Phantom limb pain—discomfort seemingly located in a missing part of the body—remains a mystery to scientists and doctors.

But Max Ortiz Catalan of Chalmers University of Technology in Sweden has a new theory that could explain the condition.

Building on previous work using machine learning and augmented reality, the electrical engineering professor described his hypothesis of “stochastic entanglement” in a paper published by the journal Frontiers in Neurology.

The idea is that after amputation, out-of-work neural circuitry related to the missing limb can become “entangled” with other neural networks—in this case, the one responsible for pain perception.

“Imagine you lose your hand,” Catalan explained. “That leaves a big chunk of ‘real estate’ in your brain, and in your nervous system as a whole, without a job. It stops processing any sensory input, it stops producing any motor output to move the hand. It goes idle—but not silent.”

The patient, missing his right arm, can see himself on screen in augmented reality, with a virtual limb (via Max Ortiz Catalan)

Neurons, he said, are never completely silent. When not busy with a particular job, they can fire at random.

“Normally, sporadic synchronized firing wouldn’t be a big deal, because it’s just part of the background noise, and it won’t stand out,” Catalan said in a statement. “But 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,” he continued. “Such a remarkable sensation could reinforce a neural connection, make it stick out, and help establish an undesirable link.”

Following the principle of Hebb’s Law—”neurons that fire together, wire together”—this new theory can explain why not all amputees suffer from the condition. The randomness, or stochasticity, suggests simultaneous firing may not occur or become linked, in all patients.

Catalan’s paper also highlights the Phantom Motor Execution (PME) treatment method he previously developed.

Electrodes attached to the residual limb catch signals intended for the missing part and translate them through AI algorithms into movements. Users can see and control the digitally rendered extremity on a screen in real time.

“The patients can start reusing those areas of the brain that had gone idle,” Catalan said. “Making use of that circuitry helps to weaken and disconnect the entanglement to the pain network.

“It’s a kind of ‘inverse Hebb’s law’—the more those neurons fire apart, the weaker their connection,” he continued. “Or, it can be used preventatively, to protect against the formation of those links in the first place.”

A fresh theory could help unravel some mysteries surrounding phantom limb pain, and, with the right treatment, offer relief to affected sufferers.

Machine learning and bionic technology have been used in the past two improve the way artificial body parts perform. Find out more about high-tech prosthetics here.

 

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