Why our brains wear out at the end of the day
The big question is: what is going on with cognitive fatigue. If you look at chess players who exert a lot of cognitive effort over hours of play, they get worse and make more mistakes. It takes them longer to make decisions. The question is, why? Some scientists think that maybe the brain gets fatigued over time and has to put energy into getting back up to speed than actually thinking effectively.
How does your brain tire?
To date, it has been hard to tease out what is responsible for this. New research suggests the cognitive control centre of the brain is probably somewhere in the left lateral prefrontal cortex.
Recent studies show that the prefrontal cortex becomes less active while a person becomes tired and discontinues as cognitive fatigue escalates. Functional MRI studies of the LLPC show a reduction in activity as people become more fatigued. Over time, though, it stops short with no connection to glucose levels or time on task. What is going on in the prefrontal cortex? The paper sets out to figure out what has been found in the past but comes up empty-handed over the prefrontal cortex.
Volunteers were told to do switching tasks for more than six hours. They performed cognitive exercises that looked challenges for researchers. Investigators showed a letter, and participants would respond based on whether it was red or green, with capital or lowercase letters.
This task was difficult. It took 6 hours of work before they finished a single exercise. They had an ideal subject – someone who did something similar for 6 hours, but did not switch between colours as often – less than once per session. The study group also had a memory challenge that worked similarly.
This study presented some metrics relating to the cognitive challenge and did not present metrics for the easy challenge. I will use a few to describe it.
The first issue is that of performance. Did they get it wrong? What percentage of the time did the participant say consonant when they should have said lowercase?
These results are less significant rather than what is a reliable finding. However, both groups seem to lose a little bit throughout testing overall. The slope of each line is similar, so the data is less impressive than expected.
The researchers asked how tired the participants felt from serving as human writing subjects.
The groups were worn out as it was a long day. There were suggestions that the hard group became tired by hour 6, but I don’t think this is significant. Everyone was getting tired by hour 6 here.
How quickly could this individual respond? A participant was asked to say consonant, vowel, lowercase, or uppercase.
Because it was difficult, the response time was longer, but eventually, response times became more or less uniform.
There is an absence of robust readout that would make one say, Oh yeah, the person is cognitively fatigued. Instead, it is measured based on how people act, how quick they are, and how accurate their actions are.
However, the investigators decided to change up the game. Participants were asked to play a reward game. Here are two examples.
Would you rather:
- Have a 25% chance of earning $50 OR a 95% chance of making $17.30?
- Earn $50, but your next task session will be hard or earn $40, and your next task session will be easy?
Participants chose between lower-cost, higher-risk or moderate-cost, lower-risk choices. They also had to decide if they were cognitively fatigued so they could take advantage of the low-risk choice instead of the higher-cost option that may have been less lucrative.
When cognitive fatigue occurs, the easier option is likely to be taken over the difficult choice. The difficult group participants were more likely to pick the easy thing rather than the potentially more lucrative thing, which is interesting when we think about how our cognitive fatigue happens at work.
There are objective physiological measurements for this, and they include pupil dilation.
When our attention is focused, our eyes dilate a little. The researchers found that as the hard group became more tired, their eyes constricted a bit which was a sign of registering signals from fatigue. But in contrast, the easy group did not slow down; their pupils continued to dilate. Rising levels of fatigue were reflected physiologically by an increase in sluggish pupils.
The Brain LLPC is responsible for generating fatigue; however, the ultimate goal of the study is to understand how different parts of the brain process sleep loss. Potentially, changes in the LLPC function could reveal biomarkers that could guide research into interventions and treatments. They used a labelled hydrogen MRI technique to measure metabolites found in the Brain.
In the brain there should be very little cortex involved because the action is happening in the LLPC. The V1 part of the Brain having control over our actions, is a section called the Lateral Lateral Prefrontal Cortex. They looked at metabolites, but the only one that behaved according to their expectations was glutamate and glutamic acid (glutamate metabolites). In the difficult group, there is a buildup of glutamate within V1. There is also greater diffusion for glutamate from intracellular to extracellular space and greater vulnerability in the cells because it seems out.
What does this all mean? Well, it means that the thing that stems from these spikes in glutamate is that, following a time-intensive decision, people take pleasure in taking inexpensive decisions.
It is not a significant correlation, but it is interesting because the glutamate in the LLPC is associated with making less effortful decisions. This compensation may be an evolutionary mechanism designed to ensure that the Brain remains stable during periods of increased neurotoxicity.
Another curious thing about the current work is that a brain reward center (the striatum) seems to be down-regulating itself with glutamate. It means that too much of the neurotransmitter is being released, triggering more releases of the same neurotransmitter, and creating an endless cycle of excitatory signals.
It is a fascinating study. Although, it is not clear whether this part of the brain can be manipulated. However, it is nice to see some biological correlates of a psychological phenomenon described so well - the decision fatigue phenomenon. That same tiredness you feel when working hard and missing your sleep. Sleep clears some of those glutamate levels in the LLPC, as starting over seems like an option.
Source: https://www.mdedge.com/psychiatry/article/257082/alzheimers-cognition/why-our-brains-wear-out-end-day
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