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Natalie Dunphy, Laurel Evans, Susan Klostermann, Frank H. Durgin; Visuo-spatial alignment produces an instant rubber hand illusion. Journal of Vision 2005;5(8):749. doi: 10.1167/5.8.749.
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In the rubber hand illusion (RHI — Botvinick & Cohen, 1998), one's hand is occluded, and a rubber hand is placed in view nearby. If the real and rubber hands are then touched in synchrony, one comes to experience the tactile sensations as occurring at the surface of the rubber hand. Like prism adaptation, this results in a shift in the felt position of the hand. We wondered whether visuo-tactile synchrony is a direct source of information about localization of the body schema, or if the RHI is primarily mediated by visuo-spatial identification. To test this hypothesis, a mirror box was constructed that allowed a rubber hand to visually replace the left hand of an observer. A first-surface mirror served to occlude the hand while simultaneously providing illusory visual access to the expected visual location of the real hand. Whether the rubber hand was placed (via the mirror) in the visual space of the real hand or displaced by 15 cm, the rubber hand illusion was immediate. To test whether synchronous touching added any additional spatial shift to the illusion we measured shifts in the felt position of the unseen hand after approximately two minutes of exposure to the optically displaced rubber hand while (1) real and rubber hand were touched synchronously, (2) the rubber hand was “stroked” with the light of a “laser,” or (3) the real hand was touched while the rubber hand was not. In all three conditions, a displaced rubber hand produced shifts in felt position of the real hand (3 cm) that were statistically equivalent. Similarly, electrodermal responses to threats to the RH were indistinguishable in the three conditions (see Armel & Ramachandran, 2003). Visuo-tactile synchrony added nothing to the visual identification of the rubber hand when plausible alternative locations of the real hand (occluders) were not visually available. When the spatial location of the real hand is rendered visually empty, or is filled by the rubber hand, the RHI is instant.
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