For years now, popular publications like Wired and The Economist have chronicled the exploits of "brain hackers" — do-it-yourself-ers who use electrical gadgets to zap their scalps in hopes of benefits ranging from improved math skills to relief from depression.

Now, apparently for the first time, a group of academic scientists who research electrical brain stimulation have organized to say publicly: Hey, maybe not such a good idea to try this at home.

The warning comes in an open letter in the journal Annals of Neurology, signed by 39 researchers. It lays out several concerns, including unknown effects, both immediate and long term, and the possibility that a zapping session may produce an effect exactly the opposite of what's intended.

I spoke with the letter's co-author, Dr. Michael Fox, a brain-stimulation specialist at Harvard Medical School and Beth Israel Deaconess Medical Center. He says the impetus for the letter came from an Institute of Medicine workshop that included a talk on the emerging field of DIY brain stimulation by MIT graduate student Anna Wexler, who studies and writes about the growing DIY movement.

She "presented evidence of people doing protocols that really kind of surprised academic people that do non-invasive brain stimulation experiments," Fox says. "People that were administering brain stimulation much more frequently and for much longer periods of time than we've ever studied.

"Some people were administering it to their kids," he says. "And we began to realize that there was a discrepancy between what we appreciate as neurologists and as academics that do non-invasive stimulation and what might be the perception of non-invasive brain stimulation on the part of the broader community."

Our conversation, lightly edited:

What were your concerns about that perception?

The academic community has published a lot of papers on non-invasive brain stimulation and tDCS [transcranial Direct Current Stimulation] in particular, with a lot of claims — showing that tDCS can enhance cognition, enhance math ability, treat anxiety, treat depression. And I think that a lot of these academic papers can be interpreted as overly positive by those in the broader community. Obviously, academics have a vested self-interest in trying to sell their science. But there's a danger to that. I think some of the downsides of non-invasive brain stimulation have not really been emphasized and are not immediately apparent from a lot of the published articles.

How would you sum up the potential downsides?

The biggest risk is that we just don't know what all the risks are. I think that's the biggest risk.

Some of the risks are obvious. People know that if you connect too much electricity to these wires that you can burn your scalp. And if you burn your scalp you're going to know about it very quickly.

But when you’re actually modulating brain activity -- there’s so little that we understand about that — the risk that you do something that you’re not intending to do is quite high. And some of these risks, unlike burns on the skin, might not be obvious.

To give you an example, there was a nice paper by Roy Kadosh: What he found was that you can stimulate your brain to improve one function, but if you measure enough other things, that can come at a cost of making another brain function worse.

He looked at the rate of learning new materials and showed that he could find a certain tDCS protocol that, across a large group of people, would improve the rate at which you could learn new material, but that same exact tDCS protocol made it harder to process material that you already learned.

And there was a different tDCS setting that helped you process learned material but that came at the cost of the rate of learning other material.

So this was an important paper that opened some people's eyes, which said: Gee, even if we are able to figure out tDCS parameters that, in a group, improve a particular cognitive function, how many other cognitive functions might we be hurting? And the answer is for the most part it's not known, because to really know the answer to that question, you'd have to measure lots and lots of different cognitive functions.

Are there a couple of practices you're most concerned about?

I'd say we're most concerned about when people experiment for much longer and much more frequently than we've experimented with. And the reason I say that is we know that tDCS can have very funny effects on brain activity. You might think that if one milliamp of current did this, then I can turn it up to two milliamps and get twice as much of that. Or if I stimulate for 20 minutes rather than 10 minutes I'll get double the effect. But what can happen is you can actually reverse the effect by increasing the current or increasing the duration.

Some of the protocols people are doing at home are taking that to the extreme — stimulating daily for 90 minutes at a time every day for months. And we simply have no idea what that does to brain activity. I don't know that any hospital review board would actually let us do that experiment, so we might never know what that does to brain activity.

What are your other concerns?

One of the biggest ones is how much subject-to-subject variability there is in tDCS results. And that's something that every experimenter who does tDCS knows and appreciates: That when we see these effects of tDCS, these effects are at the group level, and you have to study lots and lots of people to see an effect.

The effects in individual subjects are highly variable. Thirty percent of people will actually have a change in their brain that goes the opposite direction of everybody else. So if you study enough people you can see these small effects that are significant across a large group of people, but that doesn't mean that a single person trying to replicate that experiment on themselves is going to have that result.

We’re just now beginning to study all these different sources of variability. We now know that if you take the same subject and do tDCS with exactly the same settings on different days, they can have very different responses.

We know there's a huge amount that can actually change what effect tDCS has. What you’re doing at the time tDCS is administered, or before tDCS is administered, has an effect. There are so many different things that can have an effect -- your age, your gender, your hormones, whether you drank coffee that morning, whether you've had exposure to brain stimulation previously, your baseline neurotransmitter level — all of this stuff can affect what tDCS does to your brain. And some of those things vary on a day-to-day basis.

It's to the point that you wonder, are these effects even real?

Flat out, they might not be. So tDCS has come under a huge amount of criticism lately. Effects that were there on average in 30 subjects, if you repeat that same experiment, they might not be there in another group of 30 subjects.

One possibility is that the tDCS effects are so tiny that you have to have a very large group of subjects to see them. Another possibility is that the effects of tDCS are actually large, but they're so variable in different subjects that it takes a large number of people to see a small average effect. And we don't really know which one of those is actually the correct answer right now.

But it is real, physiologically?

We're fairly confident that tDCS has a physiological effect on the brain. However even that has actually been a point of controversy recently. Some people have put electrodes in cadavers and said they didn't think enough electricity gets into the brain to actually do anything. But we're pretty confident that there is a neurophysiological effect. Experiments that do tDCS over motor cortex see consistent changes, at the group level, in motor cortex excitability. These effects have been reproducible because you can actually measure the change in motor cortex excitability pretty easily.

I imagine some will respond to the letter that you're just being a sort of a 'nanny neurologist' because you don't have evidence of real harm...

The evidence of harm would be the evidence that you can hurt some cognitive functions with the same stimulation protocols that help another cognitive function. But they're completely correct that we don't have any evidence saying you're definitely hurting yourself. We do have evidence that you're definitely changing your brain. Now, it could be that they’re changing their brain in good ways that have no negative consequences but there's certainly reason to think those brain changes that they're causing do have negative consequences.

What do you hope the effect of the letter will be?

It's really providing information. People are going to do what they're going to do independent of what a 'nanny neurologist' is going to recommend. So people out there are going to make their own decisions. However, they pay attention to the scientific literature to guide those decisions. What we're trying to do with this letter is provide balance to the scientific literature, so it's not just a constant stream of of papers saying, 'Look at this great thing tDCS can do!' 'Look at this next great thing that tDCS can do!' 'Oh, look at this wonderful effect we got with tDCS!'

It's making it clear to people that there's another side out there, that there are parts that are not emphasized in these papers that someone who is thinking about do-it-yourself tDCS should be aware of. It's also a guide to physicians. We'd like people to go in and ask their primary care doctor or neurologist what they think about this, and physicians might not know what they should base that opinion on. We published this letter in a medical journal, in a neurological journal, to give information both to neurologists but also to people who might be considering whether this would be a good idea for them.

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