The news that scientists stimulated the brain by applying electricity and increased the numerical ability of a handful of subjects does not come as a surprise.
It's a little known fact that the brain already produces tiny electricity pulses for movement, thinking, cognition etc. Two excellent, well written (albeit rather old) answers can be found here.
What's interesting though is that the electrical stimulation doesn't cause any apparent discomfort, that its effects are hot short lives, that it doesn't appear to affect other brain functions, that it can be done without any visible effects (no scars or implants) and that changing polarity may have the opposite effect (doing worse rather than better).
An interesting parallel is that of PC overclockers who often increase the default voltage of a computer processor to allow a more stable overclocking and hence producing better performance.
And there are scores of documented studies showing how electricity can be used on the brain, not to boost its performance but to alleviate debilitating illnesses like Parkinson's.
Wired has a great introduction to the work of neurosurgeons who, a decade ago, had implanted hundreds of pacemaker-like devices in the brain of patients in the US.
But as Fergus Walsh, the medical correspondent of BBC News puts it, "Much bigger and more detailed research is required before any robust claims can be made about the electrical stimulation and maths ability”.