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Cognition: Thinking has its price

22.8.2022

Translation: machine translated

Mental exertion changes brain metabolism, a new study shows. A potentially toxic messenger substance accumulates in the nerve tissue.

Whoever works hard physically feels tired and exhausted afterwards. Demanding mental activities have the same effect - even though they are usually done in a sedentary position and without any significant use of muscles. Why? A research team led by Antonius Wiehler of the Paris Brain Institute has now found answers. Intensive thinking causes a substance to accumulate in the brain that can disrupt nerve metabolism and is toxic in high concentrations. The group reports on this in the scientific journal "Current Biology".

Wiehler and his team had 40 experimental participants work at a computer screen. The subjects were shown letters every second, which they had to classify according to various characteristics - such as colour or whether they were vowels or consonants. Sometimes they had to indicate in memory tests whether the letter shown had been seen shortly before. 24 participants had to cope with difficult versions of these tasks, the remaining 16 enjoyed easy ones. To exhaust themselves to the point of fatigue, the test subjects completed several thousand tests a day. This kept them busy for around six and a half hours, which is equivalent to a strenuous office job.

The longer the participants sat at the screen, the more exhausted they felt and the more mistakes they made. At the end of a day of testing, most of them said they were very exhausted. And that was regardless of whether they had solved difficult or easy tasks.

Fast money, slow money

From time to time, the research team had the subjects choose between two amounts of money: a small sum that could be obtained with little effort or after a short wait, and a larger amount that required more effort or a longer wait. In this way, the scientists tested how much discipline the participants showed. Those who struggled with difficult tasks, i.e. worked harder mentally, voted significantly more often for the small reward that was easier to obtain. A sign of fatigue and diminishing self-conquest.

While they were doing the tests, the subjects had themselves examined from time to time by magnetic resonance spectroscopy (MRS). This procedure allows the concentration of selected types of molecules in the nervous system to be measured. In participants who were mentally active, they found that the substance glutamate accumulated in the front cerebral cortex. The brain areas located there work to regulate emotions and are significant for self-control and action planning and control.

Glutamate is a substance found in the anterior cerebral cortex.

Glutamate is one of the most important excitatory neurotransmitters in the brain. It is also involved in the detoxification of ammonia, serves as a precursor for other messenger substances and has a toxic effect in higher concentrations. Therefore, the organism must precisely regulate the glutamate balance in the brain. In excessive amounts, the substance disrupts cell metabolism, impairs neuronal information transmission and, in extreme cases, leads to signs of poisoning.

Wiehler's team suspects that the accumulation of glutamate in the front cerebral cortex, while the thinking organ is running at full speed, distorts the brain metabolism and makes it increasingly difficult to activate the nerve cells there. This is accompanied by symptoms of fatigue and exhaustion and restricts self-control.

Can this effect be reversed?

Can this effect be prevented? "Not really, I'm afraid," says brain researcher Mathias Pessiglione, a colleague of Wiehler. He recommends rest and sleep to renew mental capacity. "There is convincing evidence that the accumulated glutamate is broken down again during sleep."

Fritjof Helmchen of the Brain Research Institute at the University of Zurich, who was not involved in the work, finds the study convincing: "That the glutamate balance in the brain is important and has to do with exhaustion has been suspected for a long time; the studies by Wiehler and his group now prove that."The messenger substance plays a central role in brain activity and nerve metabolism; its concentration must therefore always remain finely balanced. It is not clear whether an increased glutamate content causally leads to dwindling nerve activity in the anterior cerebral cortex. But the study shows that the two are connected. "How the system regenerates again is an exciting question," says Helmchen, "especially whether sleep has the function of restoring the balance in the brain."

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