New study warns of mental cost to cognitive enhancement.

Cognitive neuroscientists from the University of Oxford find that gains in cognition through transcranial electrical stimulation often leads to deficits in other parts of the brain.
The researchers found that the volunteers who had the parietal area involved in numerical cognition stimulated learned the new number system more quickly than those who got sham stimulation. However, "at the end of the weeklong study reaction times were slower when they had to put their newfound knowledge to use to solve a new task that they hadn't seen during the training sessions."

In other words, artificially stimulating one's brain may lead to a short-term enhancement but to a weaker consolidation of learning and poorer long-term results.