A paper published in Nature details an experiment with three Rhesus macaque monkeys that had previously been involved in accidents that resulted in amputation of their arms.
Researchers from the University of Chicago implanted electrodes in the macaques’ heads in the area where the primary motor cortex is located — a region of the brain that controls movement — and then into the severed limb. For two of the monkeys, the electrodes were placed on the opposite side to the amputated arm, and for the other monkey, the electrode was on the same side as the injury.
Using a sweet drink to incentivise them, the monkeys had to move the robot arm to reach and grab a ball. The robot prosthetic wasn’t directly connected to the monkey, but instead linked to the computer that processes signals from the electrodes. When the signal for moving the limb is activated, the robot arm moves as well.
The electrodes measured the activity of the brain’s neurons to see how connected they were to one another before and after the training. The results show that the connections between the neurons on the opposite side of the amputated arm were sparse before training as the absent limb was rarely exercised – the right side of the brain controls the left arm, and vice-versa. But as the monkey was trained, the connections in the brain’s area that were used to make the robot arm reach and grasp became more dense.
But for the neurons on the same side of the amputated limb, the behaviour was reversed. The connections between neurons started off strong, and were pruned over time to build a new, dense network of cells for muscular control. In other words, the neurons were reordered. This is an example of the phenomenon known as neuroplasticity.
Karthikeyan Balasubramanian, who led the study, said the strength of the connections “were shedding off as the animal was trying to learn a new task, because there is already a network controlling some other behaviour. But after a few days it started rebuilding into a new network that can control both the intact limb and the neuroprosthetic.”
“That’s the novel aspect to this study, seeing that chronic, long-term amputees can learn to control a robotic limb,” said Dr. Hatsopoulos, professor of organismal biology and anatomy at UChicago and senior author of the study. “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.”
The researchers hope that this will one day be applicable to humans. “The brain and nervous system of the monkeys are very similar to humans, so this research is like a proof of concept for humans. One day the same technology can be used to help human amputees control robotic prosthetics with the brain,” a spokesperson told The Register.