New research has revealed that a very specific form of electrical and magnetic brain stimulation can directly change people’s perception of whether they are in control of their actions. By targeting different areas on the right side of the brain, the researchers were able to improve or decrease the ability to detect external interference during computerized tasks. The results of this study were published in the journal NeuroImage.
Imagine that when you trace a straight line with a pen, the ink appears one inch to the left. Your brain immediately recognizes that the visual result does not match the intended body movement. This basic awareness is a fundamental part of human cognition, called the sense of agency. It represents the internal subjective experience that creates and controls behavior in the real world.
To process this experience, the brain uses a continuous and automatic monitoring system. When a motor area sends a command to a muscle, it simultaneously sends a blueprint for a replica of the expected sensory outcome to other brain areas. When the physical sensations transmitted by your eyes and skin match your predictive blueprint, you experience a seamless sense of personal control. When a mismatch occurs, the brain immediately flags the action as coming from an external source.
Previous neuroimaging studies have repeatedly pointed to a specific area near the back and sides of the brain, called the right inferior parietal lobule, as the likely home of this discrepancy detection. Different types of rhythmic electrical activity appear to coordinate these sensory comparisons across different lobes. However, observational data alone cannot prove that brain regions directly control specific behaviors.
To test whether the right inferior parietal lobule actively evokes this sense of control, the researchers investigated whether manipulating its electrical rhythms altered a person’s ability to perceive external interference. The study was led by Ondrzej Bečev, a scientist at the Czech Republic’s National Institute of Mental Health, together with an interdisciplinary group of medical and technical researchers.
The researchers designed their study around two sets of behavioral experiments involving dozens of healthy adult volunteers. During each session, participants used a standard computer mouse to move a cursor around various digital obstacles for several minutes at a time. Subjects were informed that scientists sometimes use mobile applications to interfere with their cursor’s path on the Internet.
In reality, computer algorithms were responsible for subtle changes in trajectory. The program periodically changes the angle of the cursor, manipulating it slightly away from the participant’s actual physical hand movements. Participants were required to report by pressing a button when they perceived a discrepancy between their intentions and the behavior of the cursor on the screen.
In the first group of experiments, the researchers applied a method known as transcranial alternating current stimulation. This technique involves sending a weak electric current through soft electrodes that are placed gently on the scalp. The purpose of this stimulation is to encourage local groups of brain cells to fire in rhythm and synchronize at a precise rate chosen by the researchers.
The device was set to emit electrical current at 60 cycles per second, a rate intended to mimic the high-frequency brain waves typically associated with detecting paresthesias. The results of this first experiment confirmed the team’s suspicions about the purpose of the brain region. When rapid current was applied to the right inferior parietal lobule, participants were better able to detect when the computer had hijacked the cursor.
By artificially boosting activity in this brain region, the researchers effectively strengthened the participants’ internal alarm system. Subjects became highly sensitive to non-self-agency, that is, to the presence of external interference. This provided the sought-after causal relationship between targeted brain tissue and realistic sensory perception.
In the second phase of the study, the researchers used a different stimulation tool called repetitive transcranial magnetic stimulation. Instead of passing an electric current, this device relies on a special wand held directly above the head. The tool emits powerful magnetic pulses that penetrate the skull and temporarily disrupt or weaken the normal firing habits of neurons in a localized area.
The scientific team applied these magnetic pulses at different rates, ranging from 10 to 20 cycles per second. After receiving the stimulus, volunteers immediately engaged in the same cursor tracking exercise. Because magnetic stimulation has long-lasting effects, researchers were able to assess temporary changes in participants’ hand-eye coordination and subjective awareness.
The scientists also placed special sensor caps on participants’ heads and recorded their natural brain electrical activity during gameplay. This monitoring technique is known as electroencephalography. This allowed the researchers to measure exactly which frequencies changed while participants struggled or succeeded in identifying covert computer interference.
Magnetic disruption caused completely opposite behavioral effects compared to alternating current. After the high-frequency magnetic pulse, participants had a hard time noticing when the on-screen cursor moved away from their body movements. Overall accuracy decreased and subtle spatial deviations introduced by automatic algorithms were often not noticed.
By altering the brain’s rhythmic activity in two opposing ways, researchers have built strong evidence that this region actively manages our sense of agency. This area acts as a low-level automatic mismatch detector. Rather than consciously thinking about self-image, this part of your brain tissue simply functions as a biological comparison engine.
Electronic brain wave recordings also provided unexpected insights into how this engine works. Researchers initially thought that a type of rapid brain rhythm called gamma waves might be altered primarily by the damping effect of magnetic stimulation. Instead, brain recordings showed slightly slower rhythms known as beta waves that change primarily during periods of decreased perceptual ability.
Additionally, a slow rhythm called theta waves seemed to synchronize whenever participants were dealing with non-self-intrusions. These electrical signatures may represent the brain’s active attempt to suppress conflicting information during sensory mismatches. Researchers suggest that these specific brain waves act as a communication medium between sensory comparison areas and motor areas that plan the body’s next movement.
As with all scientific investigations, the current study has certain limitations. The experimental computer task turned out to be easy for the participants in several ways, creating a statistical ceiling effect. Almost all volunteers perfectly identified the moment when they had full control of the cursor. This made it mathematically difficult to measure potential improvements in detecting pure self-agency.
EEG recordings also revealed that the residual effects of magnetic stimulation disappeared very quickly. Changes in brain activity were seen 5 min after the end of the stimulation session but completely disappeared by the 9 min time point. This narrow operating window makes it difficult to see how the brain adapts to loss of agency over long periods of time.
In a healthy brain, identifying sensory discrepancies is usually an instantaneous and imperceptible process. However, in individuals with certain neurological conditions, the neural mechanisms that process these mismatched signals may function abnormally. Patients may begin to act, but when their internal communication loops are disrupted, they feel completely disconnected from the movement of their limbs.
Understanding the precise biological basis of agency has enormous relevance for clinical medicine. Several mental illnesses, from schizophrenia to alien hand syndrome to obsessive-compulsive disorder, cause patients to feel that their behavior is controlled by outside forces. Identifying the mechanistic causes of this dysfunction in the brain could ultimately pave the way for targeted therapeutic interventions.
Future research projects should extend these findings by investigating how the right inferior parietal lobule sends error signals back to other executive brain regions. Scientists want to investigate whether individuals can learn to improve their own discrepancy detection without the need for external electrical stimulation. They propose using biological feedback techniques to train people to consciously control their own relevant brain wave patterns.
The study, “High-frequency neural stimulation of the right inferior parietal cortex alters the sense of agency: Results from tACS/tRNS and rTMS-EEG studies,” was co-authored by O. Bečev, O. Laskov, E. Bakštein, J. Štrobl, J. Hubený, N. Biačková, N. Schlezingerová, T. Novák, P. Mohr, and M. Krylová.
