Researchers who threw a bucket of cold water over brain stimulation science last year have done it again.

In November, they found that transcranial direct current stimulation (tCDS) – which involves applying current to the brain to alter how likely the neurons are to fire – has no consistent physical effect. Now it seems the same may apply to its effect on the brain’s information processing.

In recent years tCDS has been shown to improve everything from memory to mathematical ability in healthy volunteers and has even found its way into commercial, performance-enhancing products. But according to Jared Horvath and his colleagues at the University of Melbourne in Australia, it might not be all that.

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.

Background noise

He is also unconvinced by the team’s assertion that most studies fail to take account of fluctuations in our cognitive prowess that happen over a period of hours or days and might affect results more than a short sharp zap in the head. Horvath says these may be influencing or even driving the documented effects, but are not controlled for in most studies. Cohen Kadosh says they are “not major factors”.

There is one thing they both agree on, however. That there isn’t nearly enough information yet to work out what is going on. “This is still a young field of research so we still need to be really careful when we interpret the results from tDCS. The real results will come when we have enough data to make meaningful conclusions,” say Cohen Kadosh.

Horvath agrees. “I’m not saying tDCS does nothing… but neither am I saying tDCS does something. I am merely saying that we do not know and the research to date is such that we can’t conclusively say anything. We need to return to basic, systematic, rigorous research that tweaks one variable at a time.”

Whatever the ultimate outcome, if you like your science cut and dried, now probably isn’t the time to order a home tDCS machine.

Journal reference: Brain Stimulation, DOI: 10.1016/j.brs.2015.01.400