After all, at what point does “you” end and the outside world begin? Now, scientists identify brain waves that define the limits of the “self”.
In a study last week in Nature Communicationsscientists have associated a specific set of brain waves, in a certain part of the brain, with the feeling of ownership of the body.
Swedish and French researchers put 106 participants through what is known as the rubber hand illusion, monitoring and stimulating their brain activity to see what effect it would have. This classic illusion involves hiding one of a participant’s hands from their field of vision and replacing it with a rubber hand. When both the real and the fake hand are repeatedly touched at the same time, the unsettling sensation that the rubber hand is part of the person’s body can be evoked.
The tests, which in one of the experiments involved EEG (electroencephalography) readings of brain activity, revealed that the feeling of body ownership appears to arise from the frequency of alpha waves in the parietal cortex – a region of the brain responsible for mapping the body, processing sensory information and building a sense of self.
“We have identified a fundamental brain process that shapes our ongoing experience of being embodied,” the research leader, told . Mariano D’Angeloneuroscientist at the Karolinska Institute (Sweden).
What are the limits of the “I”?
As the same magazine details, in the first set of experiments, participants had a robotic arm touch the index finger of their real and fake hands, either at exactly the same time or with a delay of up to 500 milliseconds between each touch.
As expected, participants reported feeling more intensely that the fake hand was part of their body when the touches were synchronized, and this sensation gradually weakened as the gap increased between what they felt and what they saw.
EEG readings from the second experiment added detail. The frequency of alpha waves in the parietal cortex appeared to correlate with participants’ ability to detect the temporal delay between taps.
Those with faster alpha waves seemed to exclude fake hands even with a minimal interval between touches, while those with slower waves were more likely to experience the fake hand as their own, even when the touches were spaced further apart in time.
Finally, the researchers analyzed whether the frequency of these brain waves effectively controls the feeling of body ownership, or whether both were perhaps the effects of another factor.
With a third group of participants, they used a non-invasive technique called transcranial stimulation by alternating current to speed up or slow down the frequency of a person’s alpha waves. And in fact, that seemed to correlate with how real the fake hand looked.
Conclusions:
- accelerate someone’s alpha waves gave him a more restricted sense of body ownership, making him more sensitive to small temporal discrepancies;
- slow down the waves had the opposite effect, making It is more difficult for people to distinguish between their own body and the outside world.
“The findings help explain how the brain solves the challenge of integrating body signals to create a coherent sense of self,” he says. Henrik Ehrssonneuroscientist at Karolinska, told Science Alert.
“In addition, they can provide new knowledge about psychiatric conditions such as schizophrenia, in which the sense of self is disturbed”, said Mariano D’Angelo.
