Understanding the Boundaries Between You and the Outside World
Have you ever stopped to consider where “you” end and the outside world begins? It may seem like a simple question, but your brain actually works hard to determine that boundary. Recent research has identified a specific set of brain waves in a particular part of the brain that are linked to a sense of body ownership.
In a series of innovative experiments conducted by scientists from Sweden and France, 106 participants were subjected to the rubber hand illusion to investigate the impact on their brain activity.
The rubber hand illusion is a classic experiment where a participant’s hand is hidden from view and replaced with a rubber hand. By simultaneously touching both the real and fake hands, it can create the illusion that the rubber hand is actually part of the individual’s body.
The study, which included EEG (electroencephalography) readings of brain activity, revealed that the sense of body ownership is associated with the frequency of alpha waves in the parietal cortex. This brain region is responsible for mapping the body, processing sensory input, and constructing a sense of self.
Lead author Mariano D’Angelo, a neuroscientist at the Karolinska Institute in Sweden, stated, “We have identified a fundamental brain process that shapes our continuous experience of being embodied. The findings may provide new insights into psychiatric conditions such as schizophrenia, where the sense of self is disturbed.”
In the experiments, participants had a robotic arm tap the index finger of their real and fake hands either simultaneously or with a delay of up to 500 milliseconds between each tap. The results showed that participants felt a stronger connection to the fake hand when the taps were synchronized, and this feeling diminished as the timing gap increased.
The EEG readings from a subsequent experiment provided additional insights. The frequency of alpha waves in the parietal cortex appeared to correlate with the participants’ ability to detect the time delay between taps.
Individuals with faster alpha waves were more adept at discerning small gaps in taps, while those with slower waves were more inclined to perceive the fake hand as their own, even with longer intervals between taps.
Further investigation revealed that manipulating the frequency of alpha waves through transcranial alternating current stimulation influenced the sensation of body ownership. Accelerating alpha waves enhanced the feeling of body ownership, making individuals more sensitive to timing discrepancies. Conversely, slowing down the waves blurred the distinction between one’s body and the external world.
Henrik Ehrsson, a neuroscientist at the Karolinska Institute, commented, “Our findings help explain how the brain solves the challenge of integrating signals from the body to create a coherent sense of self.”
These findings have implications for understanding and treating conditions where the brain’s body maps are disrupted, such as schizophrenia or phantom limb sensations in amputees. Additionally, this research could contribute to the development of more realistic prosthetic limbs and immersive virtual reality experiences.
The study was published in the journal Nature Communications.
