The sense of touch allows us to process data about our everyday world. Without it, we might have to visually calculate how wide to stretch our fingers each time we reached for the phone or how much pressure to apply when typing on a keyboard. For those with prosthetic limbs, these simplest of tasks are an ongoing struggle.
However, a new study “Behavioral Demonstration of a Somatosensory Neuroprosthesis” published in May in IEEE Transactions on Neural Systems and Rehabilitation Engineering gives hope that those with artificial hands could regain their sense of touch. By surgically connecting the brain with an artificial fingertip equipped with sensors, researchers found that it is possible for animals to feel contact and pressure with their hands, paving the way for human trial.
“Without sensory feedback, you might as well not have an arm or hand because your ability to use it is so severely compromised,” says researcher Sliman Bensmaia, assistant professor of organismal biology and anatomy at the University of Chicago.
While great advances have been made with prosthetics to date, such as the more sophisticated control of artificial limbs through brain interfaces, patients without a sense of touch lack the ability to feel the prosthetic limb as their own, or to use it effectively as a tool in their daily life.
The procedure, which requires implanting electrodes in the brain, was recently approved by the U.S. Food and Drug Administration (FDA) for human trial. While electrode implantation has already been proven safe on humans, the procedure would also require that electrical currents continuously stimulate the brain in order for the patient to feel a response on contact. Bensmaia’s group has shown that chronic electrical stimulation of the brain does not produce substantial damage beyond that caused by the implantation itself.
Bensmaia explains that the tiniest adjustments we make with our hands require close interplay between motor control and sensory feedback. This interaction shuts down when a person loses a limb.
In their approach, Bensmaia and his team of researchers imitated the way human hands receive and convey information about touch, a so-called biomimetic approach. A human hand, Bensmaia notes, has at least 13 different kinds of receptors, each with its own role, such as detecting hot and cold temperatures, pain, motion, vibrations, and pressure.
“We’re starting with the basic sensory dimensions that one needs to grasp an object and not crush or drop it,” Bensmaia says. The team experimented with a prosthetic fingertip made with built-in sensors designed by the John Hopkins Applied Physics Laboratory, in Baltimore. Two types of sensors were used in these experiments: a contact sensor, to signal the start and end of handling an object; and a force sensor, to determine how much strength to exert on an object. These two sensors alone can help make everyday tasks like picking up a glass of water or holding a pen much more fluid.
To test the sensors, the team connected the prosthetic fingertip to implanted electrodes in a rhesus monkey’s brain. The animal was trained to signal when it feels contact on its own hand by moving its eyes to one of two visual targets.
In the brain of someone who is not missing a limb, the somatosensory cortex receives input from the skin and processes information about touch. This part of the brain contains a full map of the body. A designated part of the brain represents the right-hand index finger, and next to that a part that represents the right-hand middle finger, and so on. For a patient missing a hand, the parts of the brain that used to represent the index finger, the middle finger, and the ring finger are still there.
To test whether activation of these areas could be used to elicit tactile sensations, the researchers had the animal perform the perceptual task, but instead of poking the monkey’s own fingertip, they poked the prosthetic one. They then assessed whether the animal could perform the detection tasks, or signal that it felt contact, when it received electrical stimulation to the part of the brain that is associated with the fingertip. The study showed that the animal could perform the detection task equally well whether it was its own finger or the prosthetic finger that was poked, meaning it felt contact either way.
A computer then monitors the amount of force that is exerted on the prosthetic finger, then analyzes the information and delivers an electrical stimulus to the brain, which they hope will create a sensation that mimics what a real hand would feel. The more force exerted on the hand, the stronger the sensation the monkey feels, which can be mimicked by delivering electrical stimulation with higher intensity to the electrodes in the brain.
Patients with prosthetic parts also lose the emotional feeling that comes with touching another person.
“One of the things paraplegic patients talk about when it comes to being able to feel again is not how to hold a pen properly, but the ability to touch a loved one’s hand again,” Bensmaia says. “Touch is critical to emotional communication.”
Another aspect that a patient loses is the sense of embodiment. When sensory information is lost, patients cease to feel that their prosthetic limb is part of their body. Bensmaia says regaining the ability to feel will cause the patient to embrace the prosthetic part as one’s own limb as opposed to simply a device connected to their body.
The next step for Bensmaia and his research team is to work on prosthetic sensors that could allow an animal, and eventually a human with an artificial hand, to feel shape, texture, and motion to experience richer sensory experiences of objects, including movement across the hand, or textures such as sandpaper and silk.
“The sense of touch is critical to motor control, the sense of embodiment, and emotions,” Bensmaia says. “Without it, you cease to feel this limb as your own.”