Two recent studies have documented progress on technology designed to give direct, carefully timed electrical stimulation of the brain and recreate nuanced tactile feedback to prosthetic hands. The studies build on years of collaboration among the scientists and engineers to design, build, implement, and refine brain-computer interfaces (BCIs) to restore motor control and sensation in people who have lost significant limb function.
Researchers with the Cortical Bionics Research Group, which includes the University of Pittsburgh, the University of Chicago, and Northwestern University, discovered a method for encoding natural touch sensations of the hand via specific microstimulation patterns in implantable electrodes in the brain. The discovery allowed participants with spinal cord injuries to control a bionic arm with their brains and to feel tactile edges, shapes, curvatures, and movements, that had been impossible.
For individuals with a spinal cord injury, the electrical signals coming from the hand to the brain that should allow them to feel tactile sensations are being blocked by the injury. The research aimed to improve the usability of an extracorporeal bionic limb mounted on a wheelchair or similar equipment close to the user.
For the study published in Science, two participants were fitted with implants in the sensory and motor regions of the brain that represent the arm and hand. Over the course of several years, the researchers were able to record and decode the patterns of electrical activity that occurred in the brain related to motor intention of the arm and hand.
After the decoding, the researchers typed specific stimulations directly into the users’ brains via the implants, and the participants were able to accomplish a series of complex experiments that required rich tactile sensations.
“We found a way to type these ‘tactile messages’ via microstimulation using the tiny electrodes in the brain, and we found a unique way to encode complex sensations. This allowed for more vivid sensory feedback and experience while using a bionic hand,” said Giacomo Valle, PhD, lead author of the study and now an assistant professor at Chalmers University of Technology.
For the complementary study in Nature Biomedical Engineering, the approach to prosthetic sensation involved placing tiny electrode arrays in the parts of the brain responsible for moving and feeling the hand. On one side, a participant can move a robotic arm by simply thinking about movement, and on the other side, sensors on that robotic limb can trigger pulses of electrical activity called intracortical microstimulation in the part of the brain dedicated to touch.
“Most people don’t realize how often they rely on touch instead of vision—typing, walking, picking up a flimsy cup of water,” said Charles Greenspon, PhD, a neuroscientist at the University of Chicago. “If you can’t feel, you have to constantly watch your hand while doing anything, and you still risk spilling, crushing or dropping objects.”
For about a decade, Greenspon explained, this stimulation of the touch center could only provide a simple sense of contact in different places on the hand.
“We could evoke the feeling that you were touching something, but it was mostly just an on/off signal, and often it was pretty weak and difficult to tell where on the hand contact occurred,” he said.
By delivering short pulses to individual electrodes in participants’ touch centers and having them report where and how strongly they felt each sensation, the researchers created detailed “maps” of brain areas that corresponded to specific parts of the hand. The testing revealed that when two closely spaced electrodes are stimulated together, participants feel a stronger, clearer touch, which can improve their ability to locate and gauge pressure on the correct part of the hand.
“If I stimulate an electrode on day one and a participant feels it on their thumb, we can test that same electrode on day 100, day 1,000, even many years later, and they still feel it in roughly the same spot,” said Greenspon.
Editor’s note: This story was adapted from materials provided by Chalmers University of Technology and the University of Chicago.
The study, “Tactile edges and motion via patterned microstimulation of the human somatosensory cortex,” was published in Science.
The study, “Evoking stable and precise tactile sensations via multi-electrode intracortical microstimulation of the somatosensory cortex,” was published in Nature Biomedical Engineering.
