What exactly is, and isn't, tDCS? A recount of the modern history of noninvasive brain stimulation.
A recent article published defines all the language and terminology used in tES (transcranial electrical stimulation)—including what exactly classifies tDCS as tDCS.
tDCS in its modern form was (re)discovered by two German clinician-scientists, Michael Nitsche and Walter Paulus in 2000 (see publication). This was the original paper that showed that while tDCS does not produce a direct and/or obvious motor twitch, it does have the ability to change the underlying function of the brain motor regions—consequently changing the size of the motor twitches produced during TMS (transcranial magnetic stimulation). Dr. Marom Bikson published a nice overview of what Nitsche and Paulus’s study showed, as well as its implications for tDCS as a neuromodulator.
While Nitsche and Paulus did go on to show in later studies that 10-20 minutes of tDCS produced some of the most ideal long lasting after-effects, the 2000 paper simply applied stimulation for as little as a few seconds. It’s also interesting to note that at this point, Nitche and Paulus referred to tDCS by its full name “transcranial direct current stimulation” with no caps—as opposed to the now widely used “tDCS”; it wasn’t until 2002 (see publication) that they started using the term tDCS with the lower-cased “t”.
Going back to the original 2000 paper, one might ask: is the 4 seconds of tDCS—really tDCS as we know it today? Stimulated by the 2000 (re)discovery of tDCS, many additional non-invasive electrical brain stimulation techniques were developed (or rediscovered) and they also usually had names made with the characteristic lower case “t”. This includes transcranial Alternating Current Stimulation or tACS (see publication) and transcranial Random Noise Stimulation or tRNS (see publication). Following those were a wide range of new brain stimulation techniques with new names that sometimes seem to build on existing techniques while other times seemed to suggest new directions; a perfect case of the former was the “High-Definition” stimulation—used to make tDCS and tACS focal. An example of the latter case include new techniques such as the Slow-oscillatory transcranial direct current stimulation.
For tDCS especially, there seems to be an endless number of variants including anodal-tDCS, cathodal-tDCS, lateralized tDCS, bi-hemispheric-tDCS, and extracephalic tDCS. All this can leave one scratching their head as to “what exactly is tDCS”? Is it simply any technique with the word tDCS in the name? Does it need to be a certain duration (minutes) and intensity (mA)?
In a new paper by Bikson and colleagues, tDCS is defined alongside all these other terms and more—a welcome dose of fresh air and clarification!
For more information on what exactly constitutes tDCS, and whether slow-oscillatory transcranial direct current stimulation is really considered tDCS, check out the link to the publication PDF below: