The more tired the brain, the more impulsive choices... Important decisions, avoid at night
“Thinking is the most difficult and tiring of all labors. So many people avoid thinking.”
These are the words of American author Wallis Wattles (1860-1911) listed in an online collection of quotes. In fact, if you sit down and concentrate on something for a few hours, you will feel a lot of fatigue.
Although it weighs 2% of the body, it uses 20% of the energy for a long period of time because the brain was abused for a long time.
Scientists at the Paris Brain Institute (PBI) in France have recently discovered a metabolic mechanism in the brain that could explain this.
Finding the identity of substances that accumulate in the brain as a result of long-term mental labor. The research team reported in the international scientific journal 'Current Biology' that glutamate (glutamic acid) accumulates in the prefrontal cortex in the front of the brain when you concentrate on work for a long time, and this appears to cause fatigue. Researchers found that when glutamate accumulates in large amounts, it becomes difficult to concentrate and work or make decisions.
When a lot of glutamate builds up in the prefrontal cortex, it becomes difficult to concentrate and work or make decisions.
What is glutamate?
To find the source of mental fatigue, the research team used magnetic resonance spectroscopy (MRS) technology to measure chemicals secreted in the metabolic process of living tissues.
The researchers used the device to see what happens in the lateral prefrontal cortex, which is responsible for high-level mental tasks such as making decisions and making plans.
The researchers first recruited 40 experimental participants and divided them into a group dealing with a difficult task and a group dealing with an easy task.
Then, each of them was given a task and asked them to lie down on a brain scanner and perform the task. For example, a different number was displayed on the screen at 1.6-second intervals and asked to say whether the currently displayed character is the same as the previous one. 26 people were given a difficult problem judging whether they match the three previously shown characters, and 14 people were given an easier problem.
The electrical signal from the nerve cell itself cannot cross the synapse that connects the nerve cell. Instead, compounds such as glutamate are secreted, which transmit signals to other neurons. Glutamate is a key substance responsible for more than 90% of information transfer between neurons. The small sacs (vesicles) of the synapse, which are the junctions between neurons, contain 8,000 glutamate molecules per sac.
After the six-hour memory test, the glutamate levels of the participants who solved the high-difficulty problem increased significantly more than when the experiment began. Another indicator of fatigue, pupil dilatation, was also observed.
The glutamate levels of the participants who solved the easy problem were almost the same as at the beginning. They also complained of fatigue, but pupil dilation did not occur. Except for glutamate, seven other chemicals remained unchanged.
Location of primary visual cortex (V1) and lateral prefrontal cortex (LPFC) (A) and changes in glutamate levels in both cerebral cortex (B). Looking at the picture of B, it can be seen that glutamate was greatly increased in the lateral prefrontal cortex when performing a difficult task (orange). Current Biology
More impulsive decisions are the brain's defense strategy
The researchers also investigated whether mental fatigue affects decision-making. When people are mentally exhausted, instead of persevering, they seek immediate pleasure or act impulsively.
The researchers asked participants to choose whether they would like to receive a small amount of money right away or more money later.
Participants who felt more fatigued from solving difficult problems, and thus more glutamate, were more likely to receive a small reward immediately. The group who solved the hard problem had an 8% increase in glutamate overall and 10% more impulsive choices.
Glutamate is a key excitatory neurotransmitter in the central nervous system and plays an important role in memory. In order for the cortex to function smoothly, it must maintain a proper balance with the inhibitory neurotransmitter GABA. However, when glutamate is too much, it acts toxic to nerve cells (neurons) and causes the neurons to die. This disrupts the balance of neurotransmitters and reduces the ability to transmit information.
Glutamate is cleared from synapses while you sleep, which may be why you feel refreshed the next morning.
An indicator of how hard your brain worked
"This study shows that cognitive tasks lead to 'accumulation of harmful substances' (increased glutamate) in the brain," said Professor Matthias Pécilion of the University of Pittier-Salpetrière, a member of the research team in a press release. "It's a signal to stop what you're doing to preserve normal functioning." He added that glutamate is cleared from synapses during sleep, and that may be why you feel refreshed the next morning.
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However, Sebastian Muslik, a professor of neuroscience at Brown University, expressed his doubts in an interview with the international scientific journal 'Science' that metabolic waste might be a key causative agent of cognitive fatigue. He instead said that the increase in glutamate could be for some purpose. Organs in our body communicate with the brain to tell us when to eat, sleep, drink water, and sleep. Glutamate in the prefrontal cortex plays a role in updating the brain's internal monitoring system.
Researcher Antonius Biller, the first author of the paper, also said, “It is still difficult to say that doing mentally strenuous work is the cause of the increase in toxic glutamate in the brain.”
Professor Leto Huber of the University of Zurich, Switzerland, told the scientific journal New Scientist that the brain glutamate levels measured in this study look promising as an indicator of how hard the brain worked. “Glutamate measurement has room for use in diagnosing people with conditions that have difficulty concentrating, such as brain fog sequelae, attention deficit hyperactivity disorder (ADHD) in children, after suffering from COVID-19,” he said. There seems to be,” he said.
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