Jonathan Cohen, who is the director of the Princeton University Neuroscience Institute, and Matthew Botvinick, who is the lead researcher at Google DeepMind, are currently leading scholars in neuroscience research. The two gained fame for a fun experiment that would be popular at gatherings. This is the so-called 'rubber arm illusion' experiment published in Nature in 1998.
The principle is simple. Place both hands side by side on a table, then set up a partition so the right hand is not visible. Next to the left hand, position a rubber hand in plain sight. Then, if you alternately tickle the left hand and the rubber hand with a brush, surprisingly, both the left hand and the right hand will feel itchy. The brain mistakenly perceives the rubber hand as the right hand.
Japanese scientists have discovered for the first time that octopuses can also fall into the rubber hand illusion like humans. When a replica is placed where the legs are, the octopus showed a avoidance response upon interaction. It mistook the rubber leg for its own leg. This is the first time the rubber hand illusion phenomenon has been confirmed in animals that are not mammals.
◇First observation of the rubber hand illusion outside mammals
Yuzuru Ikeda, a professor at Ryukyu University, and his research team noted that 'we have confirmed through experiments that octopuses also fall for the rubber hand trick' in a study published on the 22nd in the international journal Current Biology. The research team explained that octopuses have bodily sensations similar to those of humans. This means that in future studies on how humans perceive their bodies, octopuses could serve as good experimental subjects.
The research team raised octopuses (scientific name Callistoctopus aspilosomatis) in a laboratory tank and conducted an octopus version of the rubber hand illusion experiment. For humans, a partition was set up to obscure one hand while touching the visible rubber hand. This time, one of the octopus's legs was covered with a partition, and a fake leg made of gel was placed in that location.
The research team stroked both the actual leg and the fake leg of the octopus simultaneously using a plastic tool. After 8 seconds, they picked up the fake leg with tweezers. Then, the octopus contracted its legs or changed color and fled. In 24 trials, all six octopuses showed this avoidance response when the fake leg was stimulated. The rubber hand illusion effect disappeared when the octopus only touched the fake leg or when the fake leg was in a different position from the real leg.
The research team explained that 'these results suggest that the octopus, an invertebrate, perceives bodily ownership at a level similar to that of mammals,' noting 'this means that this invertebrate with eight legs possesses complex cognitive abilities.' Professor Ikeda mentioned that for this reason, octopuses could serve as good experimental animals for studying the evolution of body ownership.
◇Could help in developing avatar robots
The fact that the brain perceives the rubber hand as a real hand has already been proven through experiments. A research team at the University of Turin in Italy published in 2016 in the international journal eLife that during the rubber hand illusion experiment, when measuring the motor signals the brain sends to the real hand, the signals significantly decreased as if they were not part of the body.
The research team applied mild electrical stimulation to the area of the brain that controls hand movements. This signal travels down the spinal cord to move the muscles. An electromyography (EMG) test showed that participants who mistook the rubber right hand for part of their own body experienced a sharp decline in motor signals transmitted to the real right hand. The research team interpreted this as physiological evidence that the brain treats the visible rubber hand as the real hand and the obscured real hand as a separate entity from the body.
Scientists believe that the rubber hand illusion phenomenon could be applied to remotely controlled robots. If robots can be mistaken for one's own body like the rubber hand, they could be utilized as avatars in planetary or disaster sites. This is similar to the protagonist in the movie 'Avatar' walking and running in the body of a Na'vi warrior despite being paralyzed from the waist down.
The research team at Bar-Ilan University in Israel demonstrated that possibility in an experiment in 2012. College students lay inside an fMRI (functional magnetic resonance imaging) device and practiced moving a virtual human appearing on the screen in front of them. The research team examined which areas of the brain's motor centers were activated. When certain actions were performed, blood flowed to the corresponding areas in the brain. The fMRI showed these areas as if a light had been turned on.
Next, instead of the virtual human, they moved a robot located in a laboratory in France, 2,000 kilometers away from Israel. When researchers in France called the robot, the college student thought about moving forward, and the brain signals generated at that moment were transmitted over the internet to move the robot. As a result of the training, the Israeli college student could see the world through the camera of the human-shaped robot 'HOAP-3' made by Fujitsu in Japan.
It was not just remote control. The college student in Israel felt as if they were moving when the French researchers handled the robot. They felt as if the robot was part of their body. A small illusion enjoyed as a snack at a gathering may later pave the way for exploring Mars using a borrowed robotic body.
References
Current Biology(2025), DOI: https://doi.org/10.1016/j.cub.2025.05.017
eLife(2016), DOI: https://doi.org/10.7554/eLife.1497
IEEE(2012), DOI: https://doi.org/10.1109/BioRob.2012.6290866
Nature(1998), DOI: https://doi.org/10.1038/35784