After an intense and prolonged intellectual effort, a molecule – glutamate -, accumulated in certain areas of our brain, disturbs reasoning and decision -making. One way to alert us that it is time to stop working, shows a French study.
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Even professional chess players, after four or five hours of play, can start making big mistakes. You yourself do not feel exhaustion after a day of intense intellectual effort? This cognitive fatigue, far from being a simple view of the mind, is based on physiological foundations. This is what a French study reveals, published on August 11 in Current Biology .
An intense and prolonged mental effort causes the accumulation of a by-product of the activity of neurons, glutamate, in certain areas of the lateral prefrontal cortex, a region which 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 the functioning of our brain,” sums up Mathias Pessiglione, neuroscientist at the Brain Institute (ICM, Pitié-Salpêtrière Hospital, in Paris) , who coordinated this work. It would therefore not come, as we have long believed, from a exhaustion of glucose resources provided by blood circulation.
The Parisian team recruited 40 volunteer participants (20 men and 20 women), mostly students, aged on average 24 years. They were distributed at random in two groups: the first had to accomplish cognitive tasks requiring intense effort of attention and the second the same type of tasks but easier (control group). These events lasted six hours and quarters, “with a break of ten minutes halfway up”, specifies the researcher.
Increase in the concentration of glutamate
First example of tests: the “N-Back” task. Participants must indicate whether the last letter of a list corresponds to the letter presented n positions before (for example, F-B-L-B shows a “2-back” correspondence and B-F-L-B a “3-back” correspondence). Those of the control group carried out the test in “1-back” and those of the group tested in “3-back”, a differently more difficult test.
Another example: the “N-Switch” task. 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 a capital letter or a tiny. As the letters paraded, their color alternated much more frequently in the tested group, thus subject to a more difficult test.
The tests were divided into 5 sessions of 75 minutes. The researchers compared the two groups between them but also, within each group, which was happening in the heads of the participants between the beginning and the end of the tests. During sessions 1, 3 and 5, in fact, the participants carried out these tests in the tunnel of an MRI device (magnetic resonance imaging). While a classic MRI measures blood flow through the brain (reflection of the working brain regions), here, the researchers have called on another data acquisition technique: magnetic resonance spectroscopy, which measures concentrations Differences in the brain. This technique also makes it possible to analyze the dissemination of these substances over short distances. “If a molecule has been released in synapses [spaces between neurons], it will diffuse much more easily than if it is confined inside the cells,” explains Mathias Pessiglione.
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