Much more ambitious brain-computer interfaces and neural prosthetics have been in the news lately. Last month, Elon Musk’s company Neuralink demonstrated a wireless BCI with more than a thousand flexible electrodes, designed to be inserted directly into a brain by a specialized robot surgeon. (The company has so far only shown short-term use in pigs.) Inserting electrodes is tricky; while it’s true that brain surgery isn’t exactly rocket science, it has risks whether the surgeon is a robot or not. Even flexible, thin electrodes like those that Neuralink demonstrated are invasive enough that the brain tries to defend against them, coating them with glial cells that reduce their ability to conduct the electrical impulses they’re looking for. And while implanted electrodes like those of the more commonly used “Utah array” can get clear signals from individual neurons, understanding what those signals mean is still science in progress. Plus, the brain sloshes around like jelly in a donut; fixed-in-place electrodes can damage it. But get it right and they can do more than brain research. “Locked-in” patients with ALS have used them as successful brain-computer interfaces, though they require training, maintenance, surgery, and so on.
Meanwhile, electrodes placed directly onto the scalp can pick up brain waves—electroencephalograms, or EEGs—but those lack the spatial detail of implanted electrodes. Neuroscientists know, very roughly, which part of the brain does what, but the more you know about which neurons are firing, the better you can tell what they’re firing about.
A more recent innovation, electrocorticography, places a mesh of electrodes directly onto the surface of the brain. In combination with smart spectral processing of the signals those electrodes pick up, ECoG is good enough to translate action in the part of the motor cortex that controls the lips, jaw, and tongue into text or even speech. And there are other approaches. CTRL-labs, which Facebook bought for perhaps as much as $1 billion in 2019, tries to get motor signals from neurons in the wrist. Kernel uses functional near-infrared spectroscopy on the head to sense brain activity.
Oxley and his colleagues’ stentrode, if it keeps showing good results, will fit somewhere along the spectrum between implanted electrodes and EEG. Closer to the first thing than the second, its inventors hope. But it’s still early days. “The core technology and the core idea is super cool, but given where they’re accessing the signals from, my expectation would be that this is a relatively low-fidelity signal relative to other brain-machine interface strategies,” says Vikash Gilja, who runs the Translational Neural Engineering Lab at UC San Diego. “We at least know that high-density ECoG recording from the surface of the brain can convey information beyond what is being shown in this paper.”
A possible problem: Tissue conducts electrical impulses, but the electrodes in the stent are picking up signals from the brain through the cells of the blood vessel. That lowers signal content. “If we were to take those cortical surface recordings and compare them to Utah array experiments—the bulk of clinical experience with implanted electrodes—I would say the style of recording in ECoG is a rate limiter,” Gilja says. (Just for transparency, I should point out that Gilja has done for-pay work with BCI companies including Neuralink, with whom Synchron could theoretically compete someday.)
So it might not be good enough for neuroscience, but it could be plenty useful for a person with paralysis who wants a low-maintenance BCI that doesn’t require drilling through the skull. “There’s a trade-off between how invasive you want to be and at what level you collect information,” says Andrew Pruszynski, a neuroscientist at Western University in Canada. “This is trying to get to the middle ground, to insert a catheter close to the neural activity. It’s obviously invasive, but certainly not as invasive as putting electrodes into the brain.”
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