tACS and the Future of Mental Health

By Louis Case

The most interesting thing about the brain, from an engineering standpoint, is that it runs on rhythm. Not metaphorically. Populations of neurons fire in coordinated oscillations, alpha waves around 10 Hz when you close your eyes and relax, gamma waves up near 40 Hz when you are binding a scene together into a single perception, slow oscillations under 1 Hz during deep sleep. These rhythms are not noise sitting on top of the real computation. In a growing body of work, they are part of the computation, the timing signal that lets distant regions talk to each other in phase. And a disordered brain is very often a brain whose rhythms have drifted out of tune.

Transcranial alternating current stimulation is the attempt to retune them from the outside. Two or more electrodes on the scalp, a weak sinusoidal current, usually one or two milliamps, less than a nine-volt battery pushes through a small flashlight bulb, oscillating at a chosen frequency. The premise is that if you drive the current at the alpha rhythm, the cortex underneath will tend to entrain to it, the way a room full of metronomes drifts into sync. It is one of the gentlest interventions in all of neuroscience. It is also, I will argue, one of the most underrated, and the reason it is underrated tells you something uncomfortable about how we regulate and reimburse mental health care.

I should say up front that I have skin in this. I have spent real time on the design of a tACS device, and I think the underlying research base is more mature than the public conversation reflects. So take the enthusiasm as disclosed. But the case does not rest on my say-so. It rests on what the trials are now showing.

The evidence stopped being anecdotal

For years the honest summary of tACS was "promising but early." Small samples, inconsistent montages, effects that appeared in one lab and evaporated in the next. That summary is now out of date, and the thing that changed it is scale.

Consider depression, where the strongest recent data live. In 2026, a multicenter randomized trial across three hospitals in China put 120 patients with major depressive disorder through high-definition 10 Hz stimulation over the dorsolateral prefrontal cortex, against a sham that ramped the current up and down for sixty seconds so that patients could not tell which arm they were in. The active group saw a 48 percent reduction in depression scores against 24 percent for sham, with a response rate of 43 percent versus 8 percent and remission of 27 percent versus 3 percent. Those are not subtle numbers. A remission rate nine times the sham rate, in a properly blinded multicenter design, is the kind of result that moves a technique from "interesting" to "why is this not in wider use."

Then there is the closed-loop work, which is where I think the field is actually going. A 2026 open-label trial ran a version of tACS that reads the patient's EEG in real time and delivers stimulation locked to their individual alpha frequency, rather than a fixed 10 Hz for everyone. Over twelve weeks, depressive symptoms fell by more than seven points on a standard scale, with gains in quality of life and anhedonia, the flattening of pleasure that is often the most stubborn residue of depression. It was a small study and open-label, so weight it accordingly. But the direction is the important part. A brain's alpha rhythm is not exactly 10 Hz. It is this person's 10-point-something, and it moves. A stimulator that measures the target before it aims at it is a categorically better instrument than one that assumes.

The condition closest to my own interest, PTSD, has thinner but real data. A 2025 randomized controlled trial delivered high-definition 1 Hz stimulation to the right temporal cortex in forty patients over ten sessions. The active group dropped nearly seventeen points on the PTSD checklist against the sham group's twelve, a clinically meaningful separation, and, this is the detail that caught me, salivary cortisol shifted in the treated group, suggesting the stimulation was reaching into the stress-response machinery and not just moving a questionnaire. Forty patients is small. There was no long-term follow-up. But it is a real signal in exactly the population that most needs one, and it sits on top of a far larger literature on the neighboring technique, tDCS.

The neighbor that already crossed the line

Which is worth pausing on, because in December 2025 the FDA approved the first at-home transcranial direct current stimulation device for major depression, Flow Neuroscience's FL-100, a prescription device whose pivotal trial in Nature Medicine showed active stimulation producing two to three times the remission rate of sham, with 58 percent of patients in remission by the end of a ten-week course. tDCS is the direct-current cousin of tACS. It pushes a steady current rather than an oscillating one, nudging cortical excitability up or down rather than entraining a rhythm. The mechanisms differ, but the regulatory and cultural barrier they faced was identical, and one of them has now been cleared. A patient can be handed a brain-stimulation device and told to use it on their own couch, unsupervised, and the machinery of American medicine has decided that is acceptable.

That is the hinge the whole field turns on. Once the regulator accepts an at-home neuromodulation device for depression, the question for tACS is no longer whether the category is legitimate. It is when the alternating-current version, with its better theoretical fit to the brain's own rhythms and its natural path to closed-loop control, follows the direct-current version through the same door.

Why this matters more than the trials alone suggest

Step back from any single study and look at the shape of the problem tACS is walking into.

The dominant treatments for depression and PTSD are pharmaceutical, and pharmaceuticals for these conditions have a design problem baked into their mechanism. They are systemic. An SSRI does not go to the circuit that needs help. It bathes the entire brain and body in altered serotonin signaling and lets the side effects fall where they may. That is not a criticism of the chemists. It is the nature of a molecule you swallow. The appeal of neuromodulation, across every technique in this family, is spatial: the promise that you can touch the dysregulated circuit and leave the rest of the person alone. This is the same argument I made about magnetogenetics and about DARPA's neuroplasticity work. The whole thrust of the field is precision, doing to one circuit what drugs do to the whole organism.

tACS carries a second advantage that I think is decisive over the long run. It costs almost nothing to deliver. The hardware is a current source, some electrodes, and a controller. There are no consumables in the pharmaceutical sense, no monthly prescription, no metabolic burden on the liver. A durable, well-built device is a capital cost paid once, not a recurring one paid forever. In a system that spends staggering sums on treatment-resistant depression, the patients who cycle through drug after drug and are counted as failures by each, a one-time device that produces remission in a meaningful fraction of them is not just a clinical advance. It is a budget argument, and budget arguments are what actually move care systems.

I will not pretend the picture is clean. The sham-control problem is genuine and unglamorous. It is hard to build a placebo for something the patient can sometimes feel as a faint tingle or a flicker of light at the edge of vision, and unblinding inflates apparent effects. Sample sizes outside the depression work remain small. Effects are generally not durable without repeated sessions, which means this is maintenance, not cure. And the very accessibility that makes tACS attractive, cheap hardware and at-home use, is what makes the direct-to-consumer wellness market for it a minefield of overclaiming devices with no trial behind them. The honest position is that the clinical version is arriving and the snake-oil version is already here, and the public will have trouble telling them apart.

But the trajectory is not really in doubt anymore. The brain runs on rhythm. A disordered brain is often an arrhythmic one. And we now have a gentle, cheap, increasingly precise way to conduct that rhythm from outside the skull, with blinded multicenter trials and a cleared at-home cousin to show it is not a fantasy. The future of mental health care is not going to be only pharmaceutical, because the pharmaceutical approach is fighting its own systemic nature. Some meaningful share of it is going to be a low current, tuned to a frequency that is yours and not the average of a trial population, delivered by a device that measures you before it touches you. That future is closer than the stigma around "electrical brain stimulation" would lead you to believe, and the stigma, not the science, is now the binding constraint.

References

1. Wu, Y. et al. "A multicenter randomized clinical trial of portable transcranial alternating current stimulation for major depressive disorder." npj Digital Medicine, 2026. https://www.nature.com/articles/s41746-026-02575-9
2. Schwippel, T. et al. "Sustained benefits of closed-loop transcranial alternating current stimulation (CL-tACS) on depression: a 12-week open-label clinical trial." European Archives of Psychiatry and Clinical Neuroscience, 2026. https://link.springer.com/article/10.1007/s00406-026-02240-y
3. Hatef, B., Pirzad Jahromi, G., Khaleghi, A., Radfar, S. "The Therapeutic Effect of High-definition 1-Hz Transcranial Alternating Current Stimulation in PTSD: A Randomized Controlled Trial." Clinical Psychopharmacology and Neuroscience, 2025. https://www.cpn.or.kr/journal/view.html?doi=10.9758%2Fcpn.25.1298
4. "Flow Neuroscience launches FL-100 in the US, the first FDA-approved at-home tDCS treatment for depression." September 2026. https://www.biospace.com/press-releases/flow-neuroscience-launches-fl-100-in-the-us-the-first-fda-approved-at-home-tdcs-treatment-for-depression
5. "US FDA approves home-delivered tDCS for treating depression." PubMed, 2026. https://pubmed.ncbi.nlm.nih.gov/41475514/
6. Reinhart, R. M. G., Nguyen, J. A. "Working memory revived in older adults by synchronizing rhythmic brain circuits." Nature Neuroscience, 2019. https://www.nature.com/articles/s41593-019-0371-x
7. "A meta-analysis suggests that tACS improves cognition in healthy, aging, and psychiatric populations." Science Translational Medicine. https://www.science.org/doi/abs/10.1126/scitranslmed.abo2044
8. "Transcranial alternating current stimulation for neuropsychiatric disorders: a systematic review." PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC11360874/