Stop work now – it causes toxic buildup of waste products in your brain.
People who think for a living – among whom we confidently number all Back Page readers – know the sensation of feeling drained after a day of work, without sometimes even having left your chair.
It’s a dazed feeling that can send some of us beelining towards a beer on what should have been an AFD, some towards a Mars Bar despite having spent not a kilojoule in physical activity.
The appealing ego-depletion theory, in which cognitive effort has a globally sapping effect on the body’s energy, leading to failures of willpower and self-control, has unfortunately failed to replicate.
The authors of this new paper in Current Biology say that functional explanations – the idea that brain gives the user an illusion of fatigue to make them stop what they’re doing and do something more instantly rewarding – are unsatisfying. “If there is a good reason to stop working on a task and turn to a more gratifying activity, the brain could figure it out without generating an illusion of fatigue,” they say, among their reasons.
They propose instead that cognitive fatigue comes from an increased cost of exerting cognitive control because of metabolic changes in the brain, not an increased metabolic burden overall. This extra cost might come from the need to clear out accumulated waste products or to re-up some needed resource.
The team put a few dozen participants in two groups through a daylong protocol of activities requiring high and low cognitive control, and used magnetic resonance spectroscopy to measure the levels and diffusion of metabolites of glutamate, an important excitatory transmitter, in the lateral prefrontal cortex (and in the primary visual cortex as a control). The release of glutamate would both deplete a cell of something that it needs to function and interfere with synaptic transmission, so fulfil both the cost scenarios above.
Sure enough, they found significantly more glutamate in the hard-task group’s lateral prefrontal cortices (but not the visual cortices) after a few sessions, and no changes in the easy-task group, supporting the waste-product hypothesis.
The subjects at points in the day also had to make economic choices between a small-reward/low-cost option and a big-reward/high-cost option. The hard-task group made significantly more low-risk choices than the easy-task group.
The team used eye-tracking during those choices and found less pupil dilation where there was more cognitive fatigue, suggesting lower cognitive effort.
They found that self-reported fatigue did not significantly differ between groups.
The authors note that what they have shown is only a correlation, not causation; and that they chose glutamate partly because it’s easy to measure, and that other related substances might be the actual target of elimination.
Finally, in a restful note, they say glutamate concentrations have been shown to decrease during sleep.
After reading that paper, penny@medicalrepublic.com.au will be in the pub if you need her.
