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Casey Harrell has had a set of electrodes embedded in his brain for almost three years. Harrell, who has amyotrophic lateral sclerosis (ALS) and is paralyzed, first used his brain-computer interface (BCI) to “speak” sentences with the help of a research team in 2023.

Since then, Harrell has clocked thousands of hours of use. He can use the device largely independently, once he’s been “plugged in” with the help of a carer. His team has added new features to it, and Harrell also uses it to surf the web and perform his job.

“Living with a disease like ALS, you are supposed to have diminished dreams. I do not,” Harrell tells MIT Technology Review. “Any one of these things would be an absolute godsend of improvement. To have all of them, and many, many more, is truly revolutionary.” 

Within the first 22.6 months after the device was implanted, Harrell had used it for more than 3,800 hours at home without any researchers present, the team reported today in the journal Nature Medicine. “He’s the first power user of a speech BCI,” says team member Sergey Stavisky, a neuroengineer at the University of California, Davis.

Decoding speech

Three years ago, Harrell entrusted David Brandman, an associate professor of neurological surgery at the University of California, Davis, and his colleagues with his brain. Harrell, who was 45 at the time, had already been diagnosed with ALS, a degenerative disease that robs people of the use of their muscles.

Harrell was dependent on others to control his wheelchair and to dress and feed him. He had difficulty speaking; people struggled to understand what he was saying. Then Brandman and his colleagues asked if he’d like to trial a brain implant that might help him communicate. “The industry was [on the] cusp of a transformation, and I wanted to be part of it,” says Harrell. He signed up.

In July 2023, during a five-hour operation, doctors implanted four arrays of 64 electrodes each into his brain. Each pair of arrays was wired to a “pedestal” connection point—creating two docking locations on the exterior of his skull to connect the electrodes to a computer.

The team had long been working on developing algorithms to decode brain activity into speech. Their system works by recording activity from the speech motor cortex—a region of the brain responsible for the movements that allow us to speak.

“There are 39 phonemes that make up all the sounds in the [American] English language,” says Nicholas Card, a neuroengineer at UC Davis and member of the team. Mapping neural activity related to producing each of those phonemes can allow the team to create a personalized speech decoder and software that can “speak” those words. “We first go from brain data to phonemes, and then from phonemes to words,” he says.

They started using the device around a month after the surgery. The team got Harrell’s speech decoder working on the first day, says Card. On that day in August, Harrell used the device to speak with a 50-word vocabulary, and 99.6% of the words were as he’d intended. That vocabulary was later expanded to 125,000 words with 97.5% accuracy.

At the time, it was unclear how long the device might last. Brain-computer interfaces are still new—not many people have had them implanted for long periods of time. Scar tissue can form around electrodes in a person’s brain, interfering with their ability to pick up neural activity, for example. But that doesn’t seem to be the case for Harrell.

Power user

In another advance, Harrell is now able to use the device more independently. In 2023, members of the research team would have to visit Harrell at his home and physically connect and disconnect him from the device on the days he wanted to use it. Not anymore. The team has since automated more of the system—today, Harrell’s care partner can don and doff it for him. “He’ll wake up, get plugged in, and just get going,” says Stavisky.

This is important, says Mariska Vansteesel, a BCI researcher at Utrecht Medical Center who was not involved in the trial. “For these technologies to be relevant for patients, we really need to test them in settings in which they will eventually be used … to demonstrate that it has value, that it’s usable, and that it functions well without the constant involvement of a research team,” she says.

The team has also worked to improve the system itself. It is now 99% accurate, says Stavisky. Harrell can also control a cursor—a game changer that enables him to use his personal computer to send text messages and emails, surf the web, and keep up with his job as an environmental activist.

Over the years, the team has updated the system to accommodate specific requests from Harrell. He is now able to switch on a “privacy mode”—when active, any decoded text will be automatically deleted. He can also opt to use a “profanity filter” while he’s talking to his young daughter.

“We have been able to add on to the software side of the device … improving the accuracy and adding more bells and whistles to enable me to be more independent when using the device,” says Harrell. “We are making the road as we walk it, or roll it, so to speak.”

Nothing short of revolutionary

Vansteesel cautions that while the device is working well for Harrell, there’s no guarantee it will work as well, or as long, for other people with ALS. Over the last decade, she has worked with a woman with ALS who used a fully implanted device to communicate using “brain clicks”—cursor clicks made using brain activity. The woman used her BCI for seven years, but it stopped working toward the end of that period, apparently due to brain degeneration.

At any rate, not everyone with ALS will be willing to undergo invasive brain surgery, says Jane Huggins, who is developing noninvasive BCIs at the University of Michigan and was not involved in the trial. “Long-term, independent use with efficient and accurate communication is kind of the holy grail of BCI,” she says. “But we have been finding a consistent aversion to hospital stays among people with progressive conditions like ALS.”

Harrell, however, calls the device “nothing short of revolutionary.” “This has allowed me to keep working and earn money and insurance for my family. This is reconnecting me with friends and family who are too shy or too afraid to come over and not be able to understand me,” Harrell says. “With my seven-year-old daughter, I am able to create a bond that I wasn’t before able to forge. Now I can read to them and help them sharpen their own reading skills. By doing so, I am able to share the responsibility of parenting with my wife, who does so much caregiving for me and also our daughter.”

Stavisky and his colleagues hope to improve the device further still. “We’re never satisfied,” he says. One aim is to eventually restore Harrell’s “full voice.” They are working on a “brain-to-voice” system that could directly decode brain activity to a speaking voice, complete with natural-sounding cadence, inflection and intonation—a voice that could sound happy, angry, or sarcastic, for example.

“I was quietly confident that I could get some personal benefit from the system,” says Harrell. “Never in a million years would I think that I would achieve this much.”