Wow! No less than Hank Green ( of the Vlog Brothers) covering tDCS on his SciShow Psych! He sounds a little skeptical! So we have that in common. I pretty much agree with Hank’s perspective, just a couple of thoughts I’d like to share.
Most companies (but not all) know better than to make claims about tDCS. They know they have to be careful making claims of benefits. So where you see a company making a claim, they are most likely basing the claim on legit science. But are the benefits replicable in your situation? Hank refers to the (science) literature as ‘messy’ and he’s right, with different labs coming to opposite conclusions using basically the same experiment protocols.
Hank characterizes the basic function of tDCS as being a training session for neurons. Over time, with repeated stimulation, those neurons will start to fire more. While this is a legitimate description of the approach some research takes, I think a better way to think of it is that tDCS makes it easier for neurons that are already inclined to fire, to do so. As in- you’re focused on a “Where’s Waldo” task, and with the proper stimulation, the task is easier to do, because the targeted neurons have less resistance to firing.