Mental fatigue can trick the brain into taking the easy way out

The Hindu

17 hours ago

After a long day at work, it's easier to flop on the couch than hit the gym. Scrolling over social media wins over reading a book. Our exhausted brains favour easier tasks, even if — according to a new study in the Journal of Neuroscience — those tasks offer fewer benefits.
Even sedentary work can feel exhausting when sustained for hours together. This exhaustion is called cognitive fatigue and builds up with continuous effort to maintain focus on daily tasks. While cognitive fatigue affects our performance and focus, the new study has found that it also influences decision-making by making one prefer easier tasks.
'Everybody has different capacities for work,' Vikram Chib of the Johns Hopkins School of Medicine and the lead author of the study, said. 'We all have a subjective sense of effort — what you may find easy, I might find difficult.'
The study also provides a neurobiological model to explore how fatigue can influence everyday decisions.
'Really cool'
But first, how does sustained effort build up to cognitive fatigue, and how does it affect decisions about exerting further effort?
To answer these questions, the researchers recruited healthy volunteers to repeatedly perform a working memory task. These individuals focused on a screen flashing different letters one by one. In the easy version of the task, they had to recall if the current letter matched the preceding one. As the task got harder, volunteers had to recall if the current letter matched with those displayed anywhere between two to six letters prior. After performing the harder tasks many times in a row, participants reported feeling fatigued.
Next, the participants were offered two choices: easy version of the task for less money or difficult version of the task for more money. Fatigued participants chose the easy option even if it meant earning less money, whereas they opted for harder tasks for more money when they were rested.
'It was really cool to see something we expected, but was never quantified before, that fatigue actually influenced the participants' choices,' Dr. Chib said.
While the participants made their choices, the researchers tracked their brain activity using functional magnetic resonance imaging (fMRI). This noninvasive technique allows scientists to pinpoint brain regions that are active during a task. As participants performed the working memory task and reported feeling fatigued, fMRI revealed the dorsolateral prefrontal cortex (dlPFC), located just behind the forehead, was activated, in line with its understood role.
fMRI scans also revealed the right anterior insula was active when the participants made their choices. This region judges whether the effort required for a task is worth the reward. In previous studies from Dr. Chib's group, the insula had been found to estimate the effort of performing a physical task while physically fatigued.
Pushing harder
In this study, the role of the insula was extended to evaluating cognitive effort. 'This is our current working model of where effort is evaluated in the brain. The insula receives input of fatigue resulting from working memory tasks from the dlPFC and of fatigue resulting from physical tasks from the motor cortex, and it guides decisions about future effort based on the level of fatigue,' Dr. Chib explained.
'A big strength of this study is that it finds connectivity linking exertion to choice,' Antonius Wiehler, a researcher studying motivation and cognitive fatigue at the Paris Brain Institute, said. Indeed, Dr. Chib and his team found that signals from dlPFC representing fatigue influenced the neural activity of the insula. This meant fatigue directly changed the value of effort, making the task seem more demanding than when a person was rested.
The researchers also reported a peculiar observation: while participants reported feeling fatigued as they performed the hard tasks repeatedly, their performance didn't dip. The researchers interpreted this to mean that as people performed harder tasks, the fatigue made them pick easier tasks rather than compromise their performance.
Dr. Wiehler, who wasn't part of the study, agreed: 'When the stakes are high and the benefits are clear, such as in this study, people can still use cognitive resources despite needing elevated effort. However, when benefits are uncertain or subjective, like in market decisions, the increased cost leads to behavioural changes.'
We have all experienced this in our daily lives: when a deadline looms, we push ourselves harder to finish a task at hand despite needing more effort.
The study also opens new questions. Dr. Wiehler, who induces cognitive fatigue on much longer time scales (up to six hours) in his research, wondered: 'How are short-term and long-term fatigue effects related? How are cognitive capacities restored after a night of sleep?'
Can cognitive fatigue influence effort-based choices for any task? 'We have a new study coming up where we report that cognitive fatigue, as induced here with the working memory tasks, makes participants choose even a physical task requiring less effort,' Dr. Chib said.
In other words, there may be a neurobiological reason to avoid the gym when one is mentally exhausted.
Time for a break
Cognitive fatigue is a common symptom of many neurological and psychiatric conditions. Patients who have suffered stroke and those suffering from multiple sclerosis, chronic fatigue syndrome, depression, anxiety, and schizophrenia have reported mental fatigue. Yet the way fatigue manifests in these conditions is poorly defined. The new study is a step towards developing ways to manage fatigue by understanding how it affects effort and decision-making in the brain.
How can we avoid fatigue skewing our daily decisions? Dr. Chib advised being 'mindful about taking breaks. While we haven't directly tested it, reframing tasks can also help.'
The next time you catch yourself making a poor but easy choice while mentally tired, you know it's time for a break.
Sheetal Potdar has a PhD in neuroscience and works as a science writer.