The rubber hand illusion works by making sight and touch agree. You watch a rubber hand being stroked while your own hand, hidden from view, is stroked at the same moment and in the same spot. After a few minutes most people start to feel that the rubber hand is theirs.
The brain settles the mismatch in favour of what it sees. Matching signals from two senses outweigh what the person knows to be true.
Two conditions, and the trick fails without them
Researchers at the Instituto de Sistemas Complejos de Valparaíso, in Chile, set out what the illusion requires. The two hands must be stroked in synchrony, at the same place and time. The rubber hand must also sit in a position a real arm could hold.
Break either rule and the illusion usually does not appear. Stroking the hands out of step, or not stroking the rubber hand at all, have served as control conditions in several studies for that reason.
Botvinick and Cohen first described the effect in 1998. Their participants sensed the touch on the artificial hand, and some felt that the hand belonged to their body.
How researchers measure a feeling
Two measures are the most common. One is a questionnaire. The other is “proprioceptive drift”, where people judge where their hidden index finger is, and the guess slides towards the rubber hand.
A 2004 experiment with a functional MRI scanner tracked the brain as this happened. Activity rose first in the parietal lobe. It then rose in the premotor cortex, which plans movement, once participants felt the rubber hand was theirs. When the stroking was out of step, the premotor cortex stayed quiet.
It is not all or nothing
The Chilean team interviewed participants about what they felt, rather than scoring them alone. The illusion came in shades. It changed how people perceived their real hand as well as the rubber one. It also varied widely from person to person and shifted over time.
The team also questioned the controls. Earlier work had treated out-of-step stroking as neutral, yet two studies it cited found that it could produce some form of the illusion too.
Touch is not required
A study in Scientific Reports swapped the brush for temperature. Participants felt warm or cold on their hidden hand while red or blue light fell on a fake hand in step. The illusion appeared, with vividness ratings close to the brushed version.
Colour mattered. Warm paired with red, or cold with blue, produced a stronger illusion than the mismatched pairs. The same authors note that the effect has also come from needles, from moving the hands together, and in some people from looking at a fake hand with no stimulation at all.
For some people, the eyes alone are enough to hand a rubber limb a place in the body.
