A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions, 2022, Wiehler et al

Highlights

• Cognitive fatigue is explored with magnetic resonance spectroscopy during a workday
• Hard cognitive work leads to glutamate accumulation in the lateral prefrontal cortex
• The need for glutamate regulation reduces the control exerted over decision-making
• Reduced control favors the choice of low-effort actions with short-term rewards

Summary

Behavioral activities that require control over automatic routines typically feel effortful and result in cognitive fatigue. Beyond subjective report, cognitive fatigue has been conceived as an inflated cost of cognitive control, objectified by more impulsive decisions. However, the origins of such control cost inflation with cognitive work are heavily debated.

Here, we suggest a neuro-metabolic account: the cost would relate to the necessity of recycling potentially toxic substances accumulated during cognitive control exertion. We validated this account using magnetic resonance spectroscopy (MRS) to monitor brain metabolites throughout an approximate workday, during which two groups of participants performed either high-demand or low-demand cognitive control tasks, interleaved with economic decisions. Choice-related fatigue markers were only present in the high-demand group, with a reduction of pupil dilation during decision-making and a preference shift toward short-delay and little-effort options (a low-cost bias captured using computational modeling). At the end of the day, high-demand cognitive work resulted in higher glutamate concentration and glutamate/glutamine diffusion in a cognitive control brain region (lateral prefrontal cortex [lPFC]), relative to low-demand cognitive work and to a reference brain region (primary visual cortex [V1]).

Taken together with previous fMRI data, these results support a neuro-metabolic model in which glutamate accumulation triggers a regulation mechanism that makes lPFC activation more costly, explaining why cognitive control is harder to mobilize after a strenuous workday.

Open access, https://www.cell.com/current-biology/fulltext/S0960-9822(22)01111-3

 

Why thinking hard makes us feel tired

The study, published on 11 August in Current Biology1, found that participants who spent more than six hours working on a tedious and mentally taxing assignment had higher levels of glutamate — an important signalling molecule in the brain. Too much glutamate can disrupt brain function, and a rest period could allow the brain to restore proper regulation of the molecule, the authors note. At the end of their work day, these study participants were also more likely than those who had performed easier tasks to opt for short-term, easily won financial rewards of lesser value than larger rewards that come after a longer wait or involve more effort.

The study is important in its effort to link cognitive fatigue with neurometabolism, says behavioural neuroscientist Carmen Sandi at the Swiss Federal Institute of Technology in Lausanne. But more research — potentially in non-human animals — will be needed to establish a causal link between feelings of exhaustion and metabolic changes in the brain, she adds. “It’s very good to start looking into this aspect,” says Sandi. “But for now this is an observation, which is a correlation.”

https://www.nature.com/articles/d41586-022-02161-5

 

they call it a signalling molecule but does the glutamate serve a useful purpose or is it something like waste?

 

Glutamate is essential, not only because it is the principal excitatory neurotransmitter but also because it serves as a precursor to the synthesis of GABA, the principal inhibitory neurotransmitter.

 

Another article on the paper:
Neurotransmitter Buildup May Be Why Your Brain Feels Tired
Performing complex cognitive tasks leads to glutamate accumulating in a key region of the brain, a study finds, which could explain why mental labor is so exhausting.

https://www.the-scientist.com/news-...ould-explain-why-your-brain-feels-tired-70358

 

is the glutamate's observed behavior in the study essential in that brain location? what i mean is, can we reasonably surmise that it is an evolved response to protect something, or are we talking about something like normal operation creates glutamate in that location for whatever reason and the effects are a side effect of that?

hypothetically, if a medicine is created to normalize it in that location, i can imagine militaries taking up the use of it in some cases, or salarymen becoming more productive with it, or perhaps even a bit of usefulness for us.

but if the glutamate is there for a good reason, and even if it is not, then some or all of that could be harmful.