Spinal cord injuries can result in paralysis of the legs because they sever the connection between the brain and the lower spinal cord, even if both ends of that chain may otherwise remain fully functional. Now researchers have developed a system that bypasses the injury by allowing the motor cortex of the brain to communicate wirelessly with the lower spine, and demonstrated that it can restore an almost normal walking gait in temporarily paralyzed macaques.

Over the last few years, great strides (pun intended) have been made to restore limb movement for sufferers of spinal cord injuries. Some, like the case of Rob Summers in 2011 and a wider study in 2014, use implants that stimulate the local nerve network of the lower spine. These nerves don't require input from the brain, but instead can take cues from sensory input to recover some movement, even in people who previously had zero sensation below the waist.

Other approaches have found ways for the brain signals that control movement to bypass the injured spinal cord and directly stimulate the muscles in the limbs to perform complicated tasks. In 2012, a paralyzed woman was given the ability to control a robotic arm with her thoughts, and in 2014, Ian Burkhart regained some motor control of his own arm and hand. Initially movement was limited to simple tasks like picking up a spoon, but with time and therapy, he advanced to more complex actions like stirring a drink or playing a Guitar Hero-like video game.

In cases like these, an electrode array the size of a pill is implanted into the motor cortex, which reads the brainwaves associated with movement of particular parts of the body and relays these signals to a computer. The problem is, these signals are transmitted from the brain to the computer through a thick cable running out of the top of their heads.

This new study used a wireless neurosensor to wirelessly transmit the signals gathered by the brain chip to a computer, which decodes them and then sends them back, also wirelessly, to a device hooked up to the patient's lower spine. There, a pulse generator would convert those signals to electrical pulses that mimic the normal instructions from the brain that tell the muscles how to move.

"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," says David Borton, one of the study's co-lead authors. "Doing this wirelessly enables us to map the neural activity in normal contexts and during natural behavior. 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 critical."

The team calibrated the system by first implanting the chips into the brains of uninjured monkeys, and tracking which signals corresponded with leg movements during normal walking. To determine which parts of the spine each signal should stimulate, the team used spinal maps that highlight hotspots in charge of locomotor control.

Then, the system as a whole was tested on two macaques that had lesions on their upper spine – an injury that can result in paralysis, but will generally see the monkey regaining functional control over the affected leg/s after around a month. While walking on a treadmill, the team found that the animals were able to move their temporarily paralyzed legs in a natural way that was close to the patterns of uninjured monkeys.

While it is an important step, the researchers point out that there's plenty of work still ahead before the system could be trialed in humans as a rehabilitation aid. Currently, the system is strictly one-way and cannot relay sensory information from the spine back to the brain. Measuring the animal's balance and how much pressure can be put on the leg is a goal.