Researchers at UC Irvine, Caltech and USC’s Keck School of Medicine have developed a bidirectional brain-computer interface that allows a person to control a robotic walking exoskeleton using brain signals and receive artificial leg sensation through direct electrical stimulation of the sensory cortex.
The project is the first implementation of a BDBCI for walking that incorporates bilateral interhemispheric leg sensorimotor brain areas and represents a critical step toward restoring full ambulatory function in individuals living with spinal cord injury and paraplegia. The team’s results are reported in a paper published recently in the journal Brain Stimulation.
“Millions of people worldwide suffer from paralysis from spinal cord injury, with loss of lower-extremity motor and sensory function leading to wheelchair dependence and increased risk of serious secondary conditions including heart disease, osteoporosis and pressure ulcers,” said co-author Dr. An Do, UC Irvine associate professor of neurology. “Recovering the ability to walk ranks among the highest rehabilitation priorities for paralyzed individuals.”
He said that while robotic gait exoskeletons have emerged as a promising technology for restoring walking ability, existing systems rely on manual control and provide no sensory feedback, a significant limitation as impairment of the sensation of taking steps is known to reduce gait speed and increase the risk of falls.
The team’s BDBCI apparatus directly addresses both these challenges. By decoding motor intent from electrocorticography signals recorded from the leg motor cortex and delivering artificial leg sensation through targeted electrical stimulation of the somatosensory cortex, it creates a closed-loop, brain-driven experience of walking.
“This work demonstrates that it’s feasible to restore both the motor and sensory dimensions of walking using a single, compact, embedded brain-computer interface system,” Do said. “We believe this lays a critical foundation for the development of fully implantable systems that could one day give paraplegic patients a meaningful and natural sense of movement.”
The participant in the study was a 50-year-old woman undergoing epilepsy evaluation with bilateral interhemispheric subdural electrocorticography implantation. She operated the BDBCI-controlled exoskeleton across 10 exercises, rapidly achieving a high level of performance, which demonstrated the system’s accessibility and reliability, according to the researchers.
To validate artificial sensory feedback, the subject completed a blind step-counting task, correctly identifying steps with an overall accuracy of almost 93 percent. In an additional sensory discrimination task, she identified right leg, left leg and null (no stimulation) feelings with 96, 84 and 100 percent accuracy, respectively. The participant confirmed in all 10 runs that exoskeleton steps triggered matching contralateral leg sensations and reported that the sensory feedback aided task performance. No adverse events were noted throughout the study, the researchers said.
A key innovation of this project was the use of bilateral interhemispheric electrocorticography implants to access the leg motor and sensory cortices in the medial wall of the brain along the interhemispheric fissure.
“Although interhemispheric ECoG implantation is more complex than other conventional approaches, our team demonstrated that it can be performed safely and yields superior results,” said co-author Dr. Charles Liu, professor of neurological surgery and director of the USC Neurorestoration Center at Keck School of Medicine of USC. “The leg motor cortex in the interhemispheric region provides more robust and reliable neural signal modulation associated with leg movements, making it likely the most optimal recording site for BCI walking applications.”
Artificial sensation was delivered through direct cortical electrical stimulation, a technique the researchers identify as the safest and most practical option for eliciting leg sensation in ambulatory settings. Because ECoG electrode arrays can be implanted to simultaneously cover both the motor cortex for movement decoding and the somatosensory cortex for sensory stimulation, the BDBCI system incurs no additional surgical risk. Long-term cortical electrical stimulation has also been demonstrated as safe in existing commercial devices approved by the U.S. Food and Drug Administration.
The entire BDBCI system was implemented on a compact, portable, embedded platform consisting of three 48-megahertz microcontrollers that jointly perform all system functions – including neural signal acquisition, real-time decoding, electrical stimulation and wireless communications – without reliance on a tethered computing system.
“This type of portability is necessary to be practical for patients’ everyday use. We hope that our system can serve as a prototypical example for such technologies henceforth,” said lead author Jeffrey Lim, UC Irvine postdoctoral scholar in biomedical engineering.
The robotic gait exoskeleton used in the study was the Ekso GT from Ekso Bionics, an FDA-approved powered device. The embedded design positions the BDBCI as a benchtop version of an implantable system; it’s the first of its kind to combine bilateral lower-extremity artificial sensation with motor decoding in a single BDBCI robotic gait exoskeleton.
“We are looking ahead to a fully implantable version of the BDBCI in which a skull-mounted unit connects via a subcutaneous cable to a chest wall-implanted unit housing all signal processing, stimulation and wireless communication electronics,” said co-author Payam Heydari, UC Irvine professor of electrical engineering and computer science. “Such a system would eliminate transdermal components that pose infection risks and enable chronic implantation in paraplegic spinal cord injury patients.”
He added that further miniaturization is possible through advanced multilayer printed circuit boards, smaller surface-mounted components and conventional semiconductor system-on-a-chip integration. Earlier work at UC Irvine achieved significant size reduction and energy efficiency through integrated circuit implementation.
According to co-author Zoran Nenadic, UC Irvine professor of biomedical engineering, further technical advances can be achieved via refinement of motor decoding algorithms and mitigation of electrical stimulation artifacts, enabling future systems to operate more robustly while providing continuous artificial sensation. “Our ultimate goal is to test the function of such a system on people with complete leg paralysis, demonstrating its potential to mimic the function of an intact sensorimotor loop,” he said.
Co-author Richard Andersen, Caltech’s James G. Boswell Professor of Neuroscience, said that his and other laboratories have been working for several years on bidirectional interfaces to restore somatosensations to the hands of tetraplegic subjects so they can manipulate objects with robotic hands.
“This study by UC Irvine’s Jeffrey Lim and colleagues represents an important proof of concept for a bidirectional interface for walking, providing somatosensory feedback to the legs while a paralyzed participant brain-controlled a robotic gait exoskeleton,” Andersen said. “Paraplegic subjects using exoskeletons currently lack natural somatosensory feedback and must rely on visual feedback. This research provides a new avenue for more naturalistic and effective use of walking exoskeletons.”
Other contributors to the project included Po Wang and Shravan Thaploo of UC Irvine’s Department of Biomedical Engineering and Won Joon Sohn, who was with the UC Irvine School of Medicine at the time of the study. Additional Caltech researchers were Luke Bashford, David Bjanes and Spencer Kellis. Brian Lee and Darrin Lee of Keck School of Medicine of USC also participated. Contributors from Rancho Los Amigos National Rehabilitation Center, in Downey, were Angelica Nguyen, Hui Gong, Michelle Armacost and Susan Shaw. Approved by the institutional review boards of UC Irvine and Rancho Los Amigos National Rehabilitation Center, the work was funded by the National Science Foundation.
