Researchers at the University of Munich stimulated a brain region called the dorsolateral prefrontal cortex in 31 volunteers using transcranial direct current stimulation (tDCS) while they performed a demanding memory task. The stimulated volunteers showed improved performance, reacting faster and more accurately than those receiving sham stimulation. Their average reaction time was 0.98 seconds compared with 1.12 seconds without stimulation.
However, the stimulated volunteers experienced greater mental fatigue afterward. When offered a choice between a €1 easy task or €2 harder task after testing, those who received tDCS were more likely to choose the easier, lower-paying option.
Hongmei Xia, who led the study, found that volunteers who received tDCS had higher levels of glutamate, a neurotransmitter, in their dorsolateral prefrontal cortex. "Their performance is much better; however, this good performance comes with lower motivation in the end," Xia says. Higher glutamate levels correlated with choosing the easier task. Some researchers believe mental fatigue results from glutamate buildup, which can harm cells at high levels.
Xia suggests the findings provide strong evidence for this mechanism and recommends taking breaks when mentally tired. She plans future studies examining the dorsolateral prefrontal cortex in people with depression.
Emile Radyte at Samphire Neuroscience, a UK company, says the claim that stimulation made people work harder is well supported, but notes the study does not prove that glutamate buildup caused the fatigue. Nilli Lavie at University College London suggests greater fatigue could result from increased neural energy use rather than glutamate accumulation. Lavie also notes that precisely targeting this brain region with tDCS normally requires MRI guidance.