Psychology: Scientists say too much glutamate is the reason hard thinking makes you tired | science and technology

A children’s chess tournament.Hans Neleman (Getty Images)

On September 10, 1984, one of the most exciting World Chess Championships in history began. In the competition faced the 21-year-old challenger, Garry Kasparov, and Anatoli Karpov, 12 years his senior and world champion for ten years. After four wins for Karpov and five draws, Kasparov bounced back and forced 17 draws in a row. Karpov won another match and only needed one more to claim the championship, but Kasparov forced four more draws and eventually defeated Karpov in match 32 for his first win. After another series of draws, the challenger wins games 47 and 48. Five months later, with the score still stuck at 5-3, the president of the International Chess Federation, Florencio Campomanes, ends the tournament without a winner . It was a controversial move which Campomanes justified by pointing out the players’ mental fatigue and physical deterioration. This marked the start of a 20-year rivalry between the two players that went beyond the game of chess. But the question remains: why is intense thinking tiring you?

A team of French researchers thinks they have the answer: tasks that require high mental effort produce additional quantities of molecules that are essential for the proper functioning of the brain, but which can be neurotoxic in high concentrations. Our brain helps us avoid this by creating a feeling of exhaustion that makes us stop what we are doing. This intriguing hypothesis has yet to be confirmed by other neuroscientists and researchers.

To study why mental exercise can be as tiring as physical exercise, French scientists asked 50 people to perform a series of tasks for 6.5 hours (the average workday in France). One group was asked to perform more complex tasks than the other, while the researchers studied their brains.

An eye movement tracking system was used to record pupil dilation. Other studies have shown that the eyes stop moving and the pupils dilate when the brain is performing a calculation or is in the final stages of decision making. In addition, they used a brain imaging technique (magnetic resonance spectroscopy) to measure activity in the prefrontal cortex, where executive brain functions take place, and to identify residues produced by this activity. They also developed performance tests and questionnaires to subjectively measure the level of burnout.

“Glutamate is the main excitatory neurotransmitter in the brain; it is found in many parts of the brain and is necessary for normal brain function. We have found that it increases when performing demanding mental tasks.

Antonius Wiehler, ICM Paris Brain Institute (France)

Recently published in Current Biology, the study revealed clear differences between the two groups (high-demand cognitive control tasks and low-demand tasks). They only observed signs of fatigue, including less pupil dilation, in the high-demand group. Over time, the high-demand group began to demand more immediate rewards for completing tasks. But the most compelling evidence was what they saw happening in the subjects’ brains. Those in the high demand group had higher levels of a molecule – glutamate – in the synapses (the electrochemical connection between nerve endings) of the lateral prefrontal cortex, the brain region responsible for cognitive control.

Antonius Wiehler, a researcher at the ICM Paris Brain Institute and co-author of the study, told EL PAÍS, “Glutamate is the main exciter [synapse activation] neurotransmitter in the brain; it is found in many parts of the brain and is necessary for normal brain function. We have found that it increases when performing demanding mental tasks. Glutamate molecules, not to be confused with monosodium glutamate (a food additive), are released into the synaptic cleft (the short space between the end of one neuron and the start of another), where the exchange of information takes place. Then, says Wiehler, “…brain activity in this region is down-regulated to prevent further accumulation of glutamate.” This is when the brain says it is tired.

The study results suggest that the observed physical changes and increased glutamate concentration make any further activation of the prefrontal cortex more costly, so cognitive control is more difficult after a hard day’s mental work.

The conclusions drawn by these French scientists differ from the prevailing theories on mental fatigue, in particular the theories of exhaustion. Using the analogy of energy consumption during physical exercise, they argue that cognitive control (what, how, when to do or not do something) requires energy expenditure and mental fatigue sets in. when energy resources are depleted. But their study does not identify the specific energy resources depleted by cognitive control, although blood sugar has been suggested. Questions abound – why is playing chess tiring and seeing or hearing, which also require conscious brain work, right?

“Our results show that cognitive work leads to the accumulation of harmful substances.”

Mathias Pessiglione, CHU Pitié-Salpêtrière in Paris (France)

Some psychologists and neuroscientists believe that mental fatigue is an illusion produced by the brain that functions as a warning system, just as the burning sensation of the skin is an illusion warning of the danger of fire. Mathias Pessiglione, a neuroscientist at the Pitié-Salpêtrière University Hospital in Paris (France) and co-author of the study, disagrees and says: “Some influential theories have proposed that fatigue is a kind of of illusion created by the brain to make us stop what we are doing and move on to a more rewarding activity… but our results show that cognitive work leads to real functional impairment – ​​the accumulation of harmful substances. Fatigue is indeed a signal that makes us stop working, but it has a different objective: to preserve the integrity of brain function.

Tomás Segura is the head of the neurology department at the University Hospital of Albacete (eastern Spain) and has studied patients suffering from post-Covid conditions like mental fog and fatigue. “In general, fatigue as a medical term refers to the feeling of shortness of breath associated with exercise or heart failure,” Segura said. “This is why many patients are diagnosed as suffering from non-respiratory and non-cardiac fatigue. In this sense, we can call it neurological, cognitive or mental fatigue. Mental fatigue similar to that caused by intensive cognitive tasks has been seen in a number of patients with post-Covid conditions.

“Let’s say you have to go out to buy bread and just thinking about moving your body to do so tires you out. Shortness of breath does not cause this feeling,” says Segura. “It has a lot to do with the regions of the brain where actions are planned and with the need for glutamatergic transmission to work well for the action to occur. When glutamate, one of the causes of brain damage in victims of stroke, is insufficient, it can lead to certain neurodegenerative diseases and can also produce what is called neurological fatigue.

Javier De Felipe, who heads the Cajal Cortical Circuits Laboratory at the Polytechnic University of Madrid (Spain), finds the French study very interesting and timely, but believes that its conclusions went too far. “Why thinking hard causes fatigue is a great question, but their conclusion is just a guess,” De Felipe said. He believes the study failed to demonstrate the causal relationship between glutamate concentration and mental fatigue. “Cognitive control is centered on the prefrontal cortex, but this area is hyperconnected with other regions of the brain. Why does glutamate build up in some areas and not others? he asks.

Leontxo García has been EL PAÍS’ chess specialist since 1985, the year in which the longest series of chess matches in history ended, and was present for the second round of this epic championship. García recalls that during the 1984 championship, “Karpov immediately won five games in a row and was obsessed with winning the championship 6-0 so that Kasparov would be crushed by psychological trauma from which he could never recover. Thus, instead of making risky moves to win, Karpov started to play very conservatively and waited for Kasparov to make mistakes, but Kasparov, younger and physically stronger, realized he had to wear Karpov down. Both players had sponsors at the highest level in the former Soviet Union. “The sponsors were both afraid that their man would lose, as Karpov was showing clear signs of exhaustion and Kasparov was just one game away. of defeat. Campomanes therefore decided to suspend the championship and resume it eight months later with the score restored to 0-0. So, in the 1984 World Chess Championship, Campomanes kept us from knowing whether Kasparov and mental fatigue could have finally defeated Karpov.

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