Neurolectrics (Starstim device) published a white paper (pdf) in October 2013 that nicely collects pretty much all we know to date about tDCS and cognitive enhancement. I was reminded of this while visiting their montages page on their new wiki. Quoted is from the Chi / Snyder 9 Dot study (pdf).
Brain stimulation enables the solution of an inherently difficult problem – Certain problems are inherently difficult for the normal human mind. Yet paradoxically they can be effortless for those with an unusual mind. We discovered that an atypical protocol for non-invasive brain stimulation enabled the solution of a problem that was previously unsolvable.The majority of studies over the last century find that no participants can solve the nine-dot problem – a fact we confirmed. But with 10 min of right lateralising tDCS, more than 40% of participants did so. Specifically, whereas no participant solved this extremely difficult problem before stimulation or with sham stimulation, 14 out of 33 participants did so with cathodal stimulation of the left anterior temporal lobe together with anodal stimulation of the right anterior temporal lobe. This finding suggests that our stimulation paradigm might be helpful for mitigating cognitive biases or dealing with a broader class of tasks that, although deceptively simple, are nonetheless extremely difficult due to our cognitive makeup