The 19th-century American writer Wallace D. Wattles once claimed that “thinking is the hardest and most tiring of all labors.”
On the surface, this might sound like a contentious comparison, but a new study suggests that thinking too hard and for too long can really drain your brain the same way exercise drains your body.
Heavy physical labor is obviously tiring, but the sweat on a person’s forehead or the trembling of a muscle doesn’t tell you how hard they might be thinking.
When someone says they feel mentally exhausted, we just have to take their word for it.
So scientists still don’t really understand why intense thinking can lead to cognitive fatigue. It’s not exactly a drowsy feeling. Rather, it’s a feeling that the task is getting harder and harder to complete or focus on.
Some researchers now suspect that the most abundant excitatory neurotransmitter in the brain is to blame for this lack of mental endurance.
Glutamate is an excitatory amino acid that was only correctly described in the 1950s, although it is present in more than 90 percent of interneuron communication in the human brain.
This underestimated chemical has surprised scientists for decades. For example, neurons have been found to control the strength of their signals in the brain by regulating the amount of glutamate they release to other neurons.
Glutamate can even trigger neuronal death, with as many as 8,000 glutamate molecules packed in a single pouch at a synapse, the junction where two neurons meet.
Glutamate excess is clearly a problem, which is part of the reason it’s linked to brain drain.
While monitoring the brain chemicals of 24 participants who were tasked with completing a heavy computer-based sorting task for more than six hours, the researchers found an increase in glutamate levels in the lateral prefrontal cortex. This is the part of the brain associated with higher cognitive abilities, such as short-term memory and decision-making.
In contrast, the other 16 participants who were assigned the easier task that day showed no signs of glutamate buildup in this part of their brains.
Therefore, the researchers believe that the increase in extracellular glutamate may be at least one of the limiting factors for mental endurance in humans.
Apparently, the brain also devours a lot of glucose when it’s working. Other theories suggest that this energy source may be another limiting factor, but how the loss of glucose makes thinking more difficult, biochemically, is unclear.
Some researchers have proposed that a sharp drop in glucose triggers a loss of dopamine in the brain, which makes it easier to lose interest in certain cognitive tasks.
“Influential theories suggest that fatigue is an illusion created by the brain that causes us to stop whatever we’re doing and engage in more satisfying activities,” explains Mathias Pessiglione, a clinical psychologist at the University of Pitié-Salpêtrière in Paris, France. .
“But our findings suggest that cognitive work leads to real functional changes — the accumulation of harmful substances — so fatigue is indeed a signal to stop working, but for a different purpose: to preserve the integrity of brain function.”
Pessiglione also said that there is good evidence that glutamate is eliminated from synapses during sleep.
This may be part of the reason why a night’s rest can leave a person feeling refreshed the next day.
A 2016 brain imaging study using functional magnetic resonance imaging (fMRI) also found that the lateral prefrontal cortex (lPFC) is involved in intense cognitive effort, reducing its excitability over time.
Activating this zone at the end of a long, grueling day requires more effort than at the beginning. Hence the brain drain feeling.
“Combined with previous fMRI data, these results support a neurometabolic model in which glutamate accumulation triggers a regulatory mechanism that makes activation of the lPFC more costly, explaining why cognitive control is harder after a hard work day mobilization,” concluded Pessiglione and colleagues.
Glutamate is an incredibly fast-acting neurotransmitter. This is part of what makes this amino acid so powerful. But it also makes the chemicals difficult to measure.
Studies like the current one are using new technologies to explore in greater detail the rapid effects of glutamate in our brains.
The authors now want to investigate why glutamate accumulates more in the prefrontal cortex than in other parts of the brain.
The study was published in Current Biology.