How many arms make a pair? Perceptual illusion of having an additional limb.

What are the natural constraints for the human body representation? Here I report a perceptual illusion where healthy individuals experience having two right arms, with both sensing touches applied to them. This effect reveals how visual and tactile signals from the body are integrated in a probabilistic fashion, resulting in a single limb being represented at two locations at the same time, giving rise to a perceptual duplication of this limb. This is an important observation because it suggests that even the gross morphology that we experience of ourselves is a construct resulting from dynamic and integrative processes in the perceptual systems.