A majority of people with spinal cord injury and limb amputations experience phantom sensations of excruciating pain at or below the level of their paralysis or loss. This phantom pain following a spinal cord injury is the result of hypersensitive neurons in the thalamic region of the brain that can be suppressed with specially designed molecular agents.
In a healthy human body, the perception of pain must travel through three orders of neurons. The first order carry signals from the periphery to the spinal cord. The second order relay this information from the spinal cord to the thalamus and the third order transmit the information from the thalamus to the primary sensory cortex where the information is processed, resulting in the “feeling” of pain. (Weaver, 2009)
An important 2005 study done by a group of researchers at Yale University reports that in rats with spinal cord injuries, the third order neurons in the thalamus spontaneously and abnormally fire signals in the absence of any incoming signals from the first order neurons. It also reports that these rogue neurons contain abnormally high levels of a particular type of sodium channel, called Nav1.3. Sodium channels serve as batteries during the conduction of nerve signals. (Weaver, 2009)
“This study is the first to show that thalamic neurons contain abnormally high levels of Nav1.3 after injury to the spinal cord and that suppressing the activity of Nav1.3 in these neurons can mitigate pain,” said senior author Stephen Waxman, M.D., professor and chair of neurology and director of the Veterans Administration Rehabilitation Research and Development Center in West Haven. “Although these studies must be validated in higher-order animals before testing in humans, this represents an important step forward in the understanding and treatment of phantom pain.”
Virtual Reality and Phantom Pain
Virtual reality reduces phantom body pain in paraplegics and creates the illusion that they can feel their paralyzed legs being touched again. The results could one day translate into therapies to reduce chronic pain in paraplegics.
In breakthrough research led by neuroscientist Olaf Blanke and his team at EPFL, Switzerland, the scientists show that phantom body pain can be reduced in paraplegics by creating a bodily illusion with the help of virtual reality.
“We managed to provoke an illusion: the illusion that the subject’s legs were being lightly tapped, when in fact the subject was actually being tapped on the back, above the spinal cord lesion,” explains Blanke, lead author of the study and holder of the Foundation Bertarelli Chair in Cognitive Neuroprosthetics. “When we did this, the subjects also reported that their pain had diminished.” (ScienceDaily, 2017)
The experimental setup involves a camera, VR goggles, two rods and a pair of dummy legs. The legs are filmed by the camera and in real-time, the video is relayed into the virtual reality goggles worn by the paraplegic patient. The subject sees the dummy legs viewed from above, as if looking down on their own legs. With this setup in place, the scientist taps the patient’s back with one rod while simultaneously tapping the dummy legs with the other. (Pozeg, 2017)
Thus the subject receives two stimuli: one tactile on the back, the other visual from the virtual reality display. Despite being consciously aware of being tapped on the back, the subject still begins to feel as though the tapping comes from the paralyzed legs.
This research pushes the limits of how virtual reality can be used to manipulate the brain in how it experiences the body.
Weaver, J., 2009. Phantom pain after spinal cord injury is result of hypersensitive neurons in thalamic region of the brain | News. Spinal Cord Injury Zone! Available at: https://spinalcordinjuryzone.com/news/2996/phantom-pain-after-spinal-cord-injury-is-result-of-hypersensitive-neurons-in-thalamic-region-of-the-brain [Accessed November 12, 2018]
ScienceDaily, 2017. Virtual reality reduces phantom pain in paraplegics. ScienceDaily. Available at: https://www.sciencedaily.com/releases/2017/10/171030134634.htm [Accessed November 12, 2018]
Pozeg, P. et al., 2017. Virtual reality improves embodiment and neuropathic pain caused by spinal cord injury. Neurology, 89(18), pp.1894–1903.