Electrical stimulation of a brain region linked to motivation helped volunteers perform a demanding mental task more accurately and quickly, but it also left them more mentally fatigued afterwards, according to a study led by researchers at the University of Munich in Germany. The findings suggest that a short-term boost in mental performance may come at a cost to motivation over time.
The team, led by Hongmei Xia, placed electrodes on the heads of 31 volunteers with no known health conditions. During a half-hour session inside an MRI scanner, participants spent 20 minutes completing the 3-back memory test, in which they must identify the item shown three positions earlier in an ongoing sequence of items. The task is highly demanding. Afterwards, volunteers were offered €1 for an easy test or €2 for a harder one.
During the memory test, the electrodes delivered transcranial direct current stimulation, known as tDCS, to a region of the brain called the dorsolateral prefrontal cortex. Volunteers were randomly assigned to receive the stimulation and were unaware whether they were receiving it. Other studies have shown that this region plays a role in controlling the cognitive effort people put into tasks. « Our dorsolateral prefrontal cortex helps us to work hard, » Xia says.
Volunteers who received the stimulation reacted faster and more accurately than those who received a sham version. Their average reaction time was 0.98 seconds, compared with 1.12 seconds with sham stimulation. The benefit, however, carried a trade-off. Xia says the team expects that any cognitive task would be performed better under stimulation, but that people would feel more fatigued afterwards. « Their performance is much better; however, this good performance comes with lower motivation in the end, » she says.
The researchers also used a non-invasive technique called magnetic resonance spectroscopy to measure levels of glutamate, a neurotransmitter, in the dorsolateral prefrontal cortex. After the tests, volunteers who had received tDCS showed higher glutamate levels in that region. The higher their glutamate levels, the more likely they were to choose the €1 task over the €2 one, suggesting a link between glutamate and reduced willingness to exert effort.
Some researchers think mental fatigue results from a buildup of glutamate, which can harm cells at high concentrations. Xia believes her team’s findings provide the strongest evidence for this idea so far. She says the results suggest it is important to take a break when you feel mentally tired. She also thinks future studies should examine the dorsolateral prefrontal cortex in people with conditions such as depression, which can affect motivation.
Outside researchers offered a measured response. Emile Radyte, a neuroscientist at Samphire Neuroscience, a UK-based company exploring tDCS to treat conditions such as premenstrual syndrome, says the claim that stimulation made people work harder is well supported. However, she notes that the study does not prove higher glutamate levels caused the greater fatigue. Radyte says her own trials have shown that when productivity is hindered by conditions such as premenstrual syndrome, applying tDCS over this region can restore it, but she is not sure it can boost performance in healthy people. « A 20-minute laboratory task does not establish a net productivity benefit over a working day, » she says.
Nilli Lavie, a researcher at University College London, describes the study as good but says the greater mental fatigue could instead result from the use of more neural energy rather than a buildup of glutamate. She also notes that targeting the dorsolateral prefrontal cortex precisely with tDCS is difficult without an MRI scanner to guide the placement of electrodes. Her practical advice was simple: « Drink coffee instead, » she says.
The study was published in the journal PNAS.
Source: newscientist.com
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