Cutaneous rabbit illusion

How it works

The set-up is simple. A small device taps the wrist several times in quick succession, then taps a point near the elbow the same way. Nothing touches the skin between them. Yet most people feel the taps march up the forearm in even steps, as if a rabbit were hopping toward the elbow¹.

The illusion was first described by Frank Geldard and Carl Sherrick in 1972¹. It works only when the taps are fast. If the gap between taps is longer than about a fifth of a second, the illusion breaks and you feel two separate groups of taps¹.

The brain is doing something odd here. It decides where the early taps were after the later taps arrive. This is called postdiction. The touch system assumes that things move smoothly through space, so a signal that jumps from wrist to elbow is smoothed into a path¹. Brain scans support this. In a 2006 study, Blankenburg and colleagues found that the part of the touch cortex for the middle of the forearm became active during the illusory taps, even though nothing touched that skin².

The rabbit does not stay on the skin. If you hold a stick between two fingers and the taps are applied to the stick, the hops are felt along the stick, outside the body. The brain treats the tool as part of the arm¹.

History and culture

The effect is sometimes called sensory saltation, from the Latin for “leap”. It is a standard demonstration in perception courses. Versions of it exist for hearing and for vision. Two flashes at different places, shown quickly, can be seen as a row of flashes moving across¹.