When body and brain communicate

In December 1955, a sensational article appeared in the journal BRAIN in which Canadian neurosurgeon Wilder Penfield described experiments he and a colleague had performed on epilepsy patients. The patients underwent a risky procedure: they had the regions of the brain where the seizures originated cut out. At the time, the surgeries were performed under partial anesthesia, so that electrical stimulation of the brain could be used to narrow the seizure focus. This came in handy for Penfield, who took meticulous notes on how patients responded to stimulation of different brain structures. In the case of one structure, a strange thing happened: the stimulation did not cause any part of the body to move or to feel touched. Instead, the patients felt the stimulation inside them. Some reported strange feelings in their abdomen or a scratching sensation in their stomach; others felt dizzy or nauseous, and still others experienced intestinal cramps or flatulence.

he region stimulated by Penfield is called the insula. It is located below the temples, partly hidden in a fold of the cerebral cortex. The structure is barely larger than a two-euro coin, but it performs a variety of tasks, as is now known. One of them is the processing of signals from inside the body. In technical jargon, this is called "interoception". This internal perception is extremely important. For example, when we are hungry, we know it is time to eat, and when our bladder is full, we know it is time to go to the bathroom. So interoception is a very basic process without which we could not survive. People are not aware of all interoceptive signals: breathing, for example, is largely controlled automatically, even when we are asleep or unconscious. To do this, nerve cell clusters in the brain stem continuously evaluate readings from various sensors in the body, some of which are located in the blood vessels and others in the brain itself.

Interoception helps the body maintain the physiological framework it needs to function. This has been known for 150 years. But for a long time, science paid little attention to the subject. That has changed: Over the past two decades, research has revealed more and more about everything related to body sensation, from eating disorders to autism to depression. Today, people are convinced that interoception is a prerequisite for emotional feeling. The thesis that emotions have physical roots was formulated by the American psychologist William James as early as 1880. He saw physical changes not as the result of emotional experience, but as its cause: "We don't have a lump in our throat because we are sad, but we are sad because we have a lump in our throat.

Portuguese neuroscientist António Damásio developed this theory in the 1990s at the University of Iowa. According to Damásio, emotions are always accompanied by physical reactions. Every emotional experience (for example, when we encounter a dangerous animal) activates certain structures in the brain, such as the amygdala in the case of danger. The amygdala puts the rest of the body on alert by releasing hormones and through nerve pathways: the heart beats faster, muscles tense, blood sugar is released, and blood clotting increases. Feelings arise only when we notice these physical changes.

If this is true, it is important for the experience to be able to feel the state of the body: How fast is the heart beating? How does the stomach feel? It is now known that people are able to do this to very different degrees. This is called interoceptive accuracy, or perceptual accuracy. This means that we can measure it, for example, by asking people to focus on their heartbeat under standardized conditions and count it without feeling their pulse. Other methods measure the perception of hunger and satiety signals.

Two decades ago, researchers led by Stefan Wiens at the State University of New York conducted a widely cited experiment to investigate the relationship between perceptual accuracy and emotional experience. They first assessed students' interoception with the heartbeat test described above. Then they played emotional movie clips to their subjects. Students with high interoceptive accuracy reacted more emotionally than others. This is consistent with a study on depression that pooled data from six studies. According to the study, depressed people have less insight into their bodies. The weaker their internal perception, the weaker their perception of positive emotions and the more difficult it is for them to make decisions.

Without our physical feedback, we would not be able to behave appropriately in many situations. From an early age, we learn that when we are threatened, our hands sweat and our heart beats up to the neck. António Damásio calls these symptoms "somatic markers". They help us make the right decisions in complex situations: Is the pulse rate skyrocketing? Danger is imminent, so it's better to get out of here quickly. The gut tells us where to go without conscious thought.

Our current understanding is that the brain is constantly comparing internal and external stimuli and making an assumption about the current situation. This assumption may not be correct. Your heart may be racing not because you are in danger, but because you just climbed the stairs. The fact that your stomach is queasy may mean that you are hungry or that you are nervous. At some point, the brain realizes whether it was right. If not, it adjusts the hypothesis so that the mistake will not be repeated in the future. This theory is called predictive coding. According to this theory, the human brain functions much like an adaptive software program that is fed hundreds of thousands of photos and later recognizes from new pictures whether a four-legged thing is a table or a cat. The more flawlessly this works - the smaller the prediction error - the better.

The consequences of misinterpreting physical signals can be seen, for example, in panic disorder. Sufferers notice a physical change (an increase in heart rate, a tingling sensation in the limbs) but cannot find a plausible explanation for it. They then interpret it as a sign of a threatening physical problem. That's the start of a vicious cycle: they get scared, their heart beats even faster, their stomach cramps, they start to sweat. All of this reinforces their initial assessment that something is wrong. In a short time, the reaction escalates into a panic attack. This is so uncomfortable that the person listens to their body even more closely in the future, increasing the risk of another attack. However, it is not the case that people with panic disorder generally have above-average interoception skills. They are particularly sensitive to potentially anxiety-provoking physical stimuli. In cognitive behavioral therapy, patients learn to correct the misinterpretation of internal signals - for example, by no longer avoiding the attacks but allowing them to occur without distraction. In this way, they learn that the sensations are unpleasant but not dangerous.

Interoception has many facets. One is accuracy, which can be measured objectively, for example with heartbeat tests or methods that manipulate the feeling of fullness in the stomach. Then there is interoceptive awareness: an idea of how good we are at detecting signals from inside our bodies. It is sometimes very different from reality: there are people who score rather poorly on interoceptive awareness tests and yet are rock-solid convinced that they feel their bodies correctly. This discrepancy between subjective and objective interoceptive ability is a kind of measure of prediction error: We think we perceive something in our body, but it is not there in that form.

In addition, people differ in how they emotionally evaluate perceived bodily signals, for example, whether they become anxious when their stomach clenches or simply take note of it without further action. Interoception disorders can affect any dimension. How these facets interact in different mental illnesses is not yet well understood. This is the subject of current and future research.

However, some correlations are known. For example, overweight people often do not accurately perceive the distension of their stomach and have a reduced sense of satiety. In anorexia, interoceptive accuracy is also reduced. But what is most striking is the very negative emotional evaluation of the interoceptive signals. That is, they reject their body and its needs. Whether these changes are a cause or a consequence of the disease is still unclear.

Research is now trying to normalize interoceptive abilities specifically in mental disorders, for example through mindfulness meditation or stimulation of the vagus nerve, which in turn influences interoceptive networks in the brain. Special massages that stimulate certain nerve fibers in the skin, the CT afferents, are also promising. They could also be called stroke sensors. The sensors respond specifically to very gentle, slow touches of the skin. Studies show that massages specifically designed to stimulate them can effectively relieve anxiety and depressive symptoms in depression. How this happens is still unclear. However, it is possible that gentle massage normalizes the function of the insula and thus the processing of interoceptive stimuli. This in turn could improve body awareness.

Actually, touch is considered to be an external, or exteroceptive, stimulus. However, CT afferents very effectively activate certain areas of the insular cortex, the brain structure that Wilder Penfield studied in epilepsy patients and that processes signals from inside the body. His work is now some 65 years old. However, we in the research community are only beginning to understand the importance of the insula for physical and mental health.