Cognitive fatigue, a signal that warns your brain of the danger of overheating

Even professional chess players, after four or five hours of playing, can start making big mistakes. Are you yourself not feeling tired after a day of intense intellectual effort? This cognitive exhaustion, far from being a mere figment of the mind, has physiological underpinnings. This was revealed by a French study published on August 11 in current biology.

Intense and prolonged mental effort leads to the accumulation of a byproduct of neural activity, glutamate, in certain areas of the lateral prefrontal cortex, the area that controls our higher mental functions. However, this excess glutamate alters the functioning of our neurons. “This fatigue will therefore be a signal that prompts us to stop working to keep our brain functioning properly.”, summarizes Matthias Besiglione, a neuroscientist at the Institute of the Brain (ICM, Pitié-Salpêtrière Hospital, in Paris), who coordinated this work. So it does not come, as it has long been believed, from the exhaustion of the glucose resources provided by the bloodstream.

The Parisian team recruited 40 volunteer participants (20 men and 20 women), mostly students, with an average age of 24. They were randomly divided into two groups: one had to perform cognitive tasks that required intense effort of attention and the second had the same type of tasks but easier (control group). These trials lasted six and a quarter hours. “With a ten-minute break in the middle of the road”researcher says.

Increased concentration of glutamate

First example of tests: task n return.” Participants must indicate whether the last letter of the list matches the letter presented in the n positions before (eg FBLB shows a “2-back” match and BFLB “3-back” “). Those in the control group performed the test in 1-back” and that of the group tested in the “3-back”, which is a more difficult test.

Another example: Mission n switch”. Here, the rule depends on the color of the letter presented. If it is red, the participant must say whether it is a consonant or a vowel. If it is green, if it is uppercase or lowercase. As the letters are passed, their color alternates a lot in the group tested, and therefore subject to more difficult testing.

The tests were divided into 5 sessions of 75 minutes each. The researchers compared the two groups with each other but also, within each group, what participants had in mind between the beginning and end of the tests. During sessions 1, 3 and 5, in fact, the participants performed these tests in the tunnel of an MRI machine. While conventional MRI measures blood flow through the brain (a reflection of which brain regions are working), the researchers here used another technique to obtain the data: magnetic resonance spectroscopy, which measures concentrations of various substances in the brain. This technique also makes it possible to analyze the diffusion of these substances over short distances. “If a molecule is released into the lattices [les espaces entre les neurones]It will spread more easily than if it were confined to cells.”explains Matthias Besiglione.

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