We would surely all love a way to boost our brain power. But new research suggests that one promising experimental method could come with a cost. Using a noninvasive technique to stimulate the brain, researchers found they could enhance learning when they targeted a certain spot. But that also made people worse at automaticity, or the ability to perform a task without really thinking about it. Stimulating another part of the brain had the reverse effect, on both learning and automaticity.

“This tells us something about the human brain,” says lead researcher Roi Cohen Kadosh, a neuroscientist at the University of Oxford, in England. “We can’t ask for everything without paying a price.” The findings were published Tuesday in the Journal of Neuroscience.

Cohen Kadosh and collaborators used a technique called transcranial electrical stimulation (TES), a noninvasive method for stimulating specific parts of the brain. The approach has previously been shown to enhance various brain functions, including working memory and attention, and is being used to help stroke patients regain lost language and motor skills (see “Repairing the Stroke-Damaged Brain”). But until now, little research had been done on whether improving performance on one task would come at the detriment of others.

“Very few people have thought about the real-world pragmatics of using this kind stimulation to improve function,” says Eric Wassermann, a neurologist at the National Institute of Neurological Disorders and Stroke, who was not involved in the study.

In the experiment, researchers asked volunteers to memorize the relationship between a set of numbers and a set of figures. For an hour each day over five days, they performed a test asking which symbol represented the larger number. The participant’s performance on the task quickly plateaued.

On the sixth day, participants were asked to ignore the numerical value of the symbol and report which of the two symbols was larger in physical size. The people who had become the most adept at recognizing the new symbols, a measure of automaticity, performed the worst, especially when the physical size of the symbol was at odds with its numerical value.

The researchers compared performance among three groups—those who had stimulation to the prefrontal cortex, which is linked to complex planning and decision making, stimulation to the parietal cortex, part of the brain that helps integrate different types of information, and sham stimulation, in which participants thought they were getting the treatment but were not. The parietal stimulation group learned the best but had the worst automaticity, whereas the prefrontal cortex group had the opposite pattern. (It may seem counterintuitive that learning and automaticity can be dissociated, but they can.)

Wassermann says the findings are interesting but don’t necessarily imply that stimulation is detrimental to the brain. Boosting one cognitive system may tax another, regardless of how you target the brain. (Simply changing the environment can influence how well someone learns new information, for instance.)

It’s also not yet clear how significant the findings are, especially when it comes to real-world applications. Kadosh says his team has not seen the same trade-off for some other cognitive tasks. And he says that it may be possible to reduce the effect by optimizing the parameters they use to stimulate the brain.

Moreover, he says, people who suffered language loss after a stroke are likely willing to accept some trade-offs to get back on track. “There is a lot of potential there to enhance cognition,” says Kadosh. “But we need to know what the cost is.”