Have you ever wondered why it's difficult to concentrate for a prolonged period of time? Most everyone has stories of working long nights, struggling to complete a project that must be done by morning. We've all experienced the gradual decline in cognitive abilities and decreased levels of concentration. But what is the biological cause of such fatigue?
Unfortunately, we're not quite sure yet. However, we've got some pretty good ideas.
The simplest explanation for mental fatigue is known as the depletion model. This model posits that the act of effortful thought consumes some resource (probably glucose) in the brain. As that resource is expended to the point of scarcity we then feel what we refer to as mental fatigue. The depletion model is appealing in it's simplicity, but has become widely criticized in recent years and is currently thought unlikely to be an accurate representation of mental fatigue's cause.
One of the many problems with the depletion model is it doesn't seem to fit some subjective experiences well. For example, most people have experienced waking up to a feeling of mental exhaustion. If the depletion model is accurate than why would a relatively well rested brain have difficulty with resource depletion? Situations such as these have led some people to attempt to create a wholly psychological model of mental exhaustion. This paper discusses a psychological model wherein mental fatigue is viewed as a result of the opportunity cost of executive recruitment. In other words, mental fatigue is caused by using your brain's higher functions to do something unpleasant such as difficult math problems or making conversation with an in-law. The more you'd rather avoid the task, the more rapidly mental exhaustion sets in.
Psychological models often present interesting jumping off points and areas of discussion. However, their lack of a biological mechanism limits their usefulness a great deal. Without some sort of basis in neurobiology such models are little more than thought experiments in terms of application.
The most recent and well regarded models are in some ways the opposite of the depletion models. Rather than focusing on the depletion of some neurological resource these models believe mental fatigue is caused by the accumulation of some excess product in the extracellular area (the area between cells). Each time a neuron activates it releases chemicals into the extracellular space. While the brain has mechanisms to reclaim these chemicals, when present in excess (as during extreme mental exertion), these mechanisms may not be sufficient and an accumulation could occur. This accumulation may then lead to a variety of effects including the activation of unintended astrocytes which we then experience as mental fatigue.
A growing body of evidence seems to support this recent model. For example, a recent study has found that during sleep these reclamation mechanisms go into overdrive and clean these excess chemicals from the extracellular space. This would explain why sleep is usually mentally reinvigorating. Studies have also shown that in cases of brain damage with mental fatigue as a symptom these systems are generally not working optimally due to inflammation.
Interestingly, while consensus seems to be moving towards these "excess" models, no one can seem to agree what exactly the brain is experiencing an excess of. A recent study published in Science proposed that metabolites are the culprit while others suspect glutamate (a neurotransmitter) is the main cause. Still, while the exact chemical responsible is a topic of debate most neuroscientists agree that an excess of something between cells is likely the cause of mental fatigue.
For the sake of brevity I have touched very lightly on each of these models. If you're interested in reading more regarding any of them the following links will be of interest:
Psychological/Opportunity Cost: http://www.sas.upenn.edu/~duckwort/images/publications/KurzbanDuckworthOpportunityCost.pdf
That's all for this week. Until next time stay safe and rationale.