Cognitive fatigue, a signal that warns your brain of a risk of overheating

Even professional chess players, after four or five hours of play, can start making big mistakes. Do you yourself not feel exhausted after a day of intense intellectual effort? This cognitive fatigue, far from being a mere figment of the mind, is based on physiological foundations. This is revealed by a French study, published on August 11 in Current Biology.

Intense and prolonged mental effort indeed causes the accumulation of a by-product of neuron activity, glutamate, in certain areas of the lateral prefrontal cortex, a region that governs our higher mental functions. However, this excess glutamate alters the functioning of our neurons. “This fatigue would therefore be a signal that pushes us to stop working to preserve the integrity of our brain’s functioning”, summarizes Mathias Pessiglione, neuroscientist at the Brain Institute (ICM, Pitié-Salpêtrière hospital, in Paris), who coordinated this work. It therefore does not come, as has long been believed, from a depletion of glucose resources provided by the bloodstream.

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

Increased glutamate concentration

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

Another example: the task 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’s green, if it’s uppercase or lowercase. As the letters scrolled, their color alternated much more frequently in the tested group, thus subjected to a more difficult test.

The tests were divided into 5 sessions of 75 minutes. The researchers compared the two groups to each other but also, within each group, what was going on in the minds of the participants between the start and the end of the tests. During sessions 1, 3 and 5, in fact, the participants performed these tests in the tunnel of an MRI (magnetic resonance imaging) machine. While a conventional MRI measures blood flow through the brain (a reflection of brain regions that are working), here the researchers used another data acquisition technique: magnetic resonance spectroscopy, which measures concentrations of different substances in the brain. This technique also makes it possible to analyze the diffusion of these substances over short distances. “If a molecule has been released into the synapses [les espaces entre les neurones]it will diffuse much more easily than if it is confined inside the cells”explains Mathias Pessiglione.

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