In December 1955, a sensational article appeared in the journal BRAIN, in which the Canadian neurosurgeon Wilder Penfield described experiments he had conducted with a colleague on epilepsy patients. The patients underwent a risky procedure: they had those regions of the brain from which the seizures originated cut out. At the time, the operations were performed under partial anesthesia so that the seizure focus could be narrowed down with the help of electrical stimuli in the brain. This came in handy for Penfield, who took meticulous notes on how the patients reacted to the stimulation of various brain structures. In the case of one structure, something strange happened: the stimulation did not result in the movement of any part of the body or the feeling of being touched anywhere. Instead, the patients felt the stimulation inside them. Some reported strange feelings in their abdomen or a scratching in their stomach; some felt dizzy or nauseous, and still others experienced intestinal cramps or flatulence.
The region stimulated by Penfield is called „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 piece, 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, we also speak of „interoception“. This internal perception is immensely important. For example, when we are hungry, it is time to provide energy replenishment, and when our bladder is well-filled, we should look for a toilet. Interoception is thus 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. For this purpose, nerve cell assemblies in the brainstem 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 to maintain the physiological framework it needs for its functions. Basically, 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 is revealing 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 feelings have physical roots was formulated by the U.S. psychologist William James as early as 1880. He regarded 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.
The Portuguese neuroscientist António Damásio decisively developed this theory in the 1990s at the University of Iowa. According to Damásio, emotions are always accompanied by physical reactions. With every emotional experience (for example, when we encounter a dangerous animal), certain structures in the brain become active, such as the amygdala in the case of danger. The amygdala puts the rest of the body on alert by releasing hormones and via the nerve pathways: the heart beats faster, the muscles tense up, blood sugar is released, and blood clotting is ramped up. Feelings therefore only arise when we perceive these physical changes.
Interception is measurable
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 right now? How does the stomach feel? It is now known that people are capable of this to very different degrees. In this context, we speak of interoceptive accuracy or perceptual accuracy. This means that we can measure it, for example, by asking people to concentrate on their heartbeat under standardized conditions and count it without feeling their pulse. Other methods record the perception of hunger and satiety signals, for example.
Already two decades ago, researchers around Stefan Wiens at the State University of New York investigated in a much-cited experiment how perceptual accuracy and emotional experience are related. They first assessed the interoception of students with the heartbeat test described above. Then they played emotional movie clips to their subjects. Students with high interoceptive accuracy responded more emotionally than others. This fits with a study on depression that summarizes data from six studies. According to the study, depressives have less insight into the inside of their bodies. The weaker their inner perception, the weaker their perception of positive emotions and the more difficult it is for them to make decisions.
Gut feeling tells us where to go
Without our physical feedback, we would not be able to behave adequately in many situations. From an early age, we learn that when we are threatened, our hands sweat and our heart beats up to our necks. António Damásio also calls these symptoms „somatic markers“. They help us to make the right decision in complex situations: Pulse rate skyrocketing? Danger is looming, so it’s better to get out of here fast. The gut feeling tells us where to go, without the need for conscious deliberation.
Our knowledge today says that the brain constantly compares internal and external stimuli and derives an assumption about the current situation. This assumption is not necessarily correct. Perhaps the heart is racing not because of danger, but because one has just climbed the stairs. The fact that the stomach feels queasy can mean that someone is hungry or that the person is nervous. At some point, the brain realizes if it was right. If not, it adjusts the hypothesis so that the mistake is not repeated in the future. This theory goes by the name of predictive coding. According to this theory, the human brain functions in a similar way to 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 error-free this works – the smaller the prediction error – the better.
The consequences of misinterpreting physical signals can be seen, for example, in panic disorder. Those affected 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 this as an indication of a threatening physical problem. That’s the start of a vicious circle: they get scared, their heart beats even faster, their stomach cramps, they start to sweat. All of this promotes their initial assessment that something is wrong. In a short time, the reaction escalates into a panic attack. This is then so unpleasant that the affected person listens even more closely to their body in the future, increasing the risk of another attack. However, it is not the case that people with panic disorder generally have an above-average interoception ability. They are particularly sensitive to potentially anxiety-provoking physical stimuli. In cognitive behavioral therapy, sufferers learn to correct the misinterpretation of internal signals – for example, by no longer avoiding the attacks but letting them happen without distracting themselves. In this way, they learn that the sensations are unpleasant, but not dangerous.
Interoception has several facets. Accuracy is one of them; it can be measured objectively, for example with heartbeat tests or with methods that manipulate the feeling of fullness in the stomach. In addition, there is interoceptive awareness: an idea of how good we are at detecting signals from inside our bodies. It sometimes differs greatly from reality: there are people who score rather poorly on interoceptive awareness tests and yet are rock-solidly convinced that they feel their bodies correctly. This discrepancy between subjective and objective interoceptive ability is a kind of measure of the prediction error: We believe we perceive something in our body, but it is not there in this form at all.
In addition, people differ in how they emotionally evaluate the perceived body signals, for example, whether they react fearfully to every clenching of the stomach or whether they simply take note of it without letting it influence them further. Interoception disorders can affect any dimension. How these facets interact in different mental illnesses is not yet well understood. That is the subject of current and future research.
Gentle massages improve interception
Some correlations are known, however. For example, overweight people often do not correctly perceive the distension of their stomach and have a reduced feeling of satiety. In anorexia, interoceptive accuracy is also reduced. But what is particularly evident in sufferers 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 of the disease or its consequence is still unclear.
In research, we are now trying to specifically normalize interoceptive abilities 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 stroking sensors. The sensors react 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. In what way that happens is still unclear. However, it is quite possible that the gentle massage normalizes the function of the insula and thus also the processing of interoceptive stimuli. That, in turn, could improve body awareness.
Touch is actually considered an external, i.e. exteroceptive, stimulus. However, the CT afferents very effectively activate certain areas of the insular cortex, the brain structure that Wilder Penfield studied in epilepsy patients and which processes the signals from inside the body. His work now dates back some 65 years. However, we in research are only gradually beginning to understand the importance of the insula for physical and mental health.