Zap goes the effect
The team pooled the results of more than 400 studies that reported a change in cognitive skills following a session of tDCS.
“Most studies have more than one outcome measure, such as accuracy, speed, errors made and so on,” explains Horvath. And while one study may show, for example, improved accuracy on a memory task after tDCS but no effect on speed or errors, another memory study may show improved speed, with no effect on accuracy or errors. When put together they cancel each other out. This pattern played out in studies of memory, processing speed and mathematical ability, Horvath found.
Roi Cohen Kadosh, a neuroscientist at the University of Oxford who has studied the effects of tDCS on mental arithmetic, is far from convinced by this argument. “My feeling is that it is very premature to do what they did,” he says. “They did have a large sample size, but they fractured it so that they are comparing the results of three or four studies and expecting to see something meaningful. It’s the easiest thing in science to not find results,” he says.

via Second blow to the head for effects of brain zapping – health – 29 January 2015 – New Scientist.