Researchers show that applying an electrical current to the sensory cortex can help people feel a sense of ownership over artificial limbs.
By Kate Wheeling
(Photo: Mario Tama/Getty Images)
In creating a prosthetic limb, one of the biggest challenges is to develop a device that a user might actually feel is part of their own body. Previous research has found that it’s possible to trick the brain into believing that a fake limb is part of the body; in cases where a visible rubber hand was stroked in tandem with a subject’s own hidden hand, participants mistook the prosthetics as part of their own physical self. From there, scientists have been trying to figure out how to create that illusion in individuals who can’t receive sensory inputs from real hands, due to injury or amputation.
In a new study, a team of researchers from the University of Washington and the Karolinska Institute in Stockholm have come one step closer to solving that problem—but in this case, instead of touching the participants’ real hands, they stimulated the part of the brain’s sensory cortex that represents the hand.
The researchers tapped into a 64-electrode grid array implanted into the brains of two epilepsy patients to monitor seizures. The arrays were positioned over the somatosensory cortex — the region of the brain that processes touch. The somatosensory cortex is laid out like a map of the body, and the team focused on the part of the region that corresponded to the subjects’ finger.
The researchers hid one of the volunteers’ hands behind a screen as they lay in a hospital bed, and placed a prosthetic hand in front of the screen in the patients’ view. The researchers used a touch probe linked to a cortical stimulation device to simultaneously touch the rubber hand with a brushstroke and stimulate the patients’ somatosensory cortex. The researchers touched the finger on the fake hand with the probe, and lit up the electrodes over the brain region that corresponded to that finger. During this phase of the experiment, the participants were more likely to agree with the statement “It feels as if the rubber hand were my hand.” Afterwards, they experienced something called proprioceptive drift: When the researchers blindfolded the subjects and asked them to point to their finger with the opposite hand, their guesses drifted toward the rubber hand, rather than their real hand, after the experiment.
“The brain’s system for integrating signals from different senses to shape our experience of the bodily self is more flexible than previously assumed.”
Their results show that that stimulating participants’ brains directly with a weak electrical current, while touching a rubber hand in their view, can still create an illusion of ownership over the fake limb. The findings bring researchers a step closer to understanding exactly how the brain distinguishes between the body and the world around it.
“I’m quite surprised that the illusion actually worked so well, given that electrical stimulation of the sensory cortex feels very unlike natural touch,” says Arvid Guterstam, a study author and a postdoc at the Karolinska Institute. Indeed, the patients reported feeling a “vibration” or “light pressure,” rather than a brushstroke sensation.
“Surprisingly, the illusion worked very well in both of our participants, and was actually equally vivid as the classical rubber hand illusion,” he says. “This finding tells us that the brain’s system for integrating signals from different senses to shape our experience of the bodily self is more flexible than previously assumed.”
But the illusion was strongest under very specific conditions. The touch and electrical stimulation of the brain had to occur at the same time. And when the team touched the fake finger and fired up the electrodes over the brain region corresponding to the wrist, the subjects didn’t feel the same sense of ownership over the fake limb.
It shouldn’t be overlooked that the peripheral nervous system in both subjects was intact — in other words, they didn’t have any spinal cord injuries or missing limbs, for example. The study needs to be validated in patients who physically can’t receive sensory signals from their hand, whether due to injury or amputation. But research has found that the “rubber hand illusion” works in upper limb amputees, which suggests that this electrode stimulation experiment could work in amputees as well.
“We demonstrate that it is in principle possible to create ownership of an artificial limb through brain stimulation,” Guterstam says. “Future studies are needed in order to examine if this illusion works in everyone or only a subset of individuals.”
Still, the results are an important first step toward a prosthetic that acts and feels like a real limb—and that sense of ownership could make a prosthetic easier to use and ease a patient’s emotional acceptance of a new limb.