A groundbreaking study published in Nature Medicine details how a man with ALS used an implanted brain-computer interface (BCI) at home for nearly two years to communicate via text and control a computer cursor. The participant, Casey Harrell, operated the device independently on a near-daily basis, without researcher assistance.
Device and Implantation
The BCI was developed by researchers at the University of California, Davis, in collaboration with Brown University and Mass General Brigham. Neurosurgeon David Brandman implanted four microelectrode arrays into the participant's left precentral gyrus in 2023.
The system decodes neural signals from the speech motor cortex and translates them into text displayed on a computer screen. The device also detects attempted hand movements to control a computer mouse.
Usage and Performance
Over a period of 397 days, the participant used the BCI on 364 days, totaling more than 3,800 hours of independent operation. During this time, he produced 183,060 sentences (nearly 2 million words) at an average speed of 56 words per minute.
- In controlled tests using a 125,000-word vocabulary, word accuracy exceeded 99%.
- The participant rated 92% of sentences as accurate or mostly correct.
- The system includes a privacy mode that allows disabling data transmission to researchers.
"The participant produced 183,060 sentences—nearly 2 million words—at an average speed of 56 words per minute."
Previous Research
Earlier studies on BCI typing neuroprostheses, published in Nature Neuroscience, reported that two clinical trial participants—one with ALS and another with a cervical spinal cord injury—used a similar device in their homes. That system used microelectrode sensors in the motor cortex to decode attempted finger movements corresponding to letters on a QWERTY keyboard. One participant achieved a top typing speed of 22 words per minute with a word error rate of 1.6%.
Participant Activities
Using the BCI, the participant sent emails, browsed the internet, and communicated with family and friends. The system supported his continued employment despite paralysis from ALS, which causes tetraparesis (weakness in all four limbs) and dysarthria (slurred speech).
Trial Information
The study is part of the ongoing BrainGate2 clinical trial, which is still enrolling participants. Brandman serves as the site-responsible principal investigator.
The trial produced a dataset of over 3,800 hours of brain recordings with single-neuron resolution, which researchers say may be useful for studying how the human brain produces speech.