Did you know it’s possible for scientists to trick your brain into thinking you have an additional appendage?

A participant might stand in front of a mirror placing their left hand in such a way that the thumb doesn’t show. A researcher then strokes the non-thumb side of the left hand at the same time as stroking the side with the thumb. In the space of a few moments, the participant begins to feel as though they have a second thumb on the wrong side of their hand — even though they can clearly see this is not the case .

“Your brain immediately jumps to the conclusion that it has an extra body part,” says Denise Cadete, a neuroscientist at Birkbeck, University of London. “In seconds, you can feel a body part that has a different shape, consistency [and] length.”

Strange Phantom Hand Illusions

For a study published in Cognition, Cadete and her Ph.D. supervisor, cognitive neuroscientist Matthew Longo, conducted a number of illusions to trick the body into experiencing the sensations of an altered body.

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First, the researchers had participants lay their hands on a table and placed a mirror between them. They then stroked each finger in order, starting with the thumbs. When the researchers reached the participants’ pinkies, they stroked two parts of the finger on one hand while simultaneously stroking one part of the other and an empty space beside the pinky. This created the illusion that the participants carried a sixth finger on the latter hand.

“In seconds I can make you feel like you have a sixth finger,” Cadete says.

They then took this experiment a little further, increasing or shortening the length of the strokes on the empty space beside the one hand. The participants then reported the new, phantom finger being a little longer or shorter than their actual pinky.

Since then, the team has expanded upon these illusions. In another experiment, the researchers use sound to trick the brain. They might touch the hand in one place, but play the recorded sound of the hand being touched a few inches past the participants’ finger tips. This set up gives the sensory illusion that the participants’ limbs are longer than they actually are.

“I can change the size of your arm,” Cadete says.

How Do These Illusions Work?

These experiments are more than just fancy parlor tricks. In fact, they’re designed to get down to the basic way the brain and body interact.

The somatosensory cortex enables the brain to sense what the body touches. The parietal cortex, on the other hand, is responsible for controlling the body. It relies on information from the somatosensory cortex to create a map of where, for example, the hands and fingers are located.

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Essentially, the researchers are tricking participants’ parietal cortex about the shape of their limbs by the way they touch different parts of the body.

Why Is This Research Important?

A better understanding of the way that our brain determines our sense of self has implications for further research. While most tetrapods — which includes all four-limbed, land-living vertebrates, such as frogs, turtles, hawks and humans — have five digits or fewer, the distant ancestors of amphibians, mammals, birds and reptiles used to have more.

It’s unclear whether our brain’s ability to quickly recognize a new finger is a relic from evolutionary ancestral times when we had more digits around, but Cadete notes that it’s an interesting idea to think about.

There are also practical applications for this knowledge. Cadete says a big conversation in the world of prosthetic limbs for amputees is how the brain will adapt to these devices. She and Longo’s research may have implications for alternatives to replacing hands or fingers.

“Instead of trying to replace a limb that they lost, one of the things some roboticists are doing is having an extra body part in the limb,” she says, giving the example of an extra thumb or hook. She even likes to speculate about how this might affect humans more technologically-advanced future where non-amputees are adding appendages.

Finally, many amputees experience severe phantom limb pain, which refers to the perception of pain in a limb that’s no longer there. The condition is difficult to treat since doctors can’t locate pain that in theory, amputees shouldn’t experience. Future research on how the brain recognizes limbs or fingers may help.

“We are quite behind on this research,” Cadete says of phantom limb pain.

Read More: Understanding the Disorder That Drives People to Amputate Healthy Limbs