Stimulating the brain with electrical signals can sharpen some of your faculties, but now it seems it can dim others at the same time.

Transcranial electrical stimulation (TES), delivered by electrodes on the surface of the head, has been shown to double people’s speed of learning. Now the first evidence has emerged that improvements in one aspect of learning might come at the expense of other abilities.

Roi Cohen Kadosh of the University of Oxford, showed volunteers pairs of unfamiliar symbols. Each symbol had a secret numerical value, and the volunteers’ task was to state – as quickly as possible while avoiding mistakes – which symbol in a pair had the bigger value. The correct answer was then displayed. Over six sessions in one week, it was possible to measure how quickly and efficiently the volunteers learned the value of each symbol.

Second task

In a second task, participants had to register which of each pair of symbols was physically larger, a measure of automatic thinking. “Automaticity is the skill of doing things without thinking about them, such as reading, driving or mounting stairs,” says Cohen Kadosh, who conducted the experiment with Teresa Iucalano of the Stanford Cognitive and Systems Neuroscience Laboratory in Palo Alto, California.


During the experiments, volunteers received TES to their posterior parietal cortex – vital for numerical learning – or their dorsolateral prefrontal cortex – vital for automaticity. Some unknowingly received a sham treatment.

Those who received stimulation to their posterior parietal cortex outperformed the others at the learning task, but performed worst on the automaticity task. The reverse happened in the group who received stimulation to their dorsolateral prefrontal cortex. Those receiving sham treatments had intermediate scores on both skills.

“It’s a proof of concept to show there might be a cognitive cost to TES,” says Cohen Kadosh, who is now testing procedures that may get round the problem, such as stimulating both regions but at different periods in the training. “If there is a cost, we need to optimise stimulation to limit or eliminate side effects,” he says.

Journal reference: Journal of Neuroscience, doi.org/kqm