Jared Horvath, a neuroscientist at the University of Melbourne, in Australia, looked at every study of transcranial direct current stimulation (tDCS) that reported an impact on cognitive and behavioral activities such as problem solving, learning, mental arithmetic, vision tasks, and memory games. He then excluded results that had not been replicated by other researchers, as well as any experiments lacking a “sham condition” control group—where participants were connected to the device but didn’t receive current. While many of the more than 200 individual studies that remained claimed to have found significant effects, those effects disappeared after Horvath’s number crunching. “When I pulled out the 20 studies looking at tDCS and working memory, for example, they all found something, but they all found something different,” says Horvath.
One study may have found an effect on accuracy, another on reaction time, and a third on response confidence. “But when I brought them together, they just canceled each other out, and I was left with nothing,” he says. It was a similar story for more than 100 other cognitive and behavioral outcomes. “It looks like the evidence says tDCS is not doing anything.”