The brain likes a logical and coherent world. When it detects an action, it expects a reaction based on experience, even if it is detected by different senses. For example, when we see a person hitting a gigantic gong, we expect to be immediately enveloped by a deafening sound, or when we finally (after much fighting) manage to open the plastic wrapper on a wedge of cheese, we expect our nostrils to be filled with the smell of the dairy product. Even if the stimuli are separated by several seconds, like the time that passes between lightning and thunder, it relaxes our brain to listen to it, since it is moving forward to what is going to happen.
This fact is called sensory integration and allows us to reduce the uncertainty of the world around us. However, our brain is not constantly integrating all the signals we receive, but over millions of years of evolution it has specialized in separating which ones should be integrated and which ones should be separated.
Let’s take an example of an early human. If this human integrated all the signals, such as the murmur of a stream, the chirping of each of the species of birds, the tickling of a blade of grass, etc. Etc…. it is more likely that he would not focus on the crunch of a branch that has broken under the weight of a claw, nor the smell that has enveloped the environment and therefore that the enormous cave bear could return home with a full belly.
That is why our brain, in a fraction of a second, keeps the most important and integrates sensory signals that most likely come from the same object and separates those that do not. So, if the crunch and smell occur in what is called the ‘temporary join window‘, the brain will become alert, because if everything has happened at the same moment, it is possible that there is a bear, and that would be more urgent than listening to the birds.
When stimuli come from within
But of course, when the stimuli come from our own body, this problem becomes more complicated. Since relatively recently, Neuroscientists know that our brain is constantly integrating and discriminating stimuli to know what is part of our body and what is not.. To do this, it combines both visual and tactile information, as well as that of our ‘sixth sense’: proprioception. But it was by tricking these senses that a team from the Karoliska Institute has been able to investigate how these key brain processes work.
Specifically, they have studied how 106 people reacted to ‘false hand trick’ while their brain waves were measured. To perform this trick, the person is asked to put their arms on the table. Afterwards, one of the arms is covered and exchanged with those of a mannequin. Subsequently, the real arm is stimulated while the mannequin’s arm is touched, and thus the person begins to have the sensation that the mannequin’s arm is theirs. Then, the researcher (or magician, or both) hits the fake hand with a hammer and the person will normally try to remove the hand or may even feel pain.
But some people are much more susceptible to falling for this trick, so that is why the subjects of this research carried the wave measurement instruments. These waves, which are divided into 5 types (alpha, beta, gamma, delta and theta) are the result of the activation frequency of neurons, and for each person they are different. Some produce faster waves and others slower.
More revolutions so that the body does not deceive you
The researchers were able to detect that those with faster alpha waves were harder to fool. That is, for these people to detect the hands as theirs, the researchers had to stimulate both hands with a brush at the same time and be very precise with the movements, otherwise the illusion was broken. In those with slower alpha waves, movements could be much more inaccurate.
Experimental setup and procedure for transcranial alternating current stimulation (tACS). D’Angelo, M., Lanfranco, R.C., Chancel, M. et al. Parietal alpha frequency shapes own-body perception by modulating the temporal integration of bodily signals. Nat Common 1753 (2026).
To add robustness to the hypothesis, the researchers carried out a second experiment in which they stimulated the brains of those people who were easiest to fool through a non-invasive procedure. To do this, they used a tape with electrodes capable of emitting pulses of electricity. So, increased the frequency of the participants’ waves and suddenly they became much more difficult to fool.
With this second experiment, the researchers demonstrated that, indeed, Brain waves are related to how our brain understands our body and draws the boundaries of the ‘self’. A brain rhythm capable of drawing where the body begins and where it ends.
