Don’t get too zap-happy (Image: PASIEKA/SCIENCE PHOTO LIBRARY)

Stimulating the brain with electricity improves working memory, mental maths, focused attention, creativity and could help treat depression. You can even buy DIY kits online. That’s the good news. The bad news is that the most recent investigation has found it has almost no measurable effect on the brain.

It’s a conclusion that is likely to be controversial. Over the past decade, thousands of studies have reported a beneficial effect of transcranial direct current stimulation (tDCS) on the brain, as well as on behaviour and cognition – so much so that it has become something of a hot topic in neuroscience.

The idea behind tDCS is that passing a weak current through the brain changes the electrical potential of nerve cell membranes. This alters the strength of connections between neurons, making the circuit more, or less likely to fire. It’s a tricky thing to measure directly, so any physiological effect is inferred by blood flow changes on functional MRI scans, changes in brainwaves measured by EEG, or in the strength of muscle contraction when the motor cortex is stimulated, known as an MEP.


But when Jared Horvath and his colleagues at the University of Melbourne in Australia, pooled the results of more than 100 studies reporting any or all of these measures, they found that only one was convincingly changed after tDCS. The other two were inconsistent at best.

Got no control

“There doesn’t appear to be any significant or reliable effect of tDCS on blood flow, electrical, or evoked activity within the brain,” says Horvarth. “The only measure tDCS appears to reliably modulate is MEP amplitude – a measure that fluctuates so strongly naturally that it has largely been abandoned as a clinically useful measure.”

And not only that, because just 25 of the 117 studies used a control condition, where electrodes were placed on the scalp but not switched on, it is impossible to know whether stimulation was definitely the root cause of any changes seen.

So is it time to ditch tDCS as an unproven fad? Definitely not, says Roi Cohen Kadosh, a cognitive neuroscientist at the University of Oxford who uses electrical stimulation in his work.

“There are several things that explain what they found and why I am not that concerned,” he says. “First, they didn’t take account of individual differences or variation in stimulation intensity [between studies]. I would not expect that one stimulation would work on everyone or at the same level for everyone” he says. Other studies have found that differences in brain structure affect the dose of electricity that actually gets to the brain, he points out.

DIY, FYI

However, Vincent Walsh, a cognitive neuroscientist at University College London, is less convinced. “This is an important paper,” he says, especially because it casts doubt on the aspect of this research that until now had been assumed to be the most robust – the physiology.

“In terms of cognition, which is the other aspect that people make claims about, tDCS is massively hyped. The danger is that people have been promised better memories, better reading, better maths, increased intelligence… you name it. The effects are small, short lasting, and no substantial claims have been replicated across laboratories. This paper is hopefully the beginning of a counterweight to all the bullshit.”

To that end, Hovarth and the Melbourne team are currently finalising another analysis, this time looking at the evidence for cognitive and behavioural change after tDCS. He won’t give details before publication, but Hovarth hints that many people might think the findings are controversial.

And what are the DIY stimulation enthusiasts to make of all this? “There are two options,” says Horvath. “The first is that tDCS is doing something, but we don’t know what, so take that on board. The second is a bit more innocuous: tDCS might not be doing anything to the brain, so have a good time, but temper your expectations.”

Journal reference: Neuropsychologia, DOI: 10.1016/j.neuropsychologia.2014.11.021