If you’ve ever tried to tune a musical instrument against another sound of the same frequency—another instrument or a tuning fork or a digital pure tone generator—you’ll know what I mean by interference beats. If you haven’t, here is an example:
Here’s a concert A (440 Hz):
This next sound is slightly sharp (444 Hz, about 16 cents sharp).
When both are played together, you hear a 4 Hz beat pattern: a rhythmic variation in amplitude that cycles four times per second.
OK, that was all just to make the following text intuitive. Forget sound. We’re thinking of electrical stimulation. Deep brain stimulation (DBS) uses various frequencies of electrical activity to stimulate a specific area in the brain. It can produce amazing results in a number of brain disorders. The problem is that it requires placing a very thin wire into the brain. This can generally be done safely, but it’s not a trivial intervention. Wouldn’t it be great if we could produce electrical stimulation inside the brain without having to place a wire in the brain?
Well, first of all, that’s been done for ages, as long as you want to stimulate a large part of the brain. Treatments like ECT or TMS do this. But we want to stimulate as small an area as possible that will still reduce symptoms. A few years ago, some prominent researchers proposed a method for noninvasive electrical stimulation of the brain using the same principle as the auditory beats demonstrated above.1 They called it temporal interference stimulation. The idea is that electrical waves of high enough frequency (they used a frequency 33 times higher than the 60 Hz of wall current) essentially don’t affect nerve cells, because the latter can only react so quickly, being living cells with lipid “capacitors” that use protein ion channels rather than wires to carry electrical current. They can’t respond thousands of times per second. But if you apply a 2000 Hz stimulus to one side of the brain and a 2010 Hz stimulus to the other side, in the middle you get a 10 Hz electrical beat pattern. You can steer that beat pattern around somewhat: by turning up the current on one side, the interaction will be bigger closer to that side.
The authors of that 2017 paper showed that they could move the peak interference region closer to one side or the other in a mouse brain, and showed some evidence that stimulation affected the function of the targeted area. But mice aren’t people, and people have much bigger brains, so it wasn’t obvious how readily the idea could be adapted to the brains we care about.
Last month, a group from Shanghai, China, published a report in which they aimed temporal interference electrical stimulation at the globus pallidus pars interna, a region where DBS has been used to treat Parkinson disease (PD).2 In 12 patients with mild PD, 20 minutes of this treatment reduced PD severity by an average of 15%. Side effects were “mild and tolerable.”
For some disorders, like Tourette syndrome, DBS has been used to stimulate any of several brain areas—in other words, for TS, we really don’t know yet what region of the brain is the optimal target for stimulation. In practical terms, you’d like to know where to stimulate before you put a wire in the brain. Noninvasive stimulation like that described here would be very useful for treatment research in TS and similar cases, even if it provides less benefit than DBS.
The authors appropriately warn that “future larger-scale and more definitive studies are needed to confirm the benefits.” Still, this is a potentially very exciting result, and surely research in this area will continue.
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- Grossman N et al: Noninvasive Deep Brain Stimulation via Temporally Interfering Electric Fields. Cell 169(6):1029-1041.e16, 2017. DOI: 10.1016/j.cell.2017.05.024
- Yang C et al: Transcranial Temporal Interference Stimulation of the Right Globus Pallidus in Parkinson’s Disease. Mov Disord 2024 (online ahead of print). DOI 10.1002/mds.29967