You can sense not only the sixth finger, but also its length.

The brain allocates neurons that regulate sensation or movement to parts of the body, so it can be said that the brain contains a map of the body, but studies suggest that it may not be so "reliable" at times. In 2016, a scientist conducted an experiment that made subjects think they had a sixth finger on one hand, and another shouted: "Witchcraft!" But the illusion only lasted a very short time. So another research team decided to repeat the experiment and managed to extend the duration of the illusion, which was published in 2020 in the journal Perception. The team then published a new study in 2021 in Cognition: they not only gave subjects the illusion of a sixth finger, but also managed to control the length of their perception of a finger they had never seen before.

To give the subjects this illusion, the researchers instructed the subjects to put their hands on the table and put a vertical mirror between them, so that the virtual image of the right hand in the mirror appeared in the position of the left hand. At the same time, the researchers started with the thumbs of both hands, scratched back and forth on the upper thumb, and then repeated the same steps for the fingers behind them. For the left pinkie, the researchers scratched back and forth on the inside of the left pinkie.

In the last step, they stroked back and forth on the table next to the little finger of their right hand while stroking back and forth on the outside of the little finger of their left hand. The last step was carried out 20 times in a row. After the experiment, the subjects said they felt an extra finger on their left hand that they had never seen before.

Denise Cadete, a graduate student in neuroscience and lead author of the study, at Berbeck College, University of London, said: "it's shocking to have such hallucinations even if you know what's going on."

In this new study, the length of a stroke across a table is sometimes only half the length of a typical pinky and sometimes twice the length of a typical pinky. To explore the length of the sixth finger in the illusion, the researchers asked 20 right-handed subjects to mark the length of the sixth finger with the position of the slider in the slider device. According to the subjects' self-reports, on average, their perceived finger length was either about 1.5 centimeters shorter or 3 centimeters longer than the real little finger. The difference between the length of the little finger and the real little finger indicates that this extra perceived finger is not a simple copy of the real finger, but has unique characteristics and should be viewed separately.

In addition to the interesting philosophical implications of human self-awareness, the study is also extremely useful for people who use mechanical prosthetics, Cardett said. She explained that such brain hallucinations may allow a mechanical prosthetic limb of the body to convey feelings to adjacent parts of the body, including one as complex as a Swiss Army knife.

Etienne Beidai (Etienne Burdet, who was not involved in the study) is a robotics expert at Imperial College, who studies the one-handed ability of people born with six fingers. He thinks the experiment is well done and can continue to study whether the brain can "grow" the sixth finger outside the pinkie in the future. Bildai went on to say that if he were to set up a company to make mechanical prosthetics: "I would start with a robot."