Transcranial direct brain stimulation, or tDCS, appears to have hit the big time. By big time we mean that zapping the skull (and presumably the brain) with electric current is now a science that garners serious consideration from many neuroscientists. The finer points of this technique was a hot topic at the annual neuro pow-wow held last month in Washington, DC — which is a bit surprising, considering that the Society for Neuroscience meeting is usually an opportunity to reaffirm tribal values and squash any revolutionary ideas that border on dissent. At this point in the tDCS game, we would like not only to mention a few new ideas, but also to rehash some of the basics of how it is now practiced, and — at least in theory — what it should and should not be able to do.
In short, if you’re looking for an explainer on what tDCS is, and how — or why — it works, read on.
The ol’ 9-volt-battery-and-some-wet-sponges routine
One of the simplest tDCS incarnations is to wire a 9-volt battery through a variable resistor to two wet sponges placed on your head. While there is some controversy regarding the appropriate level of stimulation, as long as the skin-electrode interface is moist and conductive, a 1K resistor will generally suffice at maintaining a reasonable current flow somewhere in the milliamp range. For typical lengths of stimulation these levels are probably not going to leave any kind of permanent mark on the skin.
If you are not comfortable putting together your own stimulator, or just want something with a few more bells and whistles, it is not too hard to find vendors willing to supply you with a turnkey solution. Once you have a device, your guess is probably as good as any as far as how best to use it. One study that was presented at the recent neurofest in DC used a half an hour of tDCS stimulation for testing sleep-deprived military personnel. They found that the stimulation increased performance on various tests to the same degree as the best drug legally available — caffeine.
Diving a little deeper into the science of tDCS
While researchers argue about the best place to put the sponges, the dowsing rod has proven to be as good a method as any for determining where they should go. In other words, it doesn’t really matter where you put them, so long as you know the exact name for the particular location. Fortunately we were were able to track down a diagram (below) which shows that these names coincide perfectly with those already well-established for placing EEG electrodes to record brainwaves. There is no good reason that we need to limit ourselves to just two sponges though. A center-tapped configuration, where the “middle” electrode is actually the ground and the other two are positive and negative could be more flexible. Four node networks of various configurations, perhaps with multiple grounds, could have their place as well.
Equally important to the polarity of the electrodes (i.e. which is positive or negative), is how you use them with regards to something called phase. More advanced stimulators, like those used in cochlear implants, don’t just deliver direct current — they pulse the current in some way in the hope that the waveform of a real world stimulus is impressed upon the auditory nerve. So-called monophasic stimulation pulses just go between ground and either positive or negative current flow. A biphasic pulse, on the other hand, is a more complex waveform where current goes successively in both directions on each pulse, all the way from positive to negative and back (or vice versa). Biphasic stimulation is generally more important for electrodes inside the brain where there are critical tissue interactions with the electrode material that benefit from being balanced. However, if your power supply is fancy enough there is no reason not to play around with both polarity and phase at the surface electrode level as well.
Now for the tough part: the red wire (note that it isn’t always red in the real world) that connects to the plus terminal on the battery or power supply is called the anode. That really tells you everything you need to know because the black wire must then be the cathode — the place where the brain-penetrating electrons are born, and to where all things positive would like to go. Inside the head this includes positively charged cations like sodium and potassium, as well as other larger things like proteins and such. By now many of you will already be thinking to yourselves that with two sponges on hand, and two sides of your head, there are two main methods to go about placing them: you can either review the exhaustive literature and videos describing the imagined effects of so-called anodal or cathodal stimulation, or you can try it for yourself.
Next page: But does tDCS actually work?
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