Brain Stimulation Can Produce Ownership of a Prosthetic Hand
Being able to replace the function of a missing or paralyzed limb with a prosthetic device that acts and feels like the user's own limb is one of the goals of applied neuroscience. Researchers have successfully used electrical stimulation of the brain to induce the feeling of ownership in two participants' artificial hands.
Previous research has shown that the rubber hand illusion, a multisensory perceptual illusion in which the sense of touch and feeling of ownership are referred to an artificial limb, can be used to induce ownership sensations of a prosthetic hand in people with upper-limb amputations. However, the classic rubber hand illusion requires that touches be delivered to the real hand, and previous studies among people with amputations have relied on tactile stimulation of the residual limb or reinnervated regions of the skin. Thus, using a modified version of the rubber hand illusion with direct stimulation to the brain, the researchers set out to determine whether it is possible to induce ownership of an artificial limb in the absence of peripheral stimulation.
The results of the study, conducted by researchers at the University of Washington and the Karolinska Institutet, Stockholm, Sweden, suggest that the brain can integrate "visual input and direct cortical-somatosensory stimulation to create the multisensory perception that an artificial limb belongs to one's own body," and that electrical brain stimulation can be used to "bypass" the peripheral nervous system to induce multisensory illusions and ownership of an artificial body part.
The study was conducted with two subjects, a 19-year-old woman and a 33-year-old man, who were undergoing electrocorticographic monitoring in preparation for surgery to determine the location in the brain where their seizures originate. The surgery entailed implantation of subdural electrodes over the cortical surfaces of their brains, the area that the researchers needed to access for their experiment.
One of each subject's hands was hidden behind a screen, and a prosthetic hand was placed in front of the screen. The distance between each participant's index finger of the real hand and that of the rubber hand was 15cm. White cloth covered the participant's upper arm to mask the gap between the shoulder and the prosthetic hand.
There were five experimental conditions-one illusion and four control-all of which featured a single finger of the rubber hand-the index finger of one subject, and the middle finger of the other-being repeatedly touched over a period of 60 seconds. The subjects were asked to continuously report the vividness of the illusion, which was defined as the degree of agreement with the statement "It feels as if the rubber hand were my hand," using a scale ranging from -3 ("I completely disagree") to 3 ("I completely agree,"); 0 indicated "I neither agree nor disagree." The touches were manually delivered to the rubber hand by a trained experimenter who did not know whether the condition in question was an illusion or a control.
The illusion was successfully created with both participants when there was electrical stimulation of the hand section of their somatosensory cortex while simultaneously touching the rubber hand. The illusion was not elicited when the electrical stimulation was delivered asynchronously or to a portion of the somatosensory cortex representing a body part other than the hand.
The study was published in the January issue of Proceedings of the National Academy of Sciences of the United States of America. www.pnas.org/content/114/1/166.full