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A recent study suggests that individuals with psychotic disorders process sensations they produce themselves, such as their own touch or heartbeat, differently from people without these conditions. This altered processing appears not only in the brain but also at the level of the spinal cord, potentially affecting the fundamental sense of self. The findings, published in the Molecular Psychiatry, provide a deeper look into the biological underpinnings of self-disturbance in psychosis.
Psychotic disorders like schizophrenia are often characterized by a disrupted sense of self. This can manifest in symptoms like hallucinations or delusions, where individuals might misattribute their own inner thoughts or actions to an outside source. Researchers have long theorized that these complex symptoms may originate from more fundamental difficulties in processing basic bodily signals.
A team of scientists, primarily from Linköping University in Sweden, sought to investigate this idea by examining how the nervous system handles sensations that are self-generated compared to those that come from the external world. Their goal was to use a variety of methods to get a comprehensive picture of self-related processing across different sensory systems.
“Schizophrenia is a complex disorder, and its underlying neurobiological mechanisms are still not understood. Especially, how hallucinations and delusions develop and are maintained remains unclear,” said study author Rebecca Böhme, an associate professor at the Center for Social and Affective Neuroscience at Linköping University.
“We hypothesized that a disturbance in the ability of identify self-produced sensations can underlie these symptoms, for example when the own thoughts are not identified as ‘self-produced,’ then they might cause the experience of voices in the head or being controlled from outside forces. Similar for touch: not identifying self-evoked tactile sensations can cause the feeling of ‘something else’ touching you, which the brain then will try to explain – potentially with a quite irrational story, because the brain always looks for causes to its experiences. It might for example come up with the idea, that an invisible demon is following and controlling you through touch.”
The research team conducted a series of experiments with 35 patients diagnosed with psychotic disorders and 35 healthy control participants who were matched for age and sex. The experiments were designed to measure neural and behavioral responses to both touch and internal body signals.
“A very common misconception is that schizophrenia means having two or more personalities,” Böhme noted. “Schizophrenia is a complicates psychiatric condition, where affected individuals experience symptoms like hallucinations and delusions, but also depression, executive dysfunction, and difficulties in social interactions.”
In one part of the study, participants underwent functional magnetic resonance imaging, or fMRI, which measures brain activity by detecting changes in blood flow. While in the scanner, they were asked to perform or receive gentle strokes on their left forearm. The conditions included touching their own arm, being touched by an experimenter, and touching a pillow as a control for movement.
The results indicated that when participants with psychosis touched themselves, a brain region known as the right superior temporal gyrus showed significantly higher activation compared to the control group. In healthy individuals, the brain tends to show reduced activity in response to self-touch, a phenomenon thought to occur because the sensation is predictable. The heightened activation in the patient group suggests there may be a mismatch between the brain’s prediction of the sensation and the sensory information it actually receives.
To investigate sensory processing at an even earlier stage, the researchers used a technique to measure somatosensory evoked potentials. This method involves delivering small, non-painful electrical pulses to a nerve in the hand and then recording the speed and strength of that signal as it travels up the spinal cord and into the brain. These measurements were taken during different conditions, including self-touch and other-touch.
In the control group, there was a measurable difference in the timing of the signal at the spinal cord level between self-touch and other-touch. For the patient group, this distinction was significantly smaller, providing evidence that the ability to differentiate between self and other may be altered at a very basic level of the nervous system.
A behavioral experiment provided further support for these neural findings. The researchers measured participants’ tactile thresholds, or the lightest touch they could feel, using a set of fine filaments. This test was also conducted during self-touch and other-touch.
Healthy controls tended to be less sensitive to the filaments while touching themselves, consistent with the idea that the brain dampens the perception of predictable, self-produced sensations. The patients with psychosis did not show this difference in sensitivity between the two conditions, suggesting an alteration in this sensory filtering mechanism.
The researchers also explored interoception, which is the sense of the internal state of the body. Participants performed a heartbeat detection task where they tried to tap a button in sync with their own heartbeat without feeling their pulse. The patients were found to be less accurate in this task compared to controls. Both groups performed equally well on a control task where they tapped along to a recorded heartbeat, indicating that the difficulty was specific to perceiving internal signals and not related to general attention or motor coordination.
Finally, the researchers measured heartbeat-evoked potentials, which are the brain’s electrical responses to the signals from the heart. The analysis showed that patients with psychosis had a reduced brain response to their own heartbeat signals.
Together, these interoceptive findings point to a broad disruption in the processing of self-generated internal signals, which are essential for maintaining a stable sense of one’s own body. The researchers also found that the degree of alteration in touch-related measures was associated with the severity of certain symptoms, and the brain activity during self-touch was a strong predictor of group membership.
The results show “that it is crucial for all of us to be able to differentiate between ‘self’ and ‘other,'” Böhme told PsyPost. “This basic self-other-distinction forms the basis of our self-experience. If this ability is altered or disrupted in some way, also our higher order sense of self will be affected.”
“Schizophrenia, the condition we studied here, is an example of such an alterations. It has been suggested before that schizophrenia can be understood as a disorder of the self. The other key take-away is that this difference is not only sensed in the brain but already affects earlier processing, like in our study the neural processing of self- or other-touch in the spinal cord.”
But the study, like all research, has some limitations. The patient sample was taking medication, which could potentially influence sensory processing, although the researchers conducted additional analyses that did not suggest a clear medication effect. The patients also had relatively low levels of active symptoms and included a mix of different psychotic disorders.
The researchers suggest that future work could examine individuals at earlier stages of the illness or before they begin treatment to see if these findings hold. Investigating these mechanisms further could open new avenues for therapies aimed at correcting the fundamental sensory and self-processing disruptions seen in psychosis.
“My lab studies the sense of self in many different psychiatric conditions and life situations,” Böhme said. “We have also investigated self-other-distinction for example in autism, ADHD, and anorexia. In my newest study, I will investigate how the sense of self is altered in times of grief, i.e. when losing a loved one, and whether people with complicated grief can be better supported using the psychedelic substance psilocybin.”
The study, “Altered processing of self-produced sensations in psychosis at cortical and spinal levels,” was authored by Paula C. Salamone, Adam Enmalm, Reinoud Kaldewaij, Marie Åman, Charlotte Medley, Michal Pietrzak, Håkan Olausson, Andrea Johansson Capusan, and Rebecca Boehme.
