Shocking the brain with electricity may bring to mind electroconvulsive therapy or James Whale’s seminal black-and-white adaptation of Frankenstein. But given that our brain cells use electricity to allow us to think, speak, and move, might there not be some way to tweak our brain’s electricity to solve our many health problems?
In the year 153, a Roman physician realized that the shock caused by touching an electric ray could dull someone’s pain. He recommended this therapy for many painful conditions, including hemorrhoids. I hesitate to ask which part of the body the fish was supposed to touch in this case.
Today, the idea has been refined into a suite of non-invasive brain stimulation methods. The most studied of them is transcranial direct current stimulation or tDCS for short. A mild current is allowed to run through your head in between two electrodes, and this bit of electrical stimulation is claimed to help with everything tied to the brain, from depression to tinnitus, from epilepsy to anxiety, from migraines to stroke rehabilitation.
It sounds like a neurological panacea, but a deep dive into the scientific literature on tDCS reveals a big problem with an equally big name: heterogeneity.
Turning a mountain into a hill
What tDCS looks like is quite simple. There are usually two electrodes, with at least one of them placed on a specific part of the scalp. The other is attached to a different region of the scalp or somewhere else on the head. In between the electrode and the skin is a sponge soaked in saltwater. The electrodes are connected to a battery which is turned on, providing a mild, steady electrical current somewhere between 1 and 2 milliamps. This current enters the head from the positive electrode and exits it from the negative electrode. The entire session lasts twenty to thirty minutes.
You can think of this mild current as water: it follows the path of least resistance. When it meets the skull, some of it manages to trickle through and it flows inside the brain tissue, generating an electric field along the way. Exactly how this impacts the brain remains to be fully understood, but there is a leading theory.
We know that nerve cells conduct electricity along their length and they do so at breakneck speed, up to 100 metres per second. This burst of electricity is triggered by a very quick change in voltage on either side of their membrane, making the cell “fire” a signal. This is the binary language of our brain: either a nerve cell is firing or it’s not. Transcranial direct current stimulation is not thought to make nerve cells fire; rather, it is believed that it makes them more or less likely to fire. It turns a mountain that needs to be climbed into a much smaller hill. The effects from a single session can last up to five hours (although multiple sessions might have a cumulative effect, at least based on work done in animals).
So far, we have a broad mechanism of action that makes sense given our knowledge of the brain. But it is possible to have an intervention that plausibly interacts with the human body but does not actually fix a specific problem in a predictable way. For example, massages, as pleasant as they can be, often fail to deliver on specific health promises. So, does tDCS actually do anything concrete?
Modern brain zapping began to be studied with great interest at the turn of this century, and fifteen years later a team in Melbourne, Australia published a review of the field arguing that this sparkly technique had no reliable effect on our nervous system and questioning the very foundation of this nascent discipline. This strong denunciation received pushback, with some accusing the Australian team of mischaracterizing studies and of comparing apples and oranges. Corrected analyses were run, with no change to the damning conclusion, and the problem of comparing apples and oranges had actually been highlighted in the original review. I reached out to its lead author, Jared Cooney Horvath, Ph.D., M.Ed., who told me that his opinion of the field has not changed: “The quality of the currently published research in this field,” he wrote to me, “is by-and-large so low that anyone can tell from even a cursory glance that the majority of findings are spurious.” I don’t think a cursory glance is enough to see noise in these allegedly positive results, but digging into the literature does reveal problems.
Heterogeneity
As with any field of study, there are plenty of examples of bad science and unwarranted hype in tDCS research. A review of studies using tDCS to treat mild cognitive impairment and Alzheimer’s disease claims, in its title, that brain zapping “enhances cognitive function,” meaning that it helps these people use their brain… except that this is clearly not true based on the data they present. A similar review for depression includes a study that single-handedly skews the overall result from negative to positive, which should always be looked at with skepticism. Its authors claim that the studies they looked at are robust… before mentioning that their quality is actually inconclusive. And when debating why other assessments of tDCS for depression came to a negative conclusion, they offer that maybe twenty sessions of brain zapping simply isn’t enough to make a dent in depression. This borders on absurdity.
But it’s a different review—this one on the use of tDCS to make older adults more functional in their everyday lives—that showcases the real problem with tDCS research so far. When crunching the data from different studies together, we do encounter this problem of comparing apples to oranges. In one of the studies looked at by the above review article, functionality in older adults is defined as not experiencing confusion or memory loss. In another, it’s walking speed. In yet another, it’s getting up from a chair, walking to a fixed point, coming back, and sitting down again. These are not the same. As Horvath, the tDCS critic, wrote in a book chapter, you can say that three studies show that an intervention improves your golf swing, but if the first only improves speed; the other, accuracy; and the third, strength, you have not shown a consistent effect of the intervention. This general halo of positivity from contradictory studies, however, is frequently seen in tDCS research.
I read dozens of systematic reviews and/or meta-analyses of tDCS for a slew of brain-related issues like depression, headaches, anxiety, fibromyalgia, and Parkinson’s disease, and their authors—experts in the field who want to embrace tDCS as a non-invasive and effective therapy—write about the technology like they are worried about getting sued for false advertising. It is “probably effective” for depression and epilepsy, they note. It “might enhance” activities of daily living following a stroke. It “seems to significantly reduce” phantom limb pain in people with amputation. This careful hedging of bets is due to a phenomenon known as heterogeneity.
When studies are said to be heterogeneous, it means that they differ in important ways, which makes comparing them like, well, comparing apples to oranges and bananas and grapes. They’re all fruits, but they look and taste very different. The word “heterogeneity” recurs over and over in the tDCS literature, and many researchers who wanted to crunch the numbers of various tDCS studies for a particular disease conclude that they can’t because of heterogeneity. The protocols for brain zapping simply are not standardized.
A paper looking at the potential use of tDCS to improve soldiers’ thinking in the line of fire lists no fewer than 26 variables that are known to affect the results of a tDCS experiment. Some of the main questions are exactly where these electrodes should be stuck (and how big they should be and what shape), and how to you ensure that you are placing them on exactly the right spot from one person to another when the shape of people’s heads can differ substantially. If two groups report that tDCS improved depression but they placed the electrodes in wildly different areas of the scalp, are we seeing a true effect or just random noise? An article from 2020 on how to improve research into tDCS concludes with a startling fact: “There are more parameter settings than there have been studies conducted.” It’s like we have done 20 experiments of a mysterious new device but there are actually 200 settings to test.
Given the need for effective and well-tolerated treatments against multiple sclerosis, tinnitus, and mood disorders, it’s no surprise that tDCS has already made its way onto the market despite an often contradictory and heterogeneous scientific literature. The risks are fairly low. Commonly, people report a tingling or itching sensation during the session. Some experience a metallic taste in their mouth. Others feel no difference. The current that is delivered to the scalp is less than 5% of the amount that is known to permanently change brain tissue: if used properly, tDCS will not burn your brain to a crisp. But there are anecdotes of people buying tDCS devices online, applying the electrodes backwards, and feeling angry or depressed during the session. There are case reports of people with mood disorders whose mania seems to have been triggered by the tDCS itself. Perhaps more worryingly, tasks in the brain are diffuse, and it is still unclear what unintended effects tDCS may have. As one team speculated, “Someone could stimulate their brain meaning to improve their ability to focus attention on a task, but change their reward sensitivity, making them more prone to risky choices.” We simply do not know.
Some tDCS devices have been approved or cleared by regulatory agencies around the world, like the Food and Drug Administration (FDA) in the United States. But as Dr. Steven Novella, a neurologist and skeptic who has written about brain zapping, is quick to point out, “the FDA has a very low threshold for electronic devices.” The risks are seen as low, the benefits as potential, and the demand as high.
Critics of tDCS like Jason Horvath have argued that the entire field is pushing the boundary of credibility with 25 years of so-called failed experiments. If you test enough people in enough ways, you’re going to stumble into a falsely positive result by chance alone. Experts in the field, who often want to have their hopes proven right, will say that there may probably appear to be something real there. Better, more rigorous studies are needed. Nearly a quarter of a century after this field was modernized, we are still waiting for these promised studies.
Take-home message:
- Transcranial direct current stimulation or tDCS is the application of electrodes to the scalp to deliver a mild current, which is believed to increase the likelihood of nerve cells firing
- tDCS is thought by experts in the field to be a promising therapy for a number of brain-related diseases and disorders, like depression and anxiety, and tDCS devices are already available on the market, as they are considered to be low risk
- However, studies of tDCS are very different and hard to compare with each other, so it is still unclear how many of tDCS’ promises will pay off one day
