It's something most of us take for granted, but our sense of touch is every bit as useful to us as our sight and hearing. Though it seems simple, picking up and holding an object requires nearly instantaneous sensation in the parts of our hands and fingers in contact with the desired object, as well as a sense of the pressure we're applying.
Many experimental efforts to simulate a sense of touch in amputees fitted with prosthetics require the subject to learn new associations between touching an object and some abstract sensation. But new research at the University of Chicago suggests that it is possible to map the individual finger pads of a prosthetic hand to the corresponding parts of the brain. In other words, prosthetic hands which offer a realistic sense of touch may theoretically be possible.
Have they actually built a prosthetic hand that can sense touch?
Not yet, and the researchers stress not to expect such a breakthrough any time soon. What they have done is to simulate the sensation of touch using brain implants: electrodes which trigger a sense of feeling in remarkably localized parts of the body, such as individual finger pads.
Whose brains are we talking about?
The researchers embedded arrays of electrodes into the brains of three 6-year-old rhesus macaques. The electrodes stimulate specific parts of the brain which can mimic not only a sense of touch, but also the apparent location of the trigger sensation along with information about the pressure applied and the duration of contact. The information isn't derived from a prosthetic, but instead comes from a computer program.
How do the macaques feel about this?
"We take great care never to cause our animals pain," University of Chicago neuroscientist Sliman Bensmaia tells Gizmag. "They enjoy performing the sensory tasks. Kind of like a game to them, and we have a staff [member] whose sole job is to keep the animals healthy and happy."
Wait. How do we know the sensations the macaque feels correspond to the correct parts of the hand?
Good question. With a human subject, you could simply ask her or him, but unfortunately, rhesus macaques don't speak human (or they keep it extremely secret if they do). The researchers effectively had to configure the system by mapping "receptive field" locations in the brain, or in other words, work out which parts of the brain correspond to which parts of the body so far as touch sensations go.
How did they do that?
They poked the macaque's hand with a poking machine in various places. By rewarding the macaques (with a drink of tasty juice, for example), they were able to train them to identify whether consecutive pokes were to the left or to the right of the previous one, which the macaques conveyed with brief glances in the corresponding direction. Once they'd trained the macaques, the researchers could replace one in each pair of stimuli with a simulated electrode "poke" instead.
Lo, the macaques responded as if they'd been physically poked, allowing the researchers to map parts of the brain to localized parts of the body, such as the afore-mentioned finger pads. The main take-away point is is that these stimuli are both useful and repeatable.
And this is surprising?
Rather, according to Bensmaia. "Given how complex the brain is, and how blunt an instrument electrical stimulation is, I thought the approach was doomed to failure," he writes (in a document provided to other reporters too, so if you see that quote elsewhere, don't blame me).
According to Bensmaia, the next task is to build even more nuanced information into the electrical stimulus which might convey information about the shape of the object, its size, and what it's made of. After that, Bensamaia would like to use human subjects who can give direct feedback. This would involve putting an array of stimulating electrodes into the sensory cortex of the brain, rather like recording arrays have been put into the motor cortexes of tetraplegic patients for the control of prosthetics.
And then there's the small matter of building all this into a prosthetic
Indeed, but again, Bensmaia says not to expect anything in the near future. That said, most of the technological barriers have already been passed. "The one exception is the implants themselves," he tells Gizmag. "It's not clear that current arrays are sufficiently robust, have the necessary longevity, to last the decades they would need to last."
Gimme the small print
I'm glad you asked. The team's research has been written up in a paper titled Restoring the sense of touch with a prosthetic hand through a brain interface published in the Proceedings of the National Academy of Sciences.