged patients have reached similar conclusions. Brain damage can sometimes affect one’s ability to perceive certain portions of space, for example ‘far’ space beyond arm’s reach or ‘near’ space within arm’s reach. When such patients use tools, the regions of space within which they show these sensory impairments can change, in a matter of minutes – tools turn ‘far space’ into ‘near space’11-14. Similar effects have been shown in human participants without brain damage – tools help to associate visual stimuli presented at a distance from the body, with the body itself, thus ‘extending peripersonal space’15,16. My work focuses on testing several behavioural predictions of the ‘tools extend peripersonal space’ hypothesis in normal human participants. My results are inconsistent with this hypothesis, however, and suggest instead that only visual stimuli presented at the tips, and not the middle, of the tools have any effect on peripersonal space.17 Furthermore, the nature and frequency of the tool-use task itself is important in modulating these effects (Holmes N P, Calvert G A, Spence C in preparation). A re-examination of the original evidence for the hypothesis18 also suggests that, to some extent, we have barking up the wrong tree, although we may be in the right neck of the woods. References 1. Gibson JJ (1966). The senses considered as perceptual systems. George Allen and Unwin Ltd, New York. 2. Weber EH (1846/1996). Tastsinn und Gemeingefühl. [On the sense of touch and ‘common sensibility’]. In Ross HE, Murray DJ (eds.) EH Weber on the tactile senses. Earlbaum UK Taylor and Francis, Hove. 3. Beck BB (1980). Animal tool behaviour: The use and manufacture of tools by animals. Garland Press, London. 4. Butler S (1874). Erehwon. London: Cape. 5. Lotze RH (1854/1885). Microcosmus: An essay concerning man and his relation to the world. T & T Clark, Edinburgh. 6. Polanyi M (1958/1973). Personal knowledge: Towards a post-critical philosophy. London: Routledge & Kegan Paul. 7. Loomis JM (1993). Distal attribution and presence. Presence, 1:119-119. 8. Head H, Holmes HG (1911-1912). Sensory disturbances from cerebral lesions. Brain, 34:102-254. 9. Iriki A, Tanaka M, Iwamura Y (1996). Coding of modified body schema during tool use by macaque postcentral neurones. NeuroReport, 7: 2325-2330. 10. Graziano MSA, Hu XT, Gross CG (1997). Visuospatial properties of ventral premotor cortex. J Neurophysiol, 77:2268-2292. 11. Berti A, Frassinetti F (2000). When far becomes near: Remapping of space by tool-use. J Cogn Neurosci, 12:415-420. 12. Farnè A, Làdavas E (2000). Dynamic size-change of hand peripersonal space following tool use. NeuroReport, 1:1645-1649. 13. Maravita A, Clarke K, Husain M, Driver J (2002). Active tool-use with contralesional hand can reduce crossmodal extinction of touch on that hand. Neurocase, 8:411-416. 14. Maravita A, Husain M, Clarke K, Driver J (2001). Reaching with a tool extends visual-tactile interactions into far space: Evidence from cross-modal extinction. Neuropsychologia, 39:580-585. 15. Maravita A, Spence C, Kennett S, Driver J (2002). Tool-use changes multimodal spatial interactions between vision and touch in normal humans. Cognition, 83:25-34. 16. Spence C, Pavani F, Maravita A, Holmes NP (2004). Multisensory contributions to the 3-D representation of visuotactile peripersonal space in humans: Evidence from the crossmodal congruency task. J Physiol (Paris), 98:171-189. 17. Holmes NP, Calvert GA, Spence C (2004). Extending or projecting peripersonal space with tools? Multisensory interactions highlight only the proximal and distal ends of tools. Neurosci Lett, 372:62-67. 18. Holmes NP, Spence C (2004). The body schema and the multisensory representation(s) of peripersonal space. Cognitive Processing, 5:94-105.