A team of researchers in California has developed a way to use ultrasound imaging to provide a new view of brain activity — through a small window in the skull of one man.

That window is not a metaphorical one, said Timmy Broderick, a disability and health care reporting fellow at STAT who wrote about the development: It’s a literal window made of a piece of polymer.

The person researchers tested this technique on, a man in his 30s, had his skull reconstructed after a traumatic brain injury two and a half years earlier.

Researchers from the California Institute of Technology, University of Southern California and University of California Riverside published their findings in the journal Science Translational Medicine last week.

“What these researchers did is basically take a part of that reconstruction and install a — it’s similar to plexiglass, basically. It’s a polymer. And that allows them to have the path to get the acoustic probes in there,” Broderick said. “They placed an ultrasound probe that allowed those acoustic waves to actually get into the skull, rather than just bouncing off the dense skull.”

The man then strummed a guitar and played video games as researchers captured images of his brain. It allowed for freedom to look around in an environment more similar to day-to-day life than, say, the confines of an MRI machine, which is what researchers and doctors often use to look at brain activity.

“They’re kind of mundane activities,” Broderick said. “This guy was moving freely. He had a lot of mobility. He was at home. So it’s a very different sort of context for this imaging that you might normally see for any other brain imaging.”

Ultrasound does not capture brain activity directly, Broderick said, but it can capture blood flow to different areas of the brain.

The polymer window allowed researchers to capture those images in a much higher resolution than would typically be available. Broderick compared it to having a crisp, detailed image from a high-quality camera, as opposed to a pixelated or blurry shot from an older cell phone.

“What you see with this new technology, with this fUSI, with this ultrasound, is just much higher resolution, almost ten times as much,” they said. “And that can give you a much better insight when we’re trying to understand conditions like epilepsy, stroke, other traumatic brain injuries.”

It can also be a path to advancements in brain-computer interfaces, Broderick said.

“That’s been in the news a lot lately because of Elon Musk’s, Neuralink, which is an invasive sort of BCI interface where they implanted electrodes directly into the brain to try and have this person manipulate things in a computer,” they said. “What this sort of technology might portend is that you can get a sense of what’s happening in the brain and potentially even manipulate the brain in ways that are much less invasive,” at least for people like the research subject who already had to have reconstructions in their skulls.

This method uses elements that are both new and very old, Broderick said.

The highly dangerous practice of creating holes in people’s skulls in the name of medicine dates back millennia, they said.

“The [ancient] Greeks were doing this, … pounding a hammer into a person’s skull because they thought it was therapeutic,” they said. “So this has a long therapeutic history, or I should say, quote-unquote therapeutic history. But the practice has mostly died out in recent years, except to relieve intracranial pressure.”

Practical implications are likely many years away, Broderick said.

“This was very much like a proof of concept. And they’ve already shown in previous studies that in non-human primates, they were able to basically install a pseudo sort of BCI with, using this ultrasound technique,” they said. “And so I think that’s really the next step: Now that they’ve shown that you can do this in humans, bumping up the population, making it far more diverse sample, and trying to see whether the images that you were able to capture in this one human can be captured in everybody, or at least most of the folks that you use with this technique.”