A sense of touch allows connection with others, enables one to interact with their surroundings, and allows one to feel their limbs are their own. But people who are paraplegics or have lost limbs have to navigate the world without this fundamental sensory input.
Fortunately, a recent scientific breakthrough may be rapidly closing the gap on developing technology which could give back the feeling of touch in new prosthetics that react similar to natural limbs.
The new model for transmitting a sense of touch to the brain that bypasses regular routes is being constructed by Silman Bensmaia at the University of Chicago, Illinois. He hopes it will be a blueprint for constructing prosthetics that can convey touch in the same way that natural limbs do.
In a series of experiments with monkeys, whose sensory limbs closely resemble those of humans, reserachers identified patterns of neural activity that occur during natural object manipulation and then successfully induced these patterns through artificial means.
To begin, Bensmaia and colleagues trained rhesus macaques to focus their gaze in different directions depending on whether their index finger or fourth finger were being prodded.
Microelectrodes were then placed in an area of the brain called the primary somatosensory cortex. This area represents an entire map of the body, with each neuron responsible for sensing when a different part of the skin is touched.
Microelectrodes are responsible for recording the activity pattern of neurons; they can also be utilized in reverse – to deliver electrical stimulation and make neurons fire.
The team then recorded what activity occurred and where it registered in the somatosensory cortex when a monkey had its index or fourth finger poked. Using this information, they then stimulated the brain using the same pattern of activity; the monkeys reacted as if they had been touched, fixing their gaze in the direction they had been taught in response to a poke.
In similar experiments, the monkeys were also able to differentiate between pokes of varying strength to a prosthetic hand that transmitted the information to their brain via the microelectrodes.
Bensmaia spoke on the importance of the findings from the studies: “Information about location and pressure of a touch is often unavailable visually or is inadequate to guide motor behavior for people with prosthetics. But it is crucial. Without it, we crush or drop objects in our grasp”.
His hope is that one day prosthetic sensors will be able to transmit signals to implants in humans that dispatch the correct pattern of electrical pulses to the brain to allow them to sense touch. Such prosthetics, Bensmaia says, will confer a greater feeling of embodiment – the sense that your limbs feel like a part of your body, and foster richer interactions with the environment.
“Maybe this will help a person touch a loved one for the first time,” he said. “That’s powerful”.
Monkeys taking part in the research have helped gain valuable information; now with foundation for further research, there are hurdles to overcome, but the researchers are optimistic.
Although electrode implants have been used in humans before, they must ensure they are safe and durable enough to remain in the brain over a long period of time, as well as adaptable enough to function as a person’s brain changes with age. Despite the obstacles, Lee Miller, at Northwestern University in Evanston, Illinois, says that Bensmaia’s biomimetic approach holds great promise for prosthetics, which have limited sensory capacity at the moment.
“Bensmaia is trying to reproduce a natural pattern of sensory activity and that’s a big distinction,” he says. “The best approach to conveying touch will likely be imitating as faithfully as possible the brain’s own signaling”.
With future research and safety precautions taken, it is likely prosthetics that allow touch connection with others will be made a reality.