Over nearly 19 months of independent use, he accumulated more than 3,800 hours, according to the study in Nature Medicine. In formal tests with pre-specified vocabulary of 125,000 words, the system consistently achieved over 99 percent word accuracy.
From implant to independent use without research support
A continuously running speech decoder translated attempted speech into word sequences, while two decoders activated by eye-tracking converted imagined hand movements into cursor control and mouse clicks. According to the study, decoded words appear as text; optional speech synthesis reads sentences aloud using a voice customized to Harrell's previous voice. At home, the system consisted of networked research computers on a mobile cart. According to Harrell and his caregivers, daily setup by caregivers took approximately 20 minutes, after which Harrell used it independently for up to 19 hours at a stretch without additional assistance. After permission requirements for the investigational device changed on day 281 post-implantation, caregivers could set up the system without research staff present, increasing average daily use to 9.5 hours. By 22.6 months post-surgery, Harrell had accumulated over 3,800 hours and used the brain-computer interface on 444 of 653 days.
How decoding worked in everyday use
Over the entire period, Harrell communicated 183,060 sentences totaling 1,960,163 words at an average rate of 56 words per minute. According to the authors, fatigue, speech rate, sentence length, and ongoing system updates influenced accuracy. Harrell also used the speech-BCI as a keyboard and cursor-BCI as mouse input to send emails and browse the internet.
Neuroscientist Sergey Stavisky, co-senior author, explained that Harrell previously could only use the neuroprosthesis with a team member present. Now it functions at home without research support. Co-author Vikash Gilja stated the potential crossing of a threshold: for years, BCIs were proof-of-concept devices in research labs, but now a paralyzed person can speak according to their own thoughts. To The Register, co-author Jaimie Henderson stated that despite his paralysis, Harrell works full-time again and has meaningful conversations with his daughter, who had never heard the sound of his voice.
What the technology means personally to Harrell
According to MIT Technology Review, Harrell himself reports that the system allows him to continue working and secure income and insurance coverage for his family. It reconnects him with friends who previously hesitated to visit due to communication difficulties.
Study limitations
The results derive from one participant. The device is an investigational product under US federal law; the underlying BrainGate2 study provides preliminary safety data and feasibility demonstration. The BrainGate2 study, ongoing since May 2009, continues recruiting participants aged 18 to 80 who live within three driving hours of a study center. The estimated study completion is December 2038. The device is investigational; the study includes a data safety monitoring board.