Vibrations make prosthetic hand seem more real

Consider for a moment the welter of unconscious judgments and adjustments you make every time your hand reaches for an object — say, a tall drink of water. Eyes, muscles, brain and digits coordinate with exquisite speed and subtlety to ensure the cup is reached, grasped around the middle, held gently but firmly, and drawn — upright and at a pace that won’t make waves — to your mouth.

Now imagine performing that quotidian task with a prosthetic hand, or a disruption in the flow of signals between hand and brain. Even with the mechanical means to clutch a glass, grasping it will require constant visual oversight and a lot of effortful calculations. Without all that tactile feedback from your muscles and digits, mistakes, frustration and a sense of loss will probably ensue.

With this in mind, researchers from the Cleveland Clinic’s Lerner Research Institute asked how they could improve the user experience for amputees fitted with a prosthetic hand. Their answer was as simple in theory as it was complex in execution: To feel in fuller command of an artificial appendage, they found, the user of a prosthetic limb may just need a little buzz.

Cleveland Clinic’s Paul Marasco and his colleagues devised a robotic system that, with every movement of an artificial hand, would deliver vibrations to the muscles in a user’s arm that controlled that hand. The location and intensity of those vibrations created for amputees an illusory “kinesthetic” sense that they were moving their own hand. Study subjects learned within minutes to use the vibratory feedback to more deftly move their mechanical hand, to better sense its position in space, and to tighten and loosen their grip on objects as needed.

In some cases, no eyes needed. Once they got the system of feedback vibrations, participants were able to carry out a wide range of hand movements blindfolded.

“Establishing a sense of agency for these devices will help amputees intrinsically feel in control of their artificial limbs, a key aspect of user acceptance,” Marasco and his colleagues reported last week in the journal Science Translational Medicine.

In the near future, the authors wrote, this approach could inspire wearable or other feedback systems that might allow amputees to guide and control their prosthetic limbs intuitively, restoring the luxury of unconscious movement.

Giving users a greater sense that they are the instigators of movement will become more important as the technology of prosthetic limbs advances, the authors wrote. Although many of those artificial limbs will be capable of independent movement, users are unlikely to accept them if they don’t feel like natural extensions of their wish to move.

To devise a menu of vibrations that would signal 22 separate movements of the hand, the researchers largely worked with six participants who had had an arm amputated. All had undergone targeted reinnervation, a procedure that enables the establishment of a link between brain and machine by redirecting amputated nerves to remaining muscles.

With a hand-held vibration unit, they delivered a slight buzz to muscles in the portion of the upper arm that remained intact. The participants then reported which complex movement they most associated with the buzz they felt.