Phantom limb pain was first recorded some 400 years ago by French surgeon Ambroise Paré. At that time, he operated on wounded soldiers, many of whom complained of a mysterious pain in amputated limbs. Despite its long history, phantom limb pain remains poorly understood.
Recently, the journal Frontiers in Neurology published a paper by Dr. Max Ortiz Catalan, Associate Professor at Chalmers University of Technology and head of the Biomechatronics and Neurorehabilitation Laboratory. Dr. Catalan’s new theory just might be the long-awaited breakthrough.
He called it “stochastic entanglement,” which describes how entangled neural circuitry causes phantom limb pain.
After an amputation, the neural circuits responsible for processing sensory input and producing motor output in the missing limb loses its role. It becomes prone to entanglement with other neural networks, such as the one responsible for pain perception.
According to Dr. Catalan, losing a limb “leaves a big chunk of ‘real estate’ in your brain” and in your nervous system. While it has lost its role, the neurons do not go silent. Instead, they might fire at random, which can result in the simultaneous firing of neurons from the sensorimotor and pain perception networks. When they fire together, pain is felt in that part of the body.
This misfiring and subsequent entangling are explained in “Hebb’s Law” which states, “Neurons that fire together, wire together.”
“Normally, sporadic synchronized firing wouldn’t be a big deal. However, in patients with a missing limb, such an event could stand out when little else is going on at the same time. This can result in a surprising, emotionally charged experience—to feel pain in a part of the body you don’t have. Such a remarkable sensation could reinforce a neural connection, make it stick out, and help establish an undesirable link,” Dr. Catalan said.
Stochastic entanglement also explains why phantom limb pain doesn’t afflict all amputees. This is due to the phenomenon’s stochasticity or randomness, which means that simultaneous firing or linking may not occur in all patients.
Phantom Motor Execution Treatment
Dr. Catalan proceeded to examine how stochastic entanglement can explain the effectiveness of Phantom Motor Execution (PME), a treatment he developed in 2017.
When patients undergo PME, they stimulate and reactivate the dormant areas of their brain through artificial intelligence (AI). During treatment, electrodes are attached to the patient's residual limb to pick up electrical signals intended for the missing limb. These signals are translated into movements for a virtual limb in real time. The patient can see his or her image on a screen with the digitally rendered limb in place of their missing one.
The treatment works because, according to Dr. Catalan, it uses the idle neural circuitry, which “helps weaken and disconnect the entanglement to the pain network.” Simply put, the treatment invokes an “inverse Hebb’s law”—the more those entangled neurons fire apart, the weaker their connection gets.
PME can also be used as a preventive measure; it can keep the networks in check and avoid their entanglement in the first place.