Mental fatigue: When the brain needs a reason to keep going

Ever get so mentally drained that your brain just says, “I’m done”? That feeling of cognitive fatigue may not be as mysterious as it seems.

In a recent study, researchers used brain scans to track what happens in our heads when fatigue sets in – and how we decide whether to keep going or give up.

The research team from Johns Hopkins University School of Medicine and the Kennedy Krieger Institute studied 28 healthy adults using functional MRI. The goal was to observe brain activity during moments of cognitive fatigue.

A closer look at mental fatigue

The volunteers – 18 women and 10 men between the ages of 21 and 29 – were paid $50 to participate. They also had the chance to earn more money depending on their performance. Before starting any tasks, each person received a baseline MRI scan.

Inside the MRI machine, participants were given memory exercises. They saw sequences of letters and had to remember where certain ones appeared. The farther back a letter was in the sequence, the harder it was to recall, making the task mentally taxing.

Before and after each exercise, the participants rated how mentally tired they felt. They also got feedback on their performance and could earn bonuses between $1 and $8 depending on the difficulty of the task they chose.

Two key brain areas light up

The scans revealed something striking. When participants felt mentally tired, two brain regions showed a noticeable increase in activity.

The right insula – a deep brain area linked to feelings of fatigue – and the dorsolateral prefrontal cortex, which helps manage working memory, both lit up. In fact, the activity in these areas more than doubled compared to the baseline.

Dr. Vikram Chib is an associate professor of biomedical engineering at the Johns Hopkins University School of Medicine and a research scientist at Kennedy Krieger Institute.

“Our lab focuses on how [our minds] generate value for effort,” said Dr. Chib. “We understand less about the biology of cognitive tasks, including memory and recall, than we do about physical tasks, even though both involve a lot of effort.”

Dr. Chib noted that while cognitive tasks clearly wear people out, there’s still a lot to learn about how and why that fatigue builds up in the brain.

Motivation matters – a lot

Participants were more likely to push through harder tasks when the rewards were higher. The research team noticed that people needed strong financial incentives to stay mentally engaged.

“That outcome wasn’t entirely surprising, given our previous work finding the same need for incentives in spurring physical effort,” said Dr. Chib.

It seems the brain weighs whether mental effort is “worth it,” and only goes the extra mile when it sees a clear benefit.

“Our study was designed to induce cognitive fatigue and see how people’s choices to exert effort change when they feel fatigue, as well as identify locations in the brain where these decisions are made,” explained Dr. Chib.

Rethinking fatigue in mental health

Fatigue is a major challenge in conditions like PTSD and depression. Understanding how it works in healthy brains is a first step toward better treatment.

“Now that we’ve likely identified some of the neural circuits for cognitive effort in healthy people, we need to look at how fatigue manifests in the brains of people with these conditions,” said Dr. Chib.

He believes the same scanning techniques could eventually help doctors classify cognitive fatigue more objectively. That could open the door for new therapies, including medication and cognitive behavioral therapy.

What the brain scans don’t show

The team is quick to point out the limitations. Functional MRI tracks blood flow in the brain, which signals general activity, but it doesn’t capture the finer details of neuron-level behavior.

“This study was performed in an MRI scanner and with very specific cognitive tasks. It will be important to see how these results generalize to other cognitive effort and real-world tasks,” said Dr. Chib.

Still, the findings offer a clearer view into how we respond to mental exhaustion – and what it takes for the brain to keep going.

The full study was published in the journal JNeurosci.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–