Recently published in the journal ‘Nature’, the study was performed by scientists and neuroengineers in an international collaboration led from the Ecole Polytechnique Federal Lausanne, Switzerland (coordinator of the WALK AGAIN and E-WALK projects), together with Brown University, US, Fraunhofer ICT-IMM, Germany (coordinator of the NEUWALK project), and Medtronic. The work builds upon technologies developed at Brown and tested in collaboration with the University of Bordeaux, Motac Neuroscience and Lausanne University Hospital.
‘The system we have developed uses signals recorded from the motor cortex of the brain to trigger coordinated electrical stimulation of nerves in the spine that are responsible for locomotion,’ commented David Borton, assistant professor of engineering at Brown and one of the study’s lead authors. Whilst the system has been tested on two macaques, it is hoped that a similar system will soon be designed for humans with spinal cord injuries.
The ability to walk is made possible by a complex interplay among neurons in the brain and spinal cord, where electrical signals originating in the brain’s motor cortex travel down to the lumbar region of the lower spinal cord. There, they activate motor neurons that coordinate the movement of muscles responsible for extending and flexing the leg. An injury to the upper spine can cut off communication between the brain and lower spinal cord, meaning the motor cortex and spinal neurons are unable to coordinate, causing the loss of the ability to walk.
The new system used a pill-sized electrode array implanted in the brain to record signals from the motor cortex and a wireless neurosensor sends the signals gathered by the brain chip wirelessly to a computer that decodes them and sends them back, again wirelessly, to an electrical spinal stimulator implanted in the lumbar spine, below the area of injury. That electrical stimulation, delivered in patterns coordinated by the decoded brain, signals to the spinal nerves that control locomotion.
To calibrate the decoding of brain signals, the brain sensor and wireless transmitter were implanted into healthy macaques. The signals relayed by the sensor could then be mapped onto the animal’s leg movements, showing that the decoder was able to accurately predict the brain states associated with extension and flexion of leg muscles. Combining their understanding of how brain signals influence locomotion with spinal maps, the researchers then tested the entire system on two macaques with lesions that spanned half the spinal cord in their thoracic spine. When the system was turned on during a period when the animals had no control of their affected leg, they began to spontaneously move their legs whilst walking on a treadmill. Kinematic comparisons with healthy controls showed that the lesioned macaques, with the aid of brain-controlled stimulation, were able to produce nearly normal locomotor patterns.
‘Doing this wirelessly enables us to map the neural activity in normal contexts and during natural behaviour,’ Borton said. ‘If we truly aim for neuroprosthetics that can someday be deployed to help human patients during activities of daily life, such untethered recording technologies will be crucial.’
Moving forward, there is now hope that a similar system will be developed for humans, however there are some hurdles that still need to be overcome. Whilst this study successfully relayed signals from the brain to the spine, the system developed lacks the ability to return sensory information to the brain. The team were also unable to test how much pressure the animals were able to apply to the affected leg.
‘There’s an adage in neuroscience that circuits that fire together wire together,’ commented Borton. ‘The idea here is that by engaging the brain and the spinal cord together, we may be able to enhance the growth of circuits during rehabilitation. That’s one of the major goals of this work and a goal of this field in general.’