Tax preparation is a mind-numbing task, but the endeavours, for many, can leave many feeling exhausted.
The same goes for reading a dense report, selecting areas of spreadsheet data, or writing a fact-filled paper.
A new study shows that feeling tired after intense meditation isn’t all in your head.
Laboratory experiments have shown that an activity that requires a lot of thinking can lead to the accumulation of toxic byproducts in a part of the brain called the prefrontal cortex, French researchers reported on August 1st. 11 in Current Biology.
This, in turn, alters decision-making, making your brain more inclined to choose easier or faster options as you get tired, the researchers say. The prefrontal cortex is the part of the brain where people process conflicting thoughts and make higher-level decisions.
“Influential theories suggest that fatigue is a kind of illusion designed to get our brain to stop what we’re doing and switch to a more enjoyable activity,” said senior researcher Mathias Pessiglione of the University of Pithier-Salpetriere in Paris. message.
“However, our findings show that cognitive work leads to a change in the real functional goal – the accumulation of harmful substances, so fatigue becomes a signal to stop our work, but in a different way: to preserve the integrity of brain function,” he added.
For this experiment, Pessiglione and his team recruited people from two groups and had them both work for just over six hours.
A group of 24 people were given a mentally demanding task in which letters were displayed on a computer every 1.6 seconds and they had to match the letters that had appeared in different ways. Another group of 16 people were asked to perform a similar but easier task.
While the participants worked, the researchers used a scanning technology called magnetic resonance spectrometry to measure the levels of glutamate stored in the prefrontal cortex of their brains.
According to the Cleveland Clinic, glutamate is a neurotransmitter that plays a key role in learning and memory. Your brain needs it in the right concentrations in the right places, but excess glutamate can overexcite neurons and cause brain cell damage or death.
The researchers found that people who performed more mentally challenging tasks had higher levels of glutamate in the prefrontal cortex. These participants also showed other signs of fatigue, including reduced pupil dilation.
Both groups were given a continuous financial choice to determine whether their brain activity affected their decision-making ability. They can choose between a smaller but easier cash prize or a larger cash prize that requires more effort or patience.
People in the more brain-intensive group shifted their economic choices toward the end of the day, often choosing lighter options that offered less effort and shorter rewards.
This experiment provided a reasonable alternative explanation for brain fatigue, said Dr. Donn Dexter, a fellow of the American Academy of Neurology and a neurologist at the Mayo Clinic in Eau Claire, Wis.
Most experts focus on energy depletion as the cause of mental fatigue, he said.
“The brain always needs glucose, but it has a very good supply, so it’s not really clear,” Dexter said. “It may make sense to create toxic byproducts that hinder production.”
If so, it would explain why sleep, even short naps, refreshes the brain, he said. There is evidence that our brain uses sleep to flush out toxins that accumulate during the day.
However, Dexter thought that financial decisions were not a good indicator of brain fatigue.
“I would be cautious about generalizing the results,” Dexter said. “I would like to see this replicated in another study.”
Dr. Thomas Wisniewski, director of NYU Langone’s Center for Cognitive Neurology in New York City, shared Dex’s concerns about the study’s design.
Wisniewski said that both MRS measurements of glutamate levels and measures of brain fatigue are “quite indirect.”
“Of course what they’re saying makes sense, but it’s a correlation. There’s no proof of causation,” Wisniewski said. “Even if what they say is possible, that’s no proof that it’s true.”