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Now we know – zapping the brain with electricity really does seem to improve some medical conditions, meaning it may be a useful tool for treating depression.

Transcranial direct current stimulation (tDCS) involves using electrodes to send a weak current across the brain. Stimulating brain tissue like this has been linked to effects ranging from accelerated learning���ٴ��� and faster recovery from .

Thousands of studies have suggested the technique may be useful for everything from schizophrenia and Parkinson’s to tinnitus and autism.

However, replicating such studies has generally been difficult, and two recent analyses found no evidence that tDCS is effective, leading some to say that the technique is largely a sham.

“There are too many folks out there right now who are using electrical brain stimulation in a cavalier way,” says , a tDCS researcher at Rio Grande Neuroscience in Santa Fe, New Mexico. “At best it has an effect that’s poorly understood, at worst it could be dangerous.”

Now a review has weighed up the best available evidence. It has found that depression, addiction and fibromyalgia are most likely to respond to tDCS treatment.

Reliably better

, a neurophysiologist at Henri Mondor Hospital in Paris, France, and his team concluded this by sifting through all tDCS studies so far. Unlike the two previous analyses, this one didn’t lump together studies of variable sizes and designs. Instead, the team chose only studies that were placebo-controlled, used tDCS as a daily medical treatment, and involved at least 10 participants.

Analysing these high-standard studies revealed that tDCS seems to reliably improve the symptoms of depression, addiction and craving, and fibromyalgia. It also uncovered that the technique does not work for tinnitus, and that the evidence for using tDCS for stroke rehabilitation was not as strong as many had thought.

This underscores the most significant finding of the analysis, says team member at  Göttingen University in Germany: tDCS seems to have the most effect when restructuring of the brain isn’t required. Simply exciting some neurons in people with tinnitus is unlikely to do much to alleviate it, he says.

As for using electric current for rehabilitation after a stroke, the team uncovered one study in which brain function deteriorated after the administration of tDCS.

Personalised treatment

“This is an important paper,” says , a biomedical engineer at City University of New York.

“It is really the type of work that could push the field further,” says Roi Cohen Kadosh at the University of Oxford.

But the study is unlikely to be enough to push tDCS closer to use in mainstream clinics. Most of the good studies had fewer than 25 participants. In contrast, a single trial of a new antipsychotic drug, for example, may involve as many as 18,000 volunteers. For now, “possible or probably effective” is the best label tDCS can hope for, says Weisend. “The community needs to push for bigger trials.”

However, the team’s findings do shed some light on what tDCS treatment may look like in the future. The analysis identified important factors, such as the exact position of the electrodes used, a person’s physiology and their particular brain chemistry. If adopted, there won’t be a one-size-fits-all tDCS treatment. Instead, tDCS will be tailored to individuals based on the drugs they are already taking – for example, antidepressants that boost serotonin in the brain – the thickness of their skull and other factors.