Mental fatigue is not an illusion: hours of deep thinking cause harmful substances to accumulate in the important brain area behind the forehead, the prefrontal cerebral cortex. As a result, it temporarily works less well, and as a result people prefer to make impulsive and less tiring choices after an intensive working day. That’s what French researchers discovered – their study appeared on Thursday in the scientific journal Current Biology.
For more than a hundred years, scientists have been looking for the mechanism behind the mental exhaustion that people experience when they have used their brains for a long time. Even professional chess players start to make mistakes after about five hours of play. But what exactly happens in the brain is still puzzling – for example, the brain hardly uses more energy for deep thinking than usual.
Just a sensation
Some theories assume that fatigue is just a sensation, an illusion that causes people to do something more satisfying. After all, when the reward of a heavy mental task is increased, you see in all kinds of tests that people get over that fatigue again.
But French brain researchers discovered that the hard thinking actually changes the function of the brain: they see a harmful build-up of glutamate in a part of the prefrontal cortex that is very active during strenuous cognitive exertion.
Glutamate is an important and common substance in the brain. It is a neurotransmitter, brain cells emit to transmit signals to other nerve cells. But if too much glutamate is released and it is not eliminated quickly enough, it is harmful to brain cells.
The excess of glutamate must first be cleared to ensure that the brain can continue to work properly. The fatigue after hours of mental effort is therefore a signal to stop the heavy thinking, the French write, to give the brain the chance to clear all by-products of the increased metabolism and prepare it for reuse.
Exhausted brain
The French exhaust the brains of their test subjects. They gave part of a group of 40 subjects a day of demanding cognitive tasks – the other participants were given easy versions of the same tasks. Most of the time, the subjects did the thinking while lying in a brain scanner: the amount of glutamate and related substances in their brains were 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 over six hours – they were given two 10-minute breaks. A new letter appeared every 1.6 seconds. For example, they had to distinguish vowels from consonants, but only if those letters were a certain color. Which color that was changed 12 times every 24 rounds; in the group that received the easy version, this only changed once. Or they had to indicate whether the newly appeared letter was the same as the one from three steps back; for the participants in the easy group, that was one step back.
The researchers found out how cognitively fatigued the participants were with tests in which economic choices had to be made. For example, they had to choose between receiving a small amount of money immediately afterwards, or 50 euros over a month. Or choose whether they wanted to do a heavy workout on an exercise bike for 50 euros afterwards, or a lighter workout for a lower reward.
The participants who completed the difficult tasks were much more likely than the others to opt for a quick or easy reward instead of a later but higher reward.
That increased impulsivity was accompanied by lower activity in a brain region that is a crucial part of the network needed for cognitive control: the left lateral prefrontal cortex. In another area they also looked at for control, the visual cortex, they didn’t see that. The same has been seen in previous studies in athletes with mild burnout from overtraining, the authors write.
Mental fatigue undermines cognitive control through glutamate accumulation, the authors conclude. Also during stressful conditions, glutamate concentrations between brain cells increase. Rest and sleep can remedy that, they think. 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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