Cutaneous rabbit illusion

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The cutaneous rabbit illusion (also known as cutaneous saltation) is a tactile illusion evoked by tapping two separate regions of the skin. Many rabbit experiments have been carried out on the forearm. A rapid sequence of taps delivered first near the wrist, and then near the elbow creates the sensation of sequential taps hopping up the arm from the wrist towards the elbow, although no physical stimulus was applied between the two actual stimulus locations. Similarly, stimuli delivered first near the elbow, then near the wrist, evoke the illusory perception of taps hopping from elbow towards wrist. The illusion was discovered by Frank Geldard and Carl Sherrick of Princeton University, in the early 1970s (Geldard & Sherrick, 1972). They likened the perception to that of a rabbit hopping along the skin, giving the phenomenon its name.

From the moment of its discovery, the illusion has piqued the curiosity of researchers. A study showed that attention directed to one skin location modifies the perceived location of the illusory taps (Kilgard & Merzenich, 1995). Another study showed that the illusory taps are associated with neural activity in the same brain areas that are activated by real taps to the skin (Blankenburg et al., 2006). Nevertheless, the specific neural mechanisms that underlie the rabbit illusion are unknown. Neural models have been proposed by several authors (e.g., Flach & Haggard, 2006), and a recent Bayesian perceptual model replicates the rabbit illusion under the assumption that the brain expects tactile stimuli to move slowly (Goldreich, 2007).