Ever come home from work feeling like your brain is fried! Can’t focus, taking forever to organize thoughts, re-reading the same paragraph over and over without comprehending, irritable, unable to do one more thing - even if it is just deciding what to eat when you get home or conversing civilly with your partner? All you can manage is to slump onto the couch and sit there staring mindlessly. And you had started the day so full of energy and ideas. What happened? Well, you experienced cognitive fatigue.
Cognitive fatigue is complex and may occur because you’ve been asking your brain to do too much. It could be from intense focus on a single task over an extended period of time, or from spreading your attention across too many things. Consider all the interactions you have, the decisions you make, the information you process, the emails you answer, the tasks you must complete, chores, etc. And, worrying about the things you have to do can just be as taxing as actually doing them. Plus, physical factors like poor nutrition, lack of sleep, or chronic stress make you more prone to cognitive overload. However, the scientific reasons underlying the experience of fatigue after engaging in prolonged cognitive effort remain unclear.
It was thought that mental work could deplete the brain of metabolic resources, since the brain accounts for about 20% of our overall energy consumption. However, studies show that energy expenditure with hard cognitive work only increases modestly – by only about 10% over resting conditions. So, cognitive tasks would not drain the body’s energy supplies in an adequately nourished individual.
Neuroimaging studies find that the prefrontal cortex (PFC), the foremost part of the brain plays a critical role in cognitive processes that underlie mental effort. One recent study suggests that cognitive fatigue after intense mental work is associated with the accumulation of the neurotransmitter glutamate in the PFC. Researchers assessed metabolic changes in the brains of individuals using a noninvasive technique called Magnetic Resonance Spectroscopy. The study measured levels of some of the products of metabolism in the PFC while participants were performing 6 hours of cognitive tasks of varying levels of difficulty. Those who did more complex cognitive tasks showed higher levels of glutamate in their PFC by the end of the experiment compared to their counterparts doing simpler tasks. Specifically, the study found a greater accumulation of glutamate and its byproducts outside of brain cells in the PFC of the complex-task group.
Glutamate is the primary brain excitatory neurotransmitter, and studies have documented the release of glutamate by neurons during tasks requiring cognitive work. It is also known that excess glutamate outside neurons can have toxic effects on surrounding tissues. In the referenced study, the authors suggest that localized excess of released glutamate in PFC during demanding cognitive tasks could trigger mechanisms to rapidly regulate its levels as a priority, and make additional mental effort more difficult for a while as a protective measure. While this is an intriguing hypothesis, the study does not establish its veracity or confirm a causal link between cognitive demand and glutamate accumulation. As always in science, more studies are needed.
The study does suggest a possible new way to measure cognitive fatigue and a potential culprit molecule that we may learn to manipulate someday to avoid mental overload. For now, since several previous studies have shown that levels of glutamate in the brain decrease after rest and sleep, perhaps a little relaxation or a nice nap are the best prescriptions to rejuvenate those mental powers.
Post by: Nadia Fike
Read more: Wiehler A, et al (2022). A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions. Curr. Bio. 2022;32:1-12. doi: 0.1016/j.cub.2022.07.010