The secret of “Body Ownership” has been discovered: alpha waves tell us which parts of the body belong to us. A breakthrough for hi-tech prosthetics and the treatment of schizophrenia.
To build the perception of belonging to our body (body ownership) the brain must continuously integrate visual, motor and tactile signals, so as to discriminate what is ours from what is external. How does he do it? Research published in Nature Communication reveals that one type of brainwave – alpha oscillations – determines how precisely the brain perceives our body as… our.
In addition to telling how a sense of self is constructed, the study could contribute to a better understanding of psychiatric disorders in which the sense of self is altered, such as schizophrenia. And also «contribute to the development of better prosthetic limbs and more realistic virtual reality experiences», explains Henrik Ehrsson, Professor of the Department of Neuroscience at Karolinska Institutet (Sweden) who supervised the analysis.
Timing is everything
Brain waves are rhythmic, repetitive oscillations of electrical activity in the brain. Those in the alpha band, characterized by a frequency ranging from 8 to 13.9 hertz, have long been considered important for visual processing. The scientists found that the frequency of alpha oscillations in the parietal cortex, a region of the brain that analyzes sensory information from the body, was linked to how accurately the brain recognized a body part as its own.
The 106 participants were involved in a great classic of body ownership studies: the rubber hand illusion. In this experiment, when researchers similarly and synchronizedly rub the subject’s real hand, hidden by a screen, and a rubber hand placed in its place, many have the feeling that the artificial hand has become their own. However, when the stimulations are asynchronous, the illusion does not occur. The “time” factor therefore has a fundamental role in the construction of the sense of self in space.
Question of frequency
The scientists analyzed the electrical activity of the participants’ brains using electroencephalography (EEG). Those who showed a higher frequency of alpha oscillations were also more sensitive to the slightest differences in temporal occurrence between what they saw and what they felt. It was as if their brain was capable of working at a more advanced temporal resolution and therefore had a more precise and defined sense of body ownership.
In contrast, a lower frequency of alpha oscillations caused non-synchronized visual and tactile sensations to be more often regarded as synchronized.
This reduced temporal precision led to the juxtaposition of unrelated stimuli and made it more difficult to distinguish sensations relating to one’s own body from external ones.
Missed connections
Using a non-invasive brain stimulation technique to modulate brain waves – tACS (Transcranial Alternating Current Stimulation) – the researchers were then able to slightly speed up or slow down the frequency of alpha oscillations in the participants’ parietal cortex. These alterations changed the precision in which experiences relating to different senses were associated over time, with repercussions on the reliability of the subjects’ sense of body ownership. The conclusion, therefore, is that the frequency of alpha waves appears to influence how accurately the brain evaluates the timing of sensory signals.
