Transcranial direct current stimulation, using a weak electric current in an attempt to boost brainpower or treat conditions, has become popular among cognitive do-it-yourselfers and the neuroscience equivalent of people selling dietary supplements, but a new University of North Carolina School of Medicine study urges caution that should be common sense.

New work found that electric brain stimulation had a statistically significant detrimental effect on IQ scores. Using less common alternating current stimulation - so-called tACS - could be a better approach. Tesla beats Edison once again.

Some believe that tACS significantly boosts creativity because it targets the brain's natural electrical alpha oscillations, which have been implicated in creative thought, but no one knows for sure. With tDCS, scientists don't target these brain waves, which represent neuronal patterns of communication throughout regions of the brain. Instead, they use tDCS to target brain structures, such particular regions of the cortex.

In the new study, Flavio Frohlich, PhD, study senior author and assistant professor of psychiatry at UNC, and colleagues recruited 40 healthy adults, each of whom took the standard WAIS-IV intelligence test, which is the most common and well-validated test of IQ. It includes tests for verbal comprehension, perceptional reasoning, working memory, and processing speed.

A week later, they divided the participants into two groups. Electrodes were placed on each side of each participant's scalp, under which sat the frontal cortex. They created imaging simulations to ensure they targeted the same parts of the cortex that previous tDCS studies had targeted. 

Then the placebo group received sham stimulation - a brief electrical current, which led participants to think they had been receiving the full tDCS. The other participants received the standard tDCS for twenty minutes - a weak electrical current of 2 millioamperes.

All participants then retook the IQ tests. They expected that most, if not all, IQ scores would improve because of the practice effect, but that tDCS would not markedly improve scores. All scores improved but the participants who did not receive tDCS saw their IQ scores increase by ten points while participants who received tDCS saw their IQ scores increase by just shy of six points, on average.\

Not only did it not help, it hurt.

When they analyzed the test scores, they saw that the scores for three of the four main kinds of cognitive tests were very similar between the two groups of participants. But the scores for perceptual reasoning were much lower among people who underwent tDCS. Perceptual reasoning tests fluid intelligence, which is defined as the ability to think logically and apply innovative problem solving to new problems. Within the category of perceptual reasoning, the researchers saw the biggest differences in the subcategory of matrix reasoning - when participants viewed two groups of symbols and had to find the one symbol missing from the other group. 

The tDCS fad started in 2000, when German scientists published a paper showing that tDCS could change the excitability of neurons in the motor cortex - the brain region that controls voluntary body movement. Since then, there's been an explosion of tDCS studies to try to make neurons more active or less active and therefore change outcomes for a variety of brain functions, such as working memory and cognitive acuity, and for illnesses, such as depression and schizophrenia.

But some of the studies that have made waves were poorly designed. Some studies were not properly double-blinded or properly placebo controlled. Other studies were very small - less than 10 people. A recent meta-analysis of a large number of tDCS papers showed that tDCS is far from a magic pill for cognitive enhancement or brain-related health conditions.

 The science world is not beholden to the precautionary principle but it seems wise to let psychiatrists continue to experiment on themselves and undergraduates getting extra credit before rolling this out to the public. The National Institutes of Health funded this study.