Why is mental exhaustion due to excessive use of the brain?

Prolonged attention spans may allow glutamate to accumulate in the brain, leading to mental fatigue. Image credit: Dmitriy Shironosov/Alamy

Hours of meditative contemplation often make people feel mentally exhausted, why is this? Recently, a new study by French researchers found that prolonged attention spans can lead to glutamate accumulation in the anterior region of the brain, and that an excess glutamate can make further mental work difficult. The findings were published in Contemporary Biology on August 11.

The study leader, Antonios Wiehler of the Paris Brain Institute, explains 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 have experienced mental fatigue. After hard work, the brain doesn’t seem to run out of energy, and even when we’re not deliberately thinking about anything specific, some brain regions are still as active as ever.

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 their attention on an area in the anterior and flanks of the brain, the lateral prefrontal cortex. Many previous studies have shown that the region is associated with complex intellectual tasks.

A total of 40 participants underwent MRS scan memory tests. This includes observing the sequence of numbers that appear on the screen and indicating whether the current number is the same as the previous one. Of those, 26 participants completed the more difficult tasks, while another 14 participants completed the simpler tasks.

The researchers also measured levels of 8 different brain chemicals, including glutamate, the main signaling chemical between neurons. Studies have shown that electrical signals cannot cross the junctions (synapses) between neurons, and can only rely on the release of tiny particles such as glutamate to transmit signals.

After completing the 6-hour memory task, participants who underwent more difficult memory tasks had higher glutamate levels in the lateral prefrontal cortex than at the beginning of the experiment. Participants who did the simpler task had glutamate levels that remained unchanged. Among all the participants, there was no increase in the other 7 brain chemicals measured.

Among participants on the more difficult task, the rise in glutamate levels coincided with the dilation of the pupils, another measure of fatigue. At the same time, participants who did the simple task did not experience glutamate rise or pupil dilation.

The researchers also investigated whether mental fatigue affects decision-making. They do this by interspersing different exercises in memory tasks, such as letting people choose between getting one sum of money directly and getting another later.

As participants felt more tired as they did harder tasks and accumulated glutamate, they turned to the option of giving small rewards immediately. This could be an example of people avoiding difficult mental tasks, such as calculating what choices to make, thus preventing the accumulation of potentially harmful glutamate.

“One way to reduce glutamate accumulation is to reduce activation of the lateral prefrontal cortex when selected.” Wiehler says that if you do, you’ve chosen an enticing option.

Reto Huber of the University of Zurich in Switzerland believes that measuring glutamate in the brain can be used to reveal the degree of effort to work in a certain area of the brain. Therefore, doctors may use it to evaluate those who have difficulty concentrating, such as childhood attention deficit hyperactivity disorder. (Source: China Science Daily Xin Yu)

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