Concentrating for long periods of time may cause glutamate to accumulate in the brain, causing mental fatigue. Image source: Dmitriy Shironosov/Alamy
Thinking hard for hours often makes people feel mentally exhausted. Why is this? Recently, a new study by French researchers found that prolonged concentration can cause glutamate to accumulate in the front area of the brain, and excess glutamate can make further mental work difficult. Relevant research results were published in Current Biology on August 11.
Antonius Wiehler, leader of the study at the Paris Brain Institute, explained that too much glutamate is potentially harmful to the human body, and the brain wants to avoid this, so it tries to reduce activity.
Many people experience mental fatigue. The brain does not seem to run out of energy after hard work, and even when we are not deliberately thinking about anything specific, some brain areas remain as active as before.
To learn more, Wiehler's team used magnetic resonance spectroscopy (MRS) technology to harmlessly measure levels of various chemicals in living tissue. The researchers focused on an area in the front and sides of the brain called the lateral prefrontal cortex. Many previous studies have shown that this area is involved in complex intellectual tasks.
A total of 40 participants took the MRS scan memory test. This involves observing a sequence of numbers that appear on the screen and stating whether the current number is the same as the previous number. Twenty-six participants completed the more difficult task, while the other 14 completed the easier task.
The researchers also measured levels of eight different brain chemicals, including glutamate, the main signaling chemical between neurons. Research shows that electrical signals cannot cross the junctions (synapses) between neurons and must rely on the release of tiny particles such as glutamate to transmit signals.
After completing the memory task for 6 hours, participants who experienced the more difficult memory task had increased glutamate levels in the lateral prefrontal cortex compared with the beginning of the experiment. Glutamate levels remained unchanged in those participants who performed easier tasks. Across all participants, there were no increases in seven other brain chemicals measured.
In participants doing the harder tasks, increases in glutamate levels coincided with pupil dilation, another measure of fatigue. Meanwhile, those participants who performed simple tasks did not experience increased glutamate levels or pupil dilation.
The researchers also investigated whether mental fatigue affects decision-making. They did this by interspersing memory tasks with different exercises, such as asking people to choose between receiving a sum of money immediately or another sum of money later.
As participants became more tired and accumulated glutamate while doing harder tasks, they switched to the option of giving a small immediate reward. This may be an example of people avoiding difficult mental tasks, such as calculating what choices to make, thereby preventing the accumulation of potentially harmful glutamate.
"One way to reduce glutamate accumulation is to reduce activation of the lateral prefrontal cortex during choice." If you do this, it means you have chosen the tempting option, Wiehler says.
Reto Huber of the University of Zurich in Switzerland believes that measuring brain glutamate can be used to reveal how hard a certain area of the brain is working. Therefore, doctors may use it to evaluate people who have difficulty concentrating, such as children with attention deficit hyperactivity disorder.
related paper information:
https://doi.org/10.1016/j.cub.2022.07.010
