But guess what would happen if someone were to numb your hand using an anesthetic? Astonishingly, we have found (in collaboration with U.C.S.D. graduate student Laura Case) that the patient now quite literally feels touch sensations in his anesthetized hand when he merely watches another person being poked. Or if the other person handles an ice cube, the patient feels the cold freezing his hand! Once you remove the touch signals from the intact hand, the patient does not merely empathize with others—he feels what they touch. The same thing happens in patients with phantom limbs. Watching another person’s hand being massaged seems to relieve pain in the patient’s absent arm or leg.
Clinically it is known that visual feedback using mirror reflections can help alleviate phantom pain and stroke paralysis, perhaps by tapping into mirror neurons. We are currently exploring whether illusions of disembodiment produced with mirrors can also be used to mimic the effects of ketamine and treat chronic pain syndromes by allowing a patient to “detach” from his body and the pain “it” experiences.
Extraordinarily, even real pain in a real arm can be cured through optical feedback. In particular, there is a cruel disorder called reflex sympathetic dystrophy in which a trifling injury leads to permanent excruciating pain, swelling and “paralysis” of an arm, a condition we have dubbed “learned pain and paralysis.” In 1995, in a lecture at the Society for Neuroscience meeting in San Diego, we suggested using mirrors to treat this disorder, and several large-scale clinical trials have since confirmed their efficacy. Even the swelling subsides—a remarkable example of mind-body interaction.
The strangest of body-image disturbances is one in which a perfectly healthy person desires to have an arm or leg amputated. In conjunction with our U.C.S.D colleagues David Brang and Paul McGeoch, we have found that touching the skin of the affected limb produces an abnormal sweating response, whereas touching the normal limb does not. Further, our brain-imaging studies indicate an impoverished representation of the affected limb in the right parietal lobe (the body image area), although the areas for touch in the somatosensory cortex remain normal. This discrepancy between accurate sensory input from the arm and a lack of arm representation in the brain creates a curious abhorrence of the limb [see “Amputee Envy,” by Sabine Mueller; Scientific American Mind, December 2007/January 2008].
Thus, studying people with brain abnormalities or manipulating sensory input in normal people using mirrors and other optical tricks can provide key insights into the way the right parietal lobe of the brain creates a vibrant image of one’s body that endures in space and time.
These observations have important implications, both theoretical and clinical. They suggest that what we call touch sensation, pain, the body or even the self results from a dynamic interplay of signals from three sources: sensory signals from the skin, muscles and gut; inhibitory signals from the prefrontal cortex; and input from mirror neurons, which respond to behavior that originates in neurons in other people’s brains! From this fluctuating mosaic of brain activity emerges your sense of an embodied self that is distinct from others and all your own.