In recent years, scientists have discovered that the brain’s image of its body, known as the body schema, is extraordinarily malleable — for instance, people can be fooled into thinking a rubber hand is part of their bodies. Now for the first time scientists have discovered monkeys can experience a version of this illusion, findings that could one day help people accept prosthetic limbs as parts of their body schema. Details of the research appeared recently in the Proceedings of the National Academy of Sciences.
The rubber hand illusion starts by hiding a person’s real hand from view, substituting it with a rubber hand. If both the real and the rubber hand are stroked with a brush at the same time for a few minutes, many people experience the unnerving illusion that the dummy hand is their hand. This illusion can even apply to objects that bear no resemblance to body parts, such as tables and invisible hands.
Learning more about the roots of the rubber hand illusion in the brain could help treat disorders of body schema, perhaps including phantom limb syndrome. However, experimenting on people’s brains to further such research is a challenge at best. Neurobiologist Miguel Nicolelis at Duke University Medical Center and his colleagues instead sought to discover if they could learn more about the rubber hand illusion with monkeys.
The researchers have long worked on brain-machine interfaces that can enable people to mentally control machines such as robotic arms. They have years of work showing they can get monkeys to mentally control robotic limbs as well.
Nicolelis and his colleagues had two monkeys observe 3-D computer avatar versions of their arms getting touched by virtual balls while they simultaneously had their real arms stroked with brushes. At the same time, the scientists recorded brain activity from the monkeys via electrodes implanted in their heads.
The scientists found that neurons in the somatosensory and motor areas of the monkey brains, which are related to the sense of touch and voluntary motions, respectively, began to respond to virtual touches alone, suggesting that these brain regions contribute to the rubber hand illusion.
“This supports a theory we have been developing for 10 years — that the brain assimilates virtual tools as an extension of its body schema,” Nicolelis says.
This is the first instance of the rubber hand illusion in monkeys, “and the first instance of recording in the somatosensory and motor cortices during the rubber hand illusion,” Nicolelis says.
Brain responses to virtual touches occurred 50 to 70 milliseconds later than to physical touches, suggesting these signals were getting routed through the visual parts of the brain, “which once and for all reveals the textbook notion of the cortex as a mosaic of segregated areas is pretty much dead now,” Nicolelis says. “The fact that you can get so-called tactile neurons driven by a visual cue is pretty exciting.”
This research suggests training people using computer avatars could help them better accept prosthetics, Nicolelis says.
“A key component of our Walk Again Project, as we build an exoskeleton to help people walk again, is to prepare patients on virtual avatars first so there’s a representation of the lower limbs in the brains of patients,” Nicolelis says. “Patients will not only learn how to control exoskeletons, but have the vivid sensation it’s part of their own bodies.”