Image via Josep Ma. Rosell on Flickr

History has watched as medical professionals and the prosthetic industry have revolutionized the lives and bodies of countless people in need of new limbs. We can now control our robot arms with our muscles and our robot legs with our nervous system. But, how much longer will it be until we will be able to feel with those robot arms and legs?

A team of comprised of researchers from the University of Chicago and Johns Hopkins University decided to tackle that question. The results of their work, reported in the newest issue of Proceedings of the National Academy of Sciences, offer a sketch as to how researchers may be able to re-establish touch sensation in those otherwise incapable by utilizing a neuroprosthesis capable of electrically stimulating the brain.

According to Sliman Bensmaia, one of the researchers and an assistant professor in the Department of Organismal Biology and Anatomy at the University of Chicago, their approach “consists of attempting to mimic naturalistic patterns of brain activation that convey information about location, pressure, and timing of contact through electrical stimulation.”

In other words, Bensmaia and his colleagues neurologically imitated the sensation of touch by stimulating the region of the brain associated with understanding tactile sensations, the primary somatosensory cortex. The three focal variables of touch—location, pressure, timing—were chosen because they represent “three of the most basic cutaneous signals that mediate object grasping and manipulation.”

To confirm the efficacy of this technique, the team worked with rhesus macaques, who were trained to understand different touches. Subsequently, actual physical contact was switched out for electrical stimulation of the brain. The goal was to see if the monkeys responded the same way to both sets of stimuli.

Researchers found that the monkeys were quiet adept at processing the electrical stimulation. They were able to tell where and how hard they were touched, which signaled to the researchers that this might be a promising approach for tetraplegic humans who have lost their tactile sense.

The import of touch seems obvious to those of imbued with the ability: we need it for mechanical tasks. But as the researchers note, this particular sensation serves purposes beyond mere physicality. Emotional communication is buoyed by touch, through gestures like hugs and holding hands. Additionally, touch confers a sense of embodiment onto our limbs, making them feel like they are indeed a part of our body.

Accordingly, the researchers note that “given the importance of somatosensation, upper-limb neuroprostheses will not be clinically relevant until they provide for somatosensory inputs.”

Some options for recreating touch already exist. “For amputees, an alternative approach would be an interface with the nerve,” Bensmaia said. “However, for tetraplegic patients, the nerve is no longer connected to the brain, so a nerve interface is not an option.” He also noted that sensory substitution is still another option, but lacks the complexity needed to communicate a range of stimuli.

Bensmaia’s technique allows for complexity and tackles the problem of a disconnected nerve. However, as exciting as his results may be for those craving the tactile world, it’s worthwhile to reiterate that this study provides only a blueprint and that we won’t be seeing these sorts of mechanisms being used by the general public anytime soon.

“The next step is to transition to human trials, for which our work lays the foundation,” Bensmaia said. “There is definitely a lot of testing to be done before we see patients equipped with sensorized neuroprostheses riding with us on the commuter train.” 

@heyiamlex