A new study carried out by neuroscientists at the University of Chicago reveals how electrodes implanted in the brain can help amputees control a robotic arm.
The study, published in the Nature Communications journal, details changes that occur in both sides of the brain used to control the amputated limb and the intact limb. The study results reveal how both areas can create new connections in order to to learn how to control the device, even many years after an amputation.
That’s the novel aspect to this study, seeing that chronic, long-term amputees can learn to control a robotic limb, but what was also interesting was the brain’s plasticity over long-term exposure, and seeing what happened to the connectivity of the network as they learned to control the device.
Nicho Hatsopoulos, PhD, professor of organismal biology and anatomy at UChicago and senior author of the study.
Although earlier experiments have shown how paralyzed patients can move robotic limbs via a brain-machine interface, the new study is one of the first to examine the potential of these devices in amputees as well.
During the study, the researchers worked with three rhesus monkeys – each of them suffered injuries at a young age and therefore, had to have an arm amputated in order to rescue them 4, 9 and 10 years ago, respectively. For the purposes of the study, their limbs were not amputated. Two of the monkeys were implanted with electrode arrays in the side of the brain contralateral (opposite) to the amputated limb. This is the side used to control the amputated limb. The third monkey was implanted with electrodes on the ipsilateral (same) side to the amputated limb, and this side still controlled the intact limb.
After that, the monkeys were trained (with generous helpings of juice) to move a robotic arm and grab a ball using only their thoughts. The researchers recorded the activity of neurons where the electrodes were positioned, and employed a statistical model to evaluate how the neurons were connected to each other before the experiments, during training and once the monkeys learnt the activity.
The neuron connections on the contralateral side—the side which had been controlling the amputated arm—were sparse before the training, probably because they had not been used for that activity in a long time. However, as training progressed, these connections became more dense and robust in the areas used for reaching and grasping.
On the ipsilateral side—the side which had been controlling the intact arm of the monkey—the connections were dense at the start of the experiments. However, the researchers observed something interesting as the training progressed: the initial connections were pruned and then the networks became thinner, before rebuilding into a new, dense network.
That means connections were shedding off as the animal was trying to learn a new task, because there is already a network controlling some other behavior, but after a few days it started rebuilding into a new network that can control both the intact limb and the neuroprosthetic.
Karthikeyan Balasubramanian, PhD, a postdoctoral researcher who headed the study.
Now the team is planning to carry on their work by combining it with research by other groups in order to equip neuroprosthetic limbs with sensory feedback about touch and the sense of proprioception where the limb is located in space.
“That’s how we can begin to create truly responsive neuroprosthetic limbs, when people can both move it and get natural sensations through the brain machine interface,” Hatsopoulos said.