by Patrick Yang ’20

Amputation or severe neural damage can trigger phantom limb pain, chronic pain felt in a limb that is no longer present. Phantom pain, which is associated with a burning or hypersensitive sensation, has a wide range of duration – it can be as short as a few minutes or as long as a couple days. According to popular theory, maladaptive plasticity of the sensorimotor cortex, or a discrepancy between our brain’s motor output and the sensory feedback, is the principal cause of this chronic pain. If this theory is true, functional restoration to a robotic prosthetic, for example, would relieve phantom limb pain because there would no longer be a discrepancy in the brain.

Unlike researchers who previously utilized drug therapy, lead researcher Dr. Takufumi Yanagisawa of Osaka University attempted to relieve phantom pain by restoring function to the brain through a robotic prosthetic hand. Nine patients with severe nerve injury and one amputee patient experiencing phantom limb pain, aged 38 to 60 years, each controlled the prosthetic hand. In order for the patients to control the prosthetic, a brain-machine interface (BMI) controlled brain and prosthetic coordination using real-time magnetoencephalography (MEG), a method for mapping brain activity. MEG waves were decoded into actual movements in real-time, which allowed patients to move the prosthetic with their phantom hand.

BMI training led to a significant increase in sensory information and activity in the sensorimotor cortex, indicating partial restoration of function. Yet despite the popular theory that functional restoration can relieve phantom limb pain, increased BMI proficiency in the affected side of the brain actually resulted in significant increase in phantom limb pain levels. This contradiction was further tested by having the patients associate their intact hand, and, therefore, a different part of the cortex, with the prosthetic. This test showed a decrease in phantom limb pain, which suggests a clear connection between sensorimotor cortex plasticity and phantom limb pain.

References:

1. T. Yanagisawa, et al., Induced sensorimotor brain plasticity controls pain in phantom limb patients. Nature Communications 7, (2016). doi: 10.1038/ncomms13209.
2. Image retrieved from: http://www.medindia.net/patients/patientinfo/images/phantom-limb-syndrome.jpg.