Pain - When it hurts

December 11, 2020

Almost one in five people in Germany suffers from chronic pain. Yet, we lack a complete understanding of where the complaints come from, which sometimes seems to come out of nowhere. It does seem clear however, that expectations play a role. Various scientists at the Max Planck Institute for Human Cognitive and Brain Sciences (MPI CBS) in Leipzig are investigating how these expectations work and how they can be influenced. They’re asking questions such as, “How can two stimuli of identical strength be perceived, at different times, as differing in terms of pain.”

Slicing tomatoes and cucumbers for salad you accidentally cut into your finger. It hurts. The classic cascade of pain is set in motion: receptors in the skin are activated, action potentials are created that run along the nerves and migrate to the spinal cord. There they are processed, sent to the brain, and finally transmitted to the cerebral cortex via various levels. The affected finger is pulled out of the danger zone, it is spared, and the pain is relieved.

Actually, according to Falk Eippert, head of the "Pain Perception" research group at the Max Planck Institute for Human Cognitive and Brain Sciences, pain is nothing more than another form of sensory perception, similar to hearing and seeing. There are pain receptors in the skin, organs, and muscles, which provide the brain with information about the stimulus. In contrast to the other senses, however, it contains an emotional, usually negative, component in addition to the sensory component. "Pain is actually never neutral," says Eippert.

The interesting thing is that the subjectively perceived pain does not always correspond to how strong the incoming signal actually is. Physically identical stimuli can sometimes be perceived as less, or sometimes as more painful. According to Eippert, expectations play an important role in this. This has a lot to do with how we perceive the world. "When we walk through the world, we 'expect' something that happens next, usually unconsciously," he explains. By constantly learning from our surroundings, we can often deduce from the current situation what will happen next. Perception is therefore not just something passive that results from external stimuli. It also feeds on existing experiences - and the expectations derived from them.

Pain has a lot to do with expectations

In terms of pain, this means that it is composed of signals that actually occur, for example through a painful movement, and those that are generated on the basis of an expectation, for example, the assumption that a stinging sensation will be felt when taking the next step. Eippert and his team therefore want to find out what contribution both components make to the actual sensation. The hope is that the results can be transferred to chronic pain and possibly used to develop more targeted therapies for those affected. Depending on the specific situation, some individuals may need to focus more on mechanical issues whereas others may need to address their expectations.

In fact, it has already been shown that the brain can process both types of signals separately. Some areas are mainly concerned with the intensity of the incoming stimulus, i.e. the physical information. Others are mainly concerned with the difference between the predicted pain and the pain actually experienced. These regions become active when a strong sting is expected but does not occur – or vice versa.

To investigate how pain is perceived and processed, MPI researchers use behavioral tests, among other things, to measure the difference between expected and actually occurring signals. Strong and slightly painful heat stimuli are used to do this, with different probabilities. For example, if a participant sees a blue screen, a small plate on her arm will give her a strongly painful stimulus, whereas a red screen will only give her a slightly painful stimulus. An association between "blue" and "strong pain" as well as "red" and "slight pain" is created. Thus, an expectation is formed for each of the colors. When the screen turns blue again, a corresponding network of neurons activate before the heat even arrives. Now, if blue screen is suddenly and covertly paired with the low intensity stimulus they normally get with a red screen, the participant experiences higher pain than they should. The difference is due to expectation, which can be quantified.

The spinal cord: The underestimated switching point for pain

The interesting thing is that these two types of information — physical and expected — are probably not first separable in the brain, but possibly already in the spinal cord. This is the first central processing point for pain prior to reaching the head region. If, according to Eippert's view, the signals could be changed at this point (the spinal cord), it would be more efficient than at higher levels where the information has already spread out across the brain.

Although it is assumed that the spinal cord is the first place where pain can be inhibited, less is known about this evolutionarily very old structure, thumb-thick, and composed of nerve cells and fibers in the spinal column. Why? It is difficult to examine. Nikolaus Weiskopf, director of the "Neurophysics" department at the MPI CBS explains in an interview why this is the case and which previously unknown insights science has nevertheless recently gained with the help of a new technology.

Music and sport against fear

Lydia Schneider also knows how decisive expectation is in dealing with pain. The doctoral student is concerned with how the fear of pain influences people’s motivation to move. "Patients who suffer from chronic pain go into a situation with a clear idea of what will be painful," she explains, "They’re afraid that when they move, the suffering will become even worse.” This is one of the reasons why they often lack the motivation to engage in physical activity.

But doing so can be disastrous. Numerous studies have shown that exercise is one of the most important remedies for chronic pain, such as prolonged back pain. Schneider is therefore looking for ways to reduce this anxiety. She is testing the influence of “Jymmin” — a mix of sport and free musical improvisation (jammin) developed at the MPI CBS.

When Jymmin, movements made on fitness machines are translated into musical sounds. In this way, different sounds are created by varying degrees of muscle contraction and movement. Essentially, the participants can play the machines like musical instruments. Composition software processes the movement in real time such that individualized accompanying music is created for each participant and each training session. This turns you into a composer, and the exercise devices into your instruments.

According to a previous study by the group, the fitness method changes the perception of pain in healthy persons. It raises pain tolerance and thus makes the participants less sensitive to physical pain. As a result, after the Jymmin, they were able to withstand significantly longer durations of pain than in a conventional training session, in which the music passively resounded from the speakers.

However, it was not clear, until now, whether this could also be transferred to pain patients. Could Jymmin be a method to reduce their fear of movement, even if it is very strong? The test showed that it really works. Especially in those who were particularly afraid of exercise. Jymmin reduced their fear more than those who were less afraid. One possible reason: Schneider suspects that the affected people focused their attention on something apart from the pain. "One suddenly concentrates on the fact that one's own body produces music." This also strengthens the feeling of being able to influence things yourself. This so-called self-efficacy increases well-being in general, and the effect is particularly strong in pain patients. According to the neuroscientist in training, the fear of losing control plays an enormous role for them. Jymmin works against this. "Suddenly the patient has the feeling of being able to influence things again, of being self-effective.”

Emotions also influence pain

But expectations are not the only thing to have a decisive influence on the perception of pain. Emotions do as well. Anger, especially suppressed anger, intensifies the suffering. This condition occurs particularly frequently in pain patients. The constant suffering and the associated restrictions in everyday life are frustrating. Women are more often affected by it than men. Schneider therefore wanted to find out how different moods in music affect the perception of pain in women. The idea behind it: Is it possible to increase the pain-relieving effect of music by producing the style that corresponds to one's own mood? From an earlier study with heavy metal music, it was known that aggressive songs in particular can dissolve anger.

The results are encouraging. "If one listens to aggressive music after provoked rage, pain tolerance rises, but not with more cheerful music," said Schneider about the preliminary results. The reason for this could be the emotional correspondence between what is heard and one's own feelings, which could have a beneficial effect. "With annoyance it is particularly important to be able to express it, otherwise it accumulates and affects the body and pain sensation in the long term.” Through music, one can express one's own feelings without becoming violent.

For her research, however, the scientist did not use heavy metal but classical music. She chose classical pieces that were stimulating and used the same instruments, but differed in their mood, one of them aggressive and the other one cheerful. In this way she wanted to ensure that patients' reactions could be easily compared. "Of course, the person's taste in music also plays a role."

She recorded their annoyance using a standardized questionnaire, the so-called Spielberg annoyance inventory. This assesses, on the basis of individual questions, how angry a person is generally, but also at the moment. "One problem, however, is that social desirability plays a large part in this. Women in particular often don't specify how angry they actually are about something," Schneider says. This is one of the challenges in the studies. Only when you know how angry someone really is can you relate it to effects of different moods in music. That's why, says Schneider, we are currently evaluating the physical signals of our study participants, such as heart rate variability. Like Eippert, she also hopes that her research will ultimately help those affected by chronic pain. "If we know more about how music and movement alleviate pain, it may be possible to replace many medications and at the same time achieve many other positive side effects.

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