A method for restoring feeling and action to amputees and others who have lost their sense of touch has been established through research published in Proceedings of National Academy of Science (PNAS) today.
By connecting artificial limbs to brains via electrodes, the research mimicked the feeling of a real hand. Experts say this is a significant step towards lifelike artificial hands, but warn that human trials are some way off.
The part of the brain responsible for touch is known as the somatosensory cortex, which contains a map of the human body. The study shows that parts of the cortex are highly specific, and can even discern between different fingers.
When a part of the body touches something, the corresponding part in the cortex tells us when and where. It also communicates pressure.
Together these three pieces of information - timing, location, and pressure - allow us to perform actions.
Gregg Tabot and Dr Sliman Bensmaia - authors on the study from the University of California - explained:
“Contact location is important because, for example, when we grasp an object, the thumb and at least one other finger must be contacting it. Contact pressure is important because you need to apply enough pressure on the object so as to not drop it but not so much that you crush it.
“Contact timing is important because, when you reach towards an object to grasp it, you preshape your hand; the instant you touch the object, you stop reaching and finish grasping.”
Previous research has found that, by stimulating parts of the somatosensory cortex, scientists could mimic the sense of touch. But no-one had applied these findings to prosthetic limbs, until now.
In the study, Rhesus macaques - a type of monkey - had their brains surgically connected to an artificial fingertip equipped with sensors.
The researchers then poked the artificial finger. By converting pressure to an electric signal, and then sending that signal to the macaque’s brain, the finger mimicked a sense of touch.
The study also compared the artificial finger to the macaques' own fingers. They found that the macaques responded in the same way to “feeling” in the artificial finger as in their real fingers.
The researchers also found that by increasing the strength of the electric signal, they could vary the pressure sensed by the macaques.
Mr Tabbot and Dr Bensmaia said the research paves the way for human trials.
But Dr Rami Khushaba at the University of Technology, Sydney, said the need for invasive brain surgery means human trials could be some way off, and further research is needed.
“The researchers justified the risk by … sensory restoration, particularly in spinal cord injury patients, for whom many less-invasive options are not available.
“Future tests should also include experiments conducted on the long term effects of these surgeries on the performance of the amputees and if such surgeries could induce any side effects.”
Dr Stephen Redmond at the University of New South Wales, who also works on prosthetic limbs, says the field faces large challenges:
“There are few commercially available sensors which are capable of replicating the density and variety of tactile receptors in the human hand.
“We have about 2,000 in each fingertip, and four different receptor types.”
He said there were also several options less invasive than surgery, such as attaching electrodes to parts of the nervous system outside the brain.