Mental fatigue is no illusion: Hours of deep thinking causes harmful substances to accumulate in the important brain area behind the forehead, the prefrontal cortex. As a result, it works less temporarily, and as a result people prefer to make impulsive and less stressful choices after an intense work day. This is what French researchers discovered – Their study appeared on Thursday in the scientific journal current biology.
For more than a hundred years, scientists have been searching for the mechanism behind the mental fatigue people experience when they use their brains for an extended period. Even professional chess players start making mistakes after about five hours of playing. But what exactly happens in the brain is still puzzling – the brain It hardly uses more energy, for example Thought more deeply than usual.
just a feeling
Some theories posit that fatigue is just a feeling, and they drive people to do something more satisfying. After all, when the reward of a heavy mental task increases, you see on all kinds of tests that people overcome that fatigue again.
But French brain researchers have discovered that serious thinking actually alters brain function: They see a harmful build-up of glutamate in a part of the prefrontal cortex that is highly active during strenuous cognitive effort.
Glutamate is an important and common substance in the brain. It is a neurotransmitter, sent by brain cells to transmit signals to other neurons. But if too much glutamate is released and not eliminated quickly enough, it is harmful to brain cells.
Excess glutamate must first be removed to ensure that the brain can continue to function properly. So fatigue after hours of mental effort is a signal to stop heavy thinking, the French wrote, to give the brain a chance to remove all the byproducts of an increased metabolism and prepare it for reuse.
The French are brain draining test subjects. They gave part of a group of 40 people daily challenging cognitive tasks – the other participants were given easy versions of the same tasks. Most of the time, people were doing the thinking while lying in a brain scanner: The amount of glutamate and related substances in their brains was imaged using magnetic resonance spectroscopy (MRS).
In the Paris lab, participants in the first group had to differentiate between sequences of letters appearing on a computer screen for more than six hours – they were given two 10-minute intervals. A new character appears every 1.6 seconds. For example, they had to distinguish vowels and consonants, but only if these letters were of a certain color. Color changed 12 times every 24 rounds; In the group that got the easy version, this only changed once. Or they had to clarify whether the newly appeared message was the same from three steps back; For the easy group participants, that was a step backwards.
The researchers discovered the extent of cognitive fatigue of the participants through tests in which economic choices must be made. For example, they had to choose between receiving a small amount of money immediately afterward, or 50 euros over the course of a month. Or choose if they want to do a heavy workout on an exercise bike for €50 afterwards, or a lighter workout for a lower reward.
Participants who completed the challenging tasks were more likely than the others to choose a quick or easy reward rather than a later but higher reward.
This increased impulsivity was accompanied by reduced activity in an area of the brain that is an important part of the network needed for cognitive control: the left lateral prefrontal cortex. In another area they also looked for control, the visual cortex, they didn’t see it. The same has been observed in previous studies of athletes with mild fatigue from overtraining, the authors wrote.
The researchers concluded that mental fatigue undermines cognitive control through the accumulation of glutamate. Glutamate concentrations between brain cells also increase during stressful conditions. They think rest and sleep can remedy that. There are indications that during sleep, when all kinds of waste products are removed from the brain, glutamate concentrations also return to normal.
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