Phantom Limb - A New Study Using Anesthesia of the Arm
Scientists now provide new information about how the brain generates phantom limbs. According to the study there is no default position that the phantom moves into after it forms.
A person has a 'phantom limb' when they continue to perceive sensations in their limb even when the body part is no longer there, such as after amputation or an accident. A phantom limb can also develop when the sensory system no longer functions properly, such as in a complete spinal cord injury or after administration of an anesthetic.
People usually perceive their phantom limbs to be in distinct positions, and often experience pain in the limb – however, the cause of this is not well understood.
The study was published today in The Journal of Physiology.
Prof Simon Gandevia and colleague Dr Lee Walsh from Neuroscience Research Australia tracked the evolution of an experimental phantom hand in a study in which they temporarily anaesthetized participants' hands to induce a phantom limb.
Normally, we know the position of a stationary limb and this information is needed for the brain to execute the limb movement. When the nerves to the limb are severed or numbed by anesthesia, some individuals experience phantom sensations in their amputated limb.
The researchers numbed one arm of the study participants, either with the wrist and fingers held straight or in bent position. Interestingly the subjects felt bent wrist and fingers in the numbed hand if the positions prior to anesthesia were straight and the perceived phantom hand was straight whenever the hand was anesthetized with the wrist and fingers in bent position.
"Our research suggests that the state of nerves in the limb at the time the phantom is forming is very important in determining how the phantom develops," says Prof. Gandevia.
Because distortions of body image, such as phantom limbs, are difficult to treat, a better understanding of the mechanisms behind their formation will help developing more effective treatments.
The team found that the state of the nerves – i.e. how much they were being stimulated – over the period when participants were losing sensation in their hand was key in determining the final perceived position of the phantom hand.
"This might also be true for phantom pain," says Prof Gandevia. "In other words, the amount and type of nerve stimulation around the time of amputation or injury could also be important in determining the type and degree of pain perceived in the phantom limb."
N. Inui, L. D. Walsh, J. L. Taylor and S. C. Gandevia. Dynamic changes in the perceived posture of the hand during ischaemic anaesthesia of the arm. The Journal of Physiology,2011 ;589, 5775-5784.
Neuroscience Research Australia
Image credit: Neuroscience Research Australia