The ability to hold and move objects relies on sensory signals sent from the hand to the brain, a faculty that amputees lack with current prosthetic limbs. But new research from the University of Chicago may one day result in touch-sensitive prosthetic limbs that communicate directly with the brain.
The researchers, led by Sliman Bensmaia, assistant professor in the Department of Organismal Biology and Anatomy at the University of Chicago, believe their research could help to increase both the dexterity and function of robotic limbs.
Through experiments with monkeys, he and his team pinpointed neural activity patterns that occur when the animals manipulate objects, and then they were able to successfully recreate these patterns artificially.
"To restore sensory motor function of an arm," Bensmaia says, "you not only have to replace the motor signals that the brain sends to the arm to move it around, but you also have to replace the sensory signals that the arm sends back to the brain."
He and his team believe that by using what they know "about how the brain of the intact organism processes sensory information," they can then "try to reproduce these patterns of neural activity through stimulation of the brain."
The research from Bensmaia and colleagues is part of a Defense Advanced Research Projects Agency (DARPA) project, called Revolutionizing Prosthetics. The aim of the project is to create an artificial upper limb that will provide natural motor control and sensation in patients with amputations.
Results of their research was published in the journal Proceedings of the National Academy of Sciences.
Same perception of artificial and genuine stimuli
One set of experiments worked with the site where skin has been touched. The researchers trained the monkeys to correctly identify different patterns of touch with their fingers.
Next, the researchers connected electrodes to areas of the monkeys' brains that correlated to each finger. But this time, rather than touching the hands, researchers sent electrical pulses to the specific areas of the brain.
The team found that the monkeys responded to the artificial stimulation in the same way they responded to physical touch.
In another set of experiments, the researchers worked with sensation of pressure by generating an algorithm that created an electrical current, which produced a feeling of pressure.
And again in this instance, the monkeys' response was the same to both artificial and genuine stimuli.
In a final set of experiments, Bensmaia and his team looked at what they call "contact events," which is when the hand first touches or lets go of an object.
They say this moment creates an activity burst in the brain, and once again, the researchers were able to artificially create these bursts in the brain with electrical stimulation.
The future of prosthetic limbs
As a result of these experiments, Bensmaia and his colleagues say that have created a set of instructions that could be used with a robotic prosthetic arm, which could give sensory feedback to the brain through a "neural interface."
This feedback, Bensmaia believes, could mean that these kind of devices may be that much closer to undergoing tests in human clinical trials.
"The algorithms to decipher motor signals have come quite a long way, where you can now control arms with seven degrees of freedom," Bensmaia says. "It's very sophisticated."
However, he says there is still work to be done:
"But I think there's a strong argument to be made that they will not be clinically viable until the sensory feedback is incorporated. When it is, the functionality of these limbs will increase substantially."
Though amputees will have to wait for these touch-sensitive limbs to come to fruition, there have been new developments over the last couple of years in the world of prosthetic limbs, such as thought-controlled robotic arms.