Pinocchio illusion

How it works

Muscle spindles are stretch sensors inside muscles. They report how long each muscle is, and the brain turns that into joint angle and limb position²³. A vibrator at about 100 Hz pressed on a tendon makes the spindles fire as if the muscle were being stretched¹. Vibrate the biceps tendon, and the brain concludes that the elbow is opening and the forearm is swinging away from the face.

Now add a touch. If the person’s fingertips rest on their nose while the tendon is vibrated, the brain gets two facts that cannot both be true. The arm is moving away. The fingers are still on the nose. It resolves the conflict by stretching the nose. People report a nose that extends far out from the face, or in some cases a nose that has moved outward, or a face that has changed shape¹.

James Lackner described the effect in 1988¹. The same conflict produces other distortions. Vibrate the tendon while the hand rests on top of the head, and the head feels taller or the hand feels sunk into it¹. The illusion works because the brain never sees the body directly. It builds the body from signals, and it trusts the touch signal most. The same mechanism explains the loss of arm position during long still meditation.