After a hard day at the office, where you were focused intently on a challenging project, you may consider a choice on the way home: impulsively splurge on a fancy dinner as a reward for that cerebral slog or save that bit of cash as planned—perhaps putting it toward a relaxing vacation next month. Despite any frugal inclinations, your weary noggin may not be able to rally your normal level of willpower, according to a new study in the Proceedings of the National Academy of Sciences.
In the study involving 52 healthy adults, researchers found that six hours of challenging cognitive tasks fatigued a part of participants' brains involved in higher thinking and willpower—the lateral prefrontal cortex (LPFC)—and increased their impulsiveness. The finding suggests that real-life scenarios, such as having a mentally taxing day at work, may have critical but unappreciated impacts on people’s economic decisions. It also may point to the need for more brain-resting periods throughout a workday.
“The number and duration of work breaks could be adapted to avoid any dramatic LPFC dysfunction,” the authors of the paper conclude. And, they note, if employees have control over their own work breaks, they might instinctively avoid this brain drain, as researchers saw in a control group.
In their experiment, researchers divided participants into three groups: one that had to perform tough cognitive exercises (involving working memory and task switching) through a period of a little longer than six hours, including two 10-minute breaks; a second that had to perform relatively easy cognitive tasks over the same period, also with the breaks; and a third group that got to read and play video games at their discretion over the same six-hour time period.
Each group had its time divided into 30 minute sessions, after which researchers had them choose between immediate monetary rewards or larger, delayed pay-offs. Also during the study period, participants had their brain activity periodically measured with functional magnetic resonance imaging, which assesses activity based on relative blood flow through various brain regions.
The researchers found that after about four and a half hours, the 27 participants performing the toughest mental tasks became more impulsive than the other groups—even taking into account each participant’s natural impulsive inclinations. Also, those participants getting the hardest mental workout showed reduced activity over time in their brains’ LPFC, which is involved in tough cognitive tasks and self-control.
The researchers speculate that the heavy mental lifting from hours of cognitive exercises tires out the brain and reduces activity in LPFC. And this cuts down on willpower, because the region is also involved in that mental feat. Thus, the authors argue, willpower and self-control may rely on limited resources that are shared with other cognitive processes.
This idea, that willpower draws on shared, limited resources, is not a new one; it has been around for decades and is called ‘ego depletion.’ But the idea has faced criticism recently after some studies and a meta-analysis failed to reproduce the effects seen in earlier works. However, the authors of the new study argue that some researchers failed to capture ego depletion because they didn’t tire people’s brains out for long enough. That’s why the authors were looking at six-hour-long tests, which also happen to be closer to real-world work schedules.
Researchers will have to validate the current study and do more tests before there’s something resembling a consensus on whether ego depletion is real. And even if the idea stands, the authors admit that scientists don’t have a clear biological explanation of how it might work in various regions of the brain. For instance, it's unclear what ‘resource’ might be limiting to undermine willpower. Blood glucose was suggested earlier, but it didn’t hold up in experiments.