Midway through the Roman empire, an unnamed court official who answered to Emperor Tiberius stepped on a fish and changed the course of medical history. The electric torpedo fish, common to the Mediterranean, shocked the administrator, who noticed afterward that the gout pain in his leg had greatly subsided. Word reached Roman physician Scribonius Largus, who then famously recommended the shocks, when applied to the head, to treat headaches. A new study explores a modern-day form of the torpedo-treatment. It’s a new kind of shock to the head under investigation for headaches, insomnia, depression and dozens of other conditions. What’s more, unless you hook up with some kind of research project, the only way to receive this procedure, transcranial electrical stimulation (TES), is to do it yourself with an at-home device. How it Works The headache sufferer straps on an electronic device (typically bought online) that range in price from $15, the study says, to hundreds of dollars. While some resemble baseball caps, others are loose assemblages of metal pads, wet sponges and a headband to hold everything in place. Over the course of a 20-to-30 minute session, TES works by sending an electric current through the brain with the hopes of doing … something. Online guides show which areas of the brain to stimulate, roughly, by positioning the electrodes, in a process like watering your cortical lawn. Most devices allow the user to crank up the juice as desired to target sections related to emotion and impulse control, for example. But is this safe? The new study, a meta-review of other research, found that despite TES’s DIY past, it has “an excellent safety record,” save for some headaches and dizziness and skin irritation where the electrodes touch the head. TES is gentle by design and uses subthreshold currents only strong enough to influence existing brain activity, not switch on new neurons. A Choreographer Skeptics of TES have long questioned its ability to influence the brain given its low-amp approach. A 2018 study using rats and human cadavers found the skull and soft tissue surrounding the brains absorbed about 75 percent of the current, putting quite a damper on TES. And in 2017, a study using epilepsy patients found that transcranial alternating-current stimulation (a form of TES) had a minimal effect on brain waves. These papers have done little to slow down TES research, however, as researchers have investigated its usefulness for “over 70 diverse conditions, including major depression, epilepsy, pain, stroke rehabilitation, Parkinson’s disease, and tinnitus,” the 2017 paper says. Some have found striking results, the new review says, such as “dramatic improvements in the symptoms of devastating psychiatric conditions,” like depression. “However, these results often prove difficult to replicate and extend, hindering their transition from the lab to the clinic.” Studies have also found TES alters, in a subtle way, the timing at which neurons fire and not their rate, like a sort of choreographer. This is enough, the team from the Montreal Neurological Institute claims, to dispel the old criticism that TES is an overwrought scalp-tickler: “We do not consider the ability of TES to influence individual neurons to be a mystery.” Spotty and Unpredictable A meta-review from 2020 came to a similar conclusion after reviewing 23 controlled trials that examined the use of transcranial direct-current stimulation (another form of TES) in depression treatment. The study found the method that sends a one-way current through the brain to be “modestly effective.” The Montreal review points to transcranial alternating-current stimulation (TACS) as the best-understood type of TES, a category that includes several more esoteric devices. TACS works by delivering an alternating current, similar in design to what powers a house. While TES may somewhat work while the device is switched on, what happens when the user disconnects? A 2015 review found evidence of spotty, unpredictable after-effects in the 20-plus TES experiments it examined, in contrast to stronger “online” effects, when the machine was switched on. The Montreal team proposes there may still be lasting effects thanks to neural plasticity and the development of new connections encouraged by the device’s currents. As the review points out, “neurons that fire together wire together.”