In a clinical trial, the researchers – also from Keck Medicine and Rancho Los Amigos National Rehabilitation Center – successfully implanted two microelectrode arrays in the posterior parietal cortex (PPC) of a tetraplegic patient. The patient, Erik Sorto, was paralysed from the neck down about 12 years ago and relies on help to perform most daily activities. The neuroprosthetic enabled Sorto to perform a fluid hand-shaking gesture, grasp a glass and take a drink, and even play "rock, paper, scissors" using a robotic arm.

By issuing "stop" and "go" brain signals for a sequence of programmed actions, Erik Sorto was able to drink a beverage at his own pace. In collaboration with Applied Physics Laboratory, Johns Hopkins. Credit: Caltech team

The PPC is a brain region in which the activity of nerve cells contains information about planned movements, as well as more abstract concepts like goals and intentions. "Signals in the PPC are more related to movement planning – what you actually intend to do – rather than the details of the movement execution," explained principal investigator Richard Andersen. "We hoped that the signals from the PPC would be easier for the patients to use, ultimately making the movement process more intuitive."

Each implanted array contains 96 active electrodes that each record the activity of a single nerve cell in the PPC. From this recorded neural activity, the researchers could predict the patient's intended movements and use this information to move a computer cursor or steer a robotic arm accordingly. After recovering from surgery, Sorto was trained to control the cursor and robotic arm with his mind. Once training was complete, the researchers observed intuitive movement of the robotic arm.

"For me, the most exciting moment of the trial was when the participant first moved the robotic limb with his thoughts. He had been paralysed for over 10 years, and this was the first time since his injury that he could move a limb and reach out to someone. It was a thrilling moment for all of us," said Andersen. "It was a big surprise that the patient was able to control the limb on day one – the very first day he tried. This attests to how intuitive the control is when using PPC activity."

Sorto was also impressed with the rapid results: "I was surprised at how easy it was," he said. "I remember just having this out-of-body experience, and I wanted to just run around and high-five everybody."

"This study has been very meaningful to me," Sorto added. "As much as the project needed me, I needed the project. The project has made a huge difference in my life. It gives me great pleasure to be part of the solution for improving paralysed patients' lives."

Future gazing

Over time, Sorto continued to refine control of his robotic arm, thus providing the researchers with more information about how the PPC works. This increased understanding should help improve future neuroprosthetic devices. "We have a unique window into the workings of a complex high-level brain area as we work collaboratively with our subject to perfect his skill in controlling external devices," Andersen explained. Ultimately, Andersen and colleagues hope that neuroprosthetics will enable paralysed patients to perform practical tasks such as being able to shave or brush their own teeth, and regain some independence.

To that end, they are now working on a strategy to help patients perform these finer motor skills, based on sensory feedback from the robotic arm. The newest devices under development feature a mechanism that relays signals from the arm back into the part of the brain that gives the perception of touch.

Writing in a perspectives article in the same issue, Andrew Pruszynski from Western University in London, ON, and Jörn Diedrichsen from University College London note that the results "represent one more step toward making brain control of a robotic limb or computing device a reality". They point out, however, that neural prosthetic devices still have a substantial way to go before becoming practical therapeutic interventions (Science 348 860).

Work is still needed to improve implant durability, refine the isolation of single nerve cells, optimize the signal interpretation algorithms and develop protocols to "write in" sensory signals from the prosthetic device into the brain. "Of particular note is the fact that current systems run wires from within the brain to the outside world – a route for potential infection," Pruszynski and Diedrichsen write. "In the long term, such systems need to become wireless and contained within the body, like modern pacemakers and cochlear implants."

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