"Cell" sub-issue: Don't want to use your brain after work? That's right! Scientists found that after 6.5 hours of high-intensity mental work, "poisons" accumulate in the brain, resulting in dull thinking and physical exhaustion

*For medical professionals only

Today is also a day of hard work! Part-time work is always exhausting and helpless
.
After working for a period of time, this brain melon ah just can't turn, and will unconsciously take out the mobile phone to look at gossip to buy things, in short, everything that needs to use the brain, retreat! Withdraw! Withdraw!

But relaxing like this really works! I will be able to continue working
in no time.
Therefore, the singularity cake raised his hand and asked: Is there a possibility that going to work to fish and buying after work is to work better?

Recently, the team of Mathias Pessiglione, the Paris Brain Institute, provided a possible explanation from a neurometabolic point of view, and published important research results in Current Biology, they found that the mechanism of cognitive fatigue may be related to metabolic changes in the brain [1].

In this regard, Mathias Pessiglione's team in-depth research found that the "metabolic waste" glutamate produced by brain processes accumulates in the area responsible for cognitive control in the brain, the lateral prefrontal cortex (lPFC), resulting in an increase in the "cost" required to activate lPFC, thereby triggering cognitive fatigue
.

Will the brain be hard when it is "tired"? NO！ In fact, your witty brain has long been thinking about how to decompress yourself, such as luring you into some "low-cost" happy behavior
that does not require effort or waiting.

To elucidate the function and biological mechanism of cognitive fatigue, the researchers recruited 40 participants and divided them into simple groups (n=16) and difficult groups (n=24)
according to how difficult it was to participate in the cognitive task.

To induce cognitive fatigue, all participants were required to perform two cognitive control tasks (N-switch and N-back)
over a 6.
25-hour period.
The N-switch task requires participants to complete the distinction between vowels and consonants or uppercase and lowercase according to letter color, and the difficulty of this task depends on the speed at which the letter color changes; The N-back task asked participants to state whether the letters on the screen were the same as those that appeared in previous experiments, and the difficulty of the task depended on the time interval between
tests.

The researchers then asked two groups of participants to make four economic decisions, each designed as a small reward/low cost (LC) or large reward/high cost (HC) model
.
For example, LC mode means that participants can get 41.
20 euros immediately, while HC mode means that participants receive 50 euros
after one month.
The cognitive tasks of the two groups of participants alternated with economic decision-making for 5 cycles
.

Diagram of the design of experiments

Subsequently, the researchers analyzed the participants' objective performance and subjective scores to assess the level
of cognitive fatigue of the participants in both groups.
The results showed that the two groups scored similarly
over time.
It is assumed that people are often unable to subjectively report fatigue related to task difficulty, i.
e.
, the degree of
load on cognitive control.

Average accuracy and subjective fatigue scores on cognitive tasks in both groups (0 indicating good state and 100 indicating exhaustion)

Nevertheless, by analyzing the economic decision-making behavior of the two groups, the researchers found that participants in the difficult group were more inclined to the LC model in economic decision-making, speculating that cognitive fatigue may predispose to choose options
with low cognitive control costs.

However, due to the lack of magnetic resonance scan data, it is currently impossible to verify whether this selection bias is associated with
reduced lPFC activity during selection.

There is no difficult research work, only brave researchers! After reviewing the literature, researchers found that pupil dilation correlates with the level of energy required for cognitive control and can be used as an indicator to measure the cost of cognitive control in economic decision-making [2-3].

So they quickly changed their research ideas and measured the changes in pupil dilation in the two groups, and the results showed that the pupil dilation of the participants in the difficult group decreased significantly over time, suggesting that the participants invested less
energy in decision-making considerations when they were in a state of cognitive fatigue.

Changes in pupil dilation during cognitive fatigue in both groups

With such effort, I finally figured out the ins
and outs of cognitive fatigue affecting decision-making.
High-difficulty cognitive tasks will lead to increased costs of cognitive control and cause cognitive fatigue, at which time our smart brains are very easy to enter the "fish" state, so they are more inclined to some choices
with short-term returns in decision-making behavior.

Speaking of which, Ben Cake suddenly realized! It turns out that there is a scientific basis for working hard all day to spend money indiscriminately! After all, as a commander-in-chief, the brain is "tired" all day, how can you still aggrieved yourself to resist? Instant fun, of course!

Closer to home, why does the cost of cognitive control increase as the difficulty of cognitive tasks increases? To explore the biology behind this, the researchers turned their focus to brain metabolism, using magnetic resonance spectroscopy (MRS) to measure
the concentration of metabolites in the brains of lPFCs and primary visual cortex (V1) in both groups.

The results showed that glutamate was the only metabolite
that showed a three-way interaction (cognitive task group, brain region, duration of task).
Moreover, compared to the simple cognitive task group and the V1 region, participants in the difficult cognitive task group had higher levels of glutamate concentration and apparent diffusion coefficient levels in the brain lPFC region of the brain
.

Top: Glutamate concentrations in the lPFC region and V1 region of the brain, normalized with total creatine

Bottom: Apparent diffusion coefficients of total Glx glutamate and glutamine in the lPFC region of the brain versus the V1 region of the two groups

Finally, the researchers evaluated the correlation
between economic decision-making behavior and brain metabolism in states of cognitive fatigue.
The results showed that the degree of decision-making inclination to LC during cognitive fatigue was positively correlated with glutamate concentration and Glx diffusion, but only the increase in Glx diffusion had a significant statistical difference, indicating that the LC option in the state of cognitive fatigue was related to the level of Glx diffusion, that is, the accumulation of glutamate
.

Correlation between participant decision-making behavior and glutamate concentration and diffusion level

Overall, this study proposes a neurometabolic model to explain the mechanism of cognitive fatigue, that is, glutamate accumulation triggers a regulatory mechanism that makes lPFC activation more expensive, making it difficult to mobilize cognitive control after exhausting work, so people's decision-making behavior is also more inclined to simple and easy "low-cost" choices
.

However, it should be noted that the Pessiglione team's results only suggest that cognitive fatigue is related to glutamate metabolism, rather than confirming that blocking glutamate accumulation can improve cognitive control, and the target of regulation may not be glutamate itself, but any substance
involved in glutamate accumulation.

In addition, the subjective reporting of fatigue in the study attracted the attention of Ben Cake, and the results suggested that there may be a deviation
between the actual fatigue of the brain's cognitive control system and our perception of fatigue.
Therefore, come out to work, don't wait for this brain to turn before remembering to rest, work for two hours, relax for two minutes, the efficiency may be higher!

References:

1.
Wiehler A, Branzoli F, Adanyeguh I, Mochel F, Pessiglione M.
A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions.
Curr Biol.
2022; 32(16):3564-3575.
e5.

2.
van der Wel P, van Steenbergen H.
Pupil dilation as an index of effort in cognitive control tasks: A review.
Psychon Bull Rev.
2018; 25(6):2005-2015.

3.
Borderies N, Bornert P, Gilardeau S, Bouret S.
Pharmacological evidence for the implication of noradrenaline in effort.
PLoS Biol.
2020; 18(10):e3000793.

ADtaxi – Remembering the ride

The author of this article Xia Tian

Responsible editorDai Siyu