What We Think We Know and Don't Know About tDCS

image: Mihály Vöröslakos / University of Szeged

“Don't Lose Your Head Over tDCS,” I warned last time. Now the infamous cadaver study has reared its ugly hot-wired head in Science News (Underwood, 2016).

The mechanism of action of transcranial direct current stimulation (tDCS) had been called into question by Dr. György Buzsáki during his presentation at the Cognitive Neuroscience Society meeting.

...Or had it?

To recap, my understanding was that an unpublished study of transcranial electrical stimulation (TES) in human cadaver heads showed a 90% loss of current when delivered through the skin vs. through the skull. This implies that a current of at least 5 mA on the scalp would be necessary to generate a 1 mV/mm electric field in the human brain. Based on his personal experience, Dr. Buzsáki reported that 4 mA was hard to tolerate even with anesthetized skin. For comparison, 2 mA is the maximum current recommended by an international panel of experts.

But Dr. Tiziana Metitieri left a comment on my post saying this is nothing new. She translated the remarks of Dr. Carlo Miniussi, who said:

...but what is reported appear to me not so “new” (http://www.ncbi.nlm.nih.gov/pubmed/?term=Miranda+PC+2006). Of course, if the findings obtained by Buzsáki are confirmed, you may think that tDCS has an effect nearly homeopathic on the brain. Certainly, these type of research is the most needed: systematic studies of animal and human models, comparable in terms of the amount of current that stimulates the brain. Luckily, they are coming out, or, well, we know they exist and we are waiting to read them, as for Buzsáki.  [read more]

Why is this important to cognitive neuroscientists? Because the behavioral effects of tDCS have been vastly overstated, according to some investigators (e.g., Horvath et al., 2015), and the “homeopathic” level of brain stimulation is one likely explanation.

But a common refrain of experts in the field [I am not an expert] is that Buzsáki's results are not surprising — the low amount of current is old hat. For instance, Dr. Marom Bikson explained in Science News that...

...many in the field already accepted that the 1 or 2 milliamps the methods use don't directly trigger firing. It can make neurons more likely to fire or form new connections, he and others believe. Unlike techniques that rely on magnetic fields or higher current to actively trigger neurons ... tDCS and tACS likely subtly alter ongoing brain activity, Bikson says. Using cadavers to test these methods is a “complicated choice” because dead tissue conducts electricity differently from living tissue, he adds.

Also quoted is Dr. Vince Clark, who...

...has found that applying 2 milliamps of current to a person’s scalp for just 30 minutes can double the speed at which they learn a game in which players must detect a concealed “threat”... Several labs have replicated those results, he says, adding that the idea that 10% or less of the current gets through to the brain is not new, and doesn’t necessarily mean the methods are ineffective. “If it works, you know 10% is enough,” Clark says.

Although some effects may be replicable, Dr. Vince Walsh dropped a stink bomb by saying that the tDCS field is “a sea of bullshit and bad science—and I say that as someone who has contributed some of the papers that have put gas in the tDCS tank.  ...  It really needs to be put under scrutiny like this.” In Wired, Walsh basically said the reason for the “sea of bullshit and bad science” is that the barrier to enter tDCS research is so darn low.


When Can TES Influence Spiking?

Returning to Buzsáki's talk, he mentioned a study in rats (Ozen et al., 2010) where a TES-induced voltage gradient of 1 mV/mm at the recording sites could phase-locked spiking (action potentials). However, the current was delivered via electrodes placed directly on the skull or even the dura covering the brain. The stimulation protocol was low frequency sinusoid patterns that mimic slow cortical oscillations, to entrain neuronal spiking activity. That was the goal in humans, but similar TES applied to the scalp produced no discernible change in oscillatory activity. Hence, the cadaver tests.

These studies used transcranial alternating current stimulation (tACS), which is designed to influence ongoing cortical oscillations by “entraining” or phase-locking to specific EEG frequency bands (as in Kanai et al., 2008). Buzsáki himself actually commented on the Science piece (which I will quote at length):

"The real question: Is the current which does reach the brain sufficient to perform this ‘extremely weak coupling’ in neural systems?" This is exactly what we investigated. Since we failed to entrain neuronal activity (local fields) repeatedly in the living human brain with the commonly used current intensities, whereas we were very successful in rodents using stimulation electrodes directly on the bone, we looked for answers. The cadaver is the next best possible thing to a living human brain if one wants to know how the currents are distributed inside the brain. We found that most current is lost by the shunting effect of the extracranial tissue. As a result, the voltage gradients that we measured in the brain were way below the values we found in rodents needed to affect population neuronal oscillations. The weak electric fields were just too weak. Of course, there is the principle of stochastic resonance and thus some super weak effect can have some effects occasionally - we cannot and do not want to deny it, but cannot prove it either, therefore cannot rely on it as an explanation for the reported behavioral effects of TES.

In his talk he mentioned possible effects on astrocytes, and my previous post cited the study of Monai et al. (2016). In his Science comment Buzsáki said, “Glia may be more sensitive to polarized currents than neurons and muscles.” He also mentioned possible effects on peripheral nerves in the scalp (edit: "like in the case of vagal stimulation"), which is something that Dr. Jamie Tyler (formerly of Thync) has said for years:

Thync tried to replicate some basic tDCS findings on cognition but could not do so. Dr Tyler now believes that tDCS may not directly stimulate the brain at all but instead modulates cranial nerves in the skull...

During the discussion period at the CNS meeting, Buzsáki was asked about the phenomenon of DIY tDCS. He compared it to alternative medicine.

On that note, I'll conclude with a nod to the tDCS reddit community, some of whom didn't trash my last critical post as much as I expected. Yay! Others? Not so much. Boo: “There are so many inaccuracies in this article, I don't know where to begin.” And then they don't bother to begin...

[EDIT April 24 2016: Later on in the reddit thread, this critic did expand on my potential inaccuracies, but I missed it. Oops, sorry. See the comment below.]

So any- and all-comers can begin by pointing out my inaccuracies in the comments section of this post.


ADDENDUM (April 23 2016): I should mention more specifically that Tyler et al. (2015) proposed that TES affects the ophthalmic and maxillary divisions of the trigeminal nerve and cervical spinal nerve afferents.


References

Horvath JC, Forte JD, Carter O. (2015). Quantitative Review Finds No Evidence of Cognitive Effects in Healthy Populations From Single-session Transcranial Direct Current Stimulation (tDCS). Brain Stimul. 8(3):535-50.

Kanai R, Chaieb L, Antal A, Walsh V, Paulus W. (2008). Frequency-dependent electrical stimulation of the visual cortex. Curr Biol. 18(23):1839-43.

Ozen, S., Sirota, A., Belluscio, M., Anastassiou, C., Stark, E., Koch, C., & Buzsaki, G. (2010). Transcranial Electric Stimulation Entrains Cortical Neuronal Populations in Rats Journal of Neuroscience, 30 (34), 11476-11485. DOI: 10.1523/JNEUROSCI.5252-09.2010

Underwood, E. (2016). Cadaver study challenges brain stimulation methods. Science, 352 (6284), 397-397 DOI: 10.1126/science.352.6284.397