The brain’s alpha-frequency rhythms in the parietal cortex help determine which sensory signals belong to the body and which do not. EEG recordings from 106 volunteers showed faster alpha waves improved timing discrimination and reduced susceptibility to the rubber-hand illusion. Noninvasive stimulation in 30 participants causally shifted alpha frequency and changed both timing judgments and body-ownership reports. Bayesian modeling suggests the effect reflects changes in sensory uncertainty rather than prior beliefs.
How Brain Rhythms Define the Boundary Between Self and World
Lead image: Stranger Man / Shutterstock
How does the brain decide where your body ends and the outside world begins? New research from Karolinska Institutet suggests the answer lies in the brain’s alpha rhythms. These rhythmic electrical oscillations, especially as they pass through the parietal cortex—a region that processes touch and body position—help the brain judge the timing of incoming sensory signals and so shape our sense of bodily ownership.
Timing, Illusions, and Alpha Waves
The familiar rubber-hand illusion demonstrates how fragile the body–world boundary can be: when a hidden real hand and a visible rubber hand are stroked in synchrony, many people begin to feel the rubber hand is their own. Small timing shifts between the touches quickly dissolve that feeling. Researchers recorded EEG from 106 volunteers while they completed variations of the rubber-hand test and separate timing-discrimination tasks. They found that individuals with faster alpha-wave frequencies were better at detecting small timing differences and were less likely to adopt the rubber hand as part of their body; slower alpha rhythms were associated with blurrier distinctions.
From Correlation to Causation
To test causality, the team ran a second experiment with 30 participants and used gentle, noninvasive brain stimulation targeted to the parietal cortex to speed up or slow down alpha activity. These manipulations changed both participants’ timing judgments and their susceptibility to the rubber-hand illusion, demonstrating a causal role for alpha frequency in shaping body ownership.
“We have identified a fundamental brain process that shapes our continuous experience of being embodied,” said lead author Mariano D’Angelo from Karolinska’s department of neuroscience. Co-author Henrik Ehrsson added that the findings help explain how the brain integrates bodily signals to construct a coherent sense of self.
Computational Insight
The researchers applied Bayesian modeling to interpret the results. The models indicate that changing alpha frequency alters the brain’s uncertainty about the source of sensory signals—not people’s underlying beliefs. Faster alpha rhythms appear to produce cleaner, more reliable timing signals; slower rhythms increase temporal noise and uncertainty about whether a stimulus originates from one’s own body.
Practical Implications
These results may shed light on psychiatric conditions, such as schizophrenia, where disturbances in the sense of self are common and where slower alpha rhythms have been observed. The findings also suggest practical steps for improving prosthetic feedback and virtual-reality experiences: precise timing of multisensory feedback should increase the sense that an artificial limb or virtual body belongs to the user.
In short: timing matters. The alpha rhythm of the parietal cortex helps the brain decide what belongs to us and what does not—so, at least in part, we are the rhythm we keep.
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