I have been following the research on transcranial direct current stimulation of the brain, with great interest. Many papers have reported positive results, suggesting that the procedure may have real benefits as a cognitive enhancer.
But now a demonstration by György Buzsáki raises questions:
“When Buzsáki and his colleague, Antal Berényi, of the University of Szeged in Hungary, mimicked an increasingly popular form of brain stimulation by applying alternating electrical current to the outside of the cadaver’s skull, the electrodes inside registered little. Hardly any current entered the brain. On closer study, the pair discovered that up to 90% of the current had been redirected by the skin covering the skull, which acted as a “shunt,” Buzsáki said.
(…)
The new, unpublished cadaver data make dramatic effects on neurons unlikely, Buzsáki says. Most tDCS and tACS devices deliver about 1 to 2 milliamps of current. Yet based on measurements from electrodes inside multiple cadavers, Buzsaki calculated that at least 4 milliamps—roughly equivalent to the discharge of a stun gun—would be necessary to stimulate the firing of living neurons inside the skull. Buzsáki notes he got dizzy when he tried 5 milliamps on his own scalp. “It was alarming,” he says, warning people not to try such intense stimulation at home.”
It is still possible that lower levels of current may be alter the threshold of neuron firing, but, there is now reason for increased skepticism.
peakmemory 
Take a large group of individuals committed to tDCS. For each one and hidden behind a screen, toss a coin to determine whether to reverse the direction of the current or not. See whether the claimed effect persists in all cases or only in the cases where the current was not reversed or disappears just because they know it might be reversed. Publish the results.