The Heart-Brain Connection

Brain and heart influence each other massively. Our perception and even our tendency to prejudice change with the heartbeat. Conversely, mental suffering as well as neurological diseases have an effect on heart health.

The 85 men had no idea that they had embarked on an experiment that would later go down in the textbooks of psychology. Part of them crossed Canada's Capilano Canyon on a sturdy wooden bridge. Another group had to cross a swaying suspension bridge to reach the other side. They could only hold on to two wire ropes during this terrifying crossing. To the left and to the right, they had to descend 70 meters.

An attractive woman was waiting in the middle of each bridge. The test subjects were asked to fill out a questionnaire and write a text in her presence. For further questions, the woman offered her private telephone number.

With the experiment, Donald Dutton and his colleague Arthur Aron from the University of British Columbia in Vancouver wanted to find out whether the exciting bridge crossing influenced the perception of and behavior toward the attractive female employee.

Weak knees or butterflies in the stomach?

In 1974, the psychologists published their experiment in the Journal of Personality and Social Psychology - with results that still make us smile today: One-third of the men who encountered that woman on the swaying suspension bridge later called her. After crossing the stable bridge, on the other hand, only eleven percent of the men made contact. Apparently, the test subjects projected their excitement onto the woman and felt attracted to her. This was also revealed by the texts the men wrote: On the wavering bridge, the writing contained more sexual references.

Psychologists refer to the incorrect attribution of a feeling to an event as "misattribution." Behind this is the fact that soft knees and butterflies in the stomach actually feel similar: In both cases, the heart beats harder, the pulse quickens. "Man" may be sweating.

Now, we're not in love every day, and we certainly don't regularly embark on a swaying suspension bridge. But the communication between heart and brain permanently influences our lives.

The ever-same heartbeat is dangerous

In a way, both organs are connected via a dedicated line, the autonomic nervous system. It is called "autonomous" because it cannot be directly influenced by will. It consists of sympaticus and parasympathicus. The two nervous systems are predominantly controlled by the brainstem and act as counterparts.

The sympathicus activates us and provides the necessary physical reactions to attack and flight. For example, it makes our heart beat faster when threatened and increases muscle tone. It makes our face red in an embarrassing situation or our eyes blur frantically during a lecture.

The parasympathetic nervous system, on the other hand, brings us to rest. It slows down breathing and makes the heart beat leisurely. In this way, relaxation and tension are constantly balanced in healthy people. As a side effect, the heartbeat is variable. It constantly adapts to the respective situation. A high heart rate variability is considered a sign of a vital heart-brain connection and ultimately of health. "Rigid heart function, on the other hand, is life-threatening and can lead to sudden death," says neurologist Arno Villringer of the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig.

An image of the heart in the brain

Villringer is interested in fine-tuned communication between the brain and the heart. To this end, researchers in his group analyze the electrocardiogram and brain waves simultaneously. They compare the pattern of electrical excitation in both organs in detail. It is noticeable that certain regions of the brain are activated synchronously with the heart. There is a certain harmony in both organs. This can be measured in the so-called heartbeat-evoked potential (HEP). As a result of the electrical excitation propagation in the heart tissue, which leads to contraction of the heart muscle, an electrical voltage is generated. This can be measured in the brain as excitation. Especially in the insular region of the brain Villringer was able to detect this rhythmic correlation with the heart. It is virtually an image of the pumping heart in the brain.

The neurologist assumes, however, that the HEP says much more. Namely, it is at different levels in different people. "We suspect that the brain has two different modes. When the HEP is high, you focus on your own body, the inner world. In the second mode, the HEP is low: you turn to the outside world, which in the evolutionary context was, for example, foraging and attacking," Villringer says. Another finding fits this interpretation: When the HEP is high, subjects perceive an external electrical stimulus on the finger more weakly. Conversely, they perceive it more intensely when the HEP is smaller. Villringer's team described this in 2020 in the scientific journal PNAS.

How the heartbeat changes thinking and feeling

It is still unclear whether the height of the HEP and thus the size of the image of the heart in the brain depends only on the situation or is also a personality trait. All that is certain is that the influence of the heart on the brain is so pronounced that even the heartbeat has an effect on perception and thinking.

Again, Villringer's team studied how strongly subjects felt an electrical stimulus on their finger. When the heart contracts and pumps blood into the body, i.e. in the so-called systolic phase, they do not feel the stimulus as intensely. The researchers suspect that this could be due to the fact that it is precisely at this moment that receptors in the large blood vessels transmit information about the blood pressure to the brain. The flood of information from these so-called baroreceptors apparently takes quite a lot out of the brain. At this moment, impressions of the outside world reach us only in a muted form. In the following diastole, on the other hand, test subjects register an external electrical stimulus more intensively. In this phase, the heart fills with blood again.

It was not the first finding of this kind. According to Villringer, an experiment by psychologists led by Ruben Azevedo of the University of London is probably the most disturbing result on the influence of the heartbeat. Azevedo showed 30 subjects photos of faces - either black or white men followed by a gun or tool - in rapid succession. As quickly as possible, the subjects were asked to assign whether a light-skinned or dark-skinned man was followed by a tool or a weapon. In the process, the experimenters also recorded the heartbeat. When the heart contracted and blood flowed into the vessels, the participants followed their prejudice significantly more often: they assigned a weapon to a black man. In this phase, the brain is so hogged by the baroreceptors that there is not enough brain left for critical self-reflection, argues Azevedo 2016 in Nature Communications.