Feeling a Touch to the Hand on the Foot

Stephanie Badde, Brigitte Röder, Tobias Heed
Current Biology: CB 2019 March 13
Where we perceive a touch putatively depends on topographic maps that code the touch's location on the skin [1] as well as its position in external space [2-5]. However, neither somatotopic nor external-spatial representations can account for atypical tactile percepts in some neurological patients and amputees; referral of touch to an absent or anaesthetized hand after stimulation of a foot [6, 7] or the contralateral hand [8-10] challenges the role of topographic representations when attributing touch to the limbs. Here, we show that even healthy adults systematically misattribute touch to other limbs. Participants received two tactile stimuli, each to a different limb-hand or foot-and reported which of all four limbs had been stimulated first. Hands and feet were either uncrossed or crossed to dissociate body-based and external-spatial representations [11-14]. Remarkably, participants regularly attributed the first touch to a limb that had received neither of the two stimuli. The erroneously reported, non-stimulated limb typically matched the correct limb with respect to limb type or body side. Touch was misattributed to non-stimulated limbs of the other limb type and body side only if they were placed at the correct limb's canonical (default) side of space. The touch's actual location in external space was irrelevant. These errors replicated across several contexts, and modeling linked them to incoming sensory evidence rather than to decision strategies. The results highlight the importance of the touched body part's identity and canonical location but challenge the role of external-spatial tactile representations when attributing touch to a limb.

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