Gotta agree with the placebo question here. When I'm having a really excellent learning day, it scares me how fast I learn. When I'm not, a 9V battery to the head doesn't sound like the worst idea...
Agreed. I listened to an article recently about a study that purported to show that wearing a 'lab coat' increased performance. Similarly, I believe that the tDCS results could be explained by a form of mental 'priming'.
Read the articles. Studies were done double blind. There is no placebo effect.