Zapping part of the brain makes you think harder – at a cost

Struggling with a difficult mental task? Zapping the brain region linked to our motivation to make a mental effort might help – for a while. People who had this part of their brain stimulated by electrical currents excelled at a difficult task, but also became more mentally fatigued afterwards.

“Their performance is much better; however, this good performance comes with lower motivation in the end,” says Hongmei Xia at the University of Munich, Germany.

Xia and her colleagues put electrodes on the heads of 31 volunteers with no known health conditions. During a half-hour session in an MRI scanner, the volunteers spent 20 minutes doing the 3-back memory test, where you have to remember and identify the third item back as an ongoing sequence is presented to you – a very demanding mental task. They were then offered €1 for doing an easy test or €2 for a harder one.

During the 3-back memory test, the electrodes delivered transcranial direct current stimulation (tDCS) to a region of the brain called the dorsolateral prefrontal cortex to randomly chosen volunteers, who were unaware they were receiving it.

Other studies have shown the dorsolateral prefrontal cortex is involved in controlling the cognitive effort we make. “Our dorsolateral prefrontal cortex helps us to work hard,” says Xia.

The volunteers who had tDCS reacted faster and more accurately than those who didn’t receive this stimulation. For instance, their average reaction time was 0.98 seconds, compared with 1.12 seconds with sham stimulation.

Xia thinks stimulating the dorsolateral prefrontal cortex will boost brain performance generally. “I can imagine any cognitive task will be better, but afterwards you will feel more fatigued,” she says.

After the tests, the volunteers who got tDCS had higher levels of the neurotransmitter glutamate in their dorsolateral prefrontal cortex, according to a non-invasive technique called magnetic resonance spectroscopy. The higher their glutamate levels, the more likely they were to choose the €1 task over the €2 one.

Some researchers think mental fatigue is a result of the buildup of glutamate, which can harm cells at high levels. Xia thinks her team’s findings are the strongest evidence of this yet. The findings suggest it is important to take a break when you feel mentally tired, she says.

Xia thinks future studies should also look at the dorsolateral prefrontal cortex in people with conditions such as depression, which can affect motivation.

“I think the claim that the stimulation made people ‘work harder’ is well substantiated,” says neuroscientist Emile Radyte at Samphire Neuroscience, a UK-based company exploring the use of tDCS to treat conditions such as premenstrual syndrome. But the study doesn’t prove that higher glutamate levels were the cause of the greater mental fatigue, she says.

Radyte says her trials have shown that if productivity is hindered by conditions such as premenstrual syndrome, applying tDCS over the dorsolateral prefrontal cortex can restore it. But she isn’t sure about using it to boost mental performance. “A 20-minute laboratory task does not establish a net productivity benefit over a working day,” says Radyte.

It is a good study, says Nilli Lavie at University College London, but the greater mental fatigue could be a result of using more neural energy, rather than the buildup of glutamate. It’s also hard to precisely target the dorsolateral prefrontal cortex with tDCS without an MRI scanner to guide you, says Lavie. “Drink coffee instead,” she says.