Have you bought your transcranial direct current stimulation (tDCS) head-set yet? You've heard of this, right? It's a device with electrodes that zap your brain through your skull, using mild electrical currents to make you smarter. The man in the picture above sports one of the first commercially available devices. Produced by Foc.us, it's available for $249, and also comes in black. This technology is far from new - Roman physician Galen was on to something similar when he slapped electric fish on his patients' heads. But tDCS is now in the process of going mainstream: there are DIY brain-zapping enthusiasts on YouTube; last year MTV editor Mary H K Choi wrote an amusing but inconclusive tDCS self-experimentation piece for Aeon; and just the other day, Oliver Burkeman included tDCS in his roundup of new brain-enhancing technologies for The Guardian.

The manufacturers claim that the tDCS headset will "overclock your brain", increase your brain's plasticity and "make your synapses fire faster". Overclocking sounds a bit dangerous, and rather than your synapses, wouldn't it be better to make your neurons fire faster? Synapses are the junctions between neurons. We usually say it's neurons that "fire" and their message is passed across one or more synapses to other neurons using chemicals. Unless the marketing people were talking specifically about electrical synapses? But sorry, I'm rambling. Must focus. "Foc.us". Need more electric current. Hang on …

Phew, better … To be fair to Foc.us, the idea of having faster synapses at the flick of switch sounds appealing, and, believe it or not, their claims for the brain-enhancing effects of tDCS are not entirely unfounded. In fact, almost each week there's a new study claiming that tDCS can boost yet another aspect of mental function. Zapping different parts of the brain has been linked with superior learning of new motor skills; better math skills; better social skills; superior learning and memory; and on the list goes. tDCS is also being investigated as a treatment for a range of psychiatric and neurological problems, but for today let's focus on mental enhancement for healthy people.

From a physiological perspective, tDCS affects brain function in two ways - by altering the baseline activity level of targeted neurons and by modifying functioning at synapses. The effect on neuron activity levels occurs while you zap; the synaptic influence is a longer-lasting after-effect. The specific changes depend on a whole range of factors, most obviously whether the current is positive ("anodal"), which increases neuronal activity, or negative ("cathodal") which suppresses it. tDCS is not to be confused with electro-convulsive therapy (for severe depression and other conditions) in which a much higher current of electricity is used to deliberately induce a brain seizure.

So the brain changes triggered by tDCS are real. And there are those findings in peer-reviewed journals showing a range of appealing cognitive benefits. What's not to like? Well, I confess I'm geeky enough to have compiled and read a number of cautionary science papers on tDCS published by experts over the last couple of years, and they certainly give pause for thought. Before you start revving up your grey matter with extra electricity, I suggest you bear in mind the following caveats and warnings:

Most studies looking at the cognitive benefits of tDCS fail to include adequate blinding and control conditions. This means the researchers and the lab rats - sorry, participants - both know who is receiving the real intervention. Big placebo effects are therefore likely because participants will have expectations of some kind of effect, and researchers could also influence outcomes with their enthusiasm or expectations. You don't just strap on a tDCS headset and become instantly smarter. Shucks. The experts say that the technique works by enhancing the effects of learning and practice. You still have to put effort in. "tDCS alone is of little use," Roi Cohen Kadosh, a leading researcher in this area, told me last year. ‘The advantage of it is when it is combined with a cognitive training, rather than just applied alone to the brain," he said. But even then it doesn't work for everyone. There's huge variability in the effects of tDCS between individuals, and probably also in the same individual from one session to another. "Unfortunately … response reliability at the level of the individual has not been explored (or at least reported) in the literature to date," say Jared Horvath and colleagues. Factors to do with fatigue and hormone levels are also likely to interact with tDCS in ways we don't yet understand. All of which makes it hard to know the optimal and safe level of zapping to use. Bad news! "Meddling with the tDCS dose is potentially as dangerous as tampering with a drug's chemical composition," say Marom Bikson and colleagues in their recent Letter to *Nature *entitled: "Transcranial devices are not playthings". Other factors that will interfere with the dose include how much hair you have on your head and whether or not you sweat a lot. Effects of brain zapping can accumulate over time and the long-term consequences of this are unknown. Researchers studying tDCS are very careful to target specific brain areas. How will you know you're zapping the right part of your brain? This is particularly important for left-handers, who can have functional hubs located on a different side of the brain than usual. What you do after a brain zapping session can modify or completely nullify any effects of the electricity. Walking around or having specific thoughts is all it takes to potentially reverse the effects. Research on this problem is still in its infancy, so there's no way you can know how best to behave after a tDCS session to preserve any potential benefits. If you enhance mental function in one area, it can actually have an negative impact on another aspect of mental function. Because the neural effects of tDCS can be long-lasting, what might be advantageous in one situation could therefore leave you impaired in a different context later. Misuse of the technology could risk seizures or scalp burns. Also watch out for itching, fatigue and nausea. Nick Davis and colleagues say it's a mistake to think of brain zapping as non-invasive. "Any technique which directly affects brain tissue to generate such powerful acute and long-lasting effects should be treated with the same respect as any surgical technique," they write. On the plus side, a 2011 paper stated that "no serious side effects have occurred" in more than 100 studies with patients and healthy controls. Photographic evidence from Foc.us suggests that too much tDCS causes a desire to squinch.

Disclaimer: Despite possible appearances to the contrary, this post was written by an under-clocked brain that's not yet been zapped by tDCS.