A sense of touch lets you connect with loved ones, makes your limbs feel your own, and helps you to interact with your surroundings. But people who are paraplegics or have lost limbs have to navigate the world without this most fundamental of sensory inputs.
Sliman Bensmaia at the University of Chicago, Illinois, is working to change that with a new model for transmitting a sense of touch to the brain that bypasses regular routes. He hopes it will be a blueprint for constructing prosthetics that convey touch in the same way that natural limbs do.
To start, Bensmaia and his 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 record the activity pattern of neurons. They can also be used in reverse – to deliver electrical stimulation to make neurons fire.
Fourth finger exercise
Next, the team recorded what activity occurred and where it registered in the somatosensory cortex when a monkey had its index or fourth finger poked.
Then they 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 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.
“Information about location and pressure of a touch is often unavailable visually or is inadequate to guide motor behaviour for people with prosthetics,” Bensmaia says. “But it is crucial. Without it we crush or drop objects in our grasp.”
He hopes 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, he 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,” Bensmaia says. “That’s powerful.”
Though electrode implants has been used in humans, Bensmaia says that hurdles remain. Implants must be 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 signalling.”