The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

MY SECOND EXPERIMENT on phantom limbs was even simpler. In a nutshell, I created a simple setup using ordinary mirrors to mobilize paralyzed phantom limbs and reduce phantom pain. To understand how this works, I first need to explain why some patients are able to “move” their phantoms but others are not.

Many patients with phantoms have a vivid sense of being able to move their missing limbs. They say things like “It’s waving goodbye” or “It’s reaching out to answer the phone.” Of course, they know perfectly well that their hands aren’t really doing these things—they aren’t delusional, just armless—but subjectively they have a realistic sensation that they are moving the phantom. Where do these feelings come from?

I conjectured that they were coming from the motor command centers in the front of the brain. You might recall from the Introduction how the cerebellum fine-tunes our actions through a servo-loop process. What I didn’t mention is that the parietal lobes also participate in this servo-loop process through essentially the same mechanism. Again briefly: Motor output signals to the muscles are (in effect) CC’ed to the parietal lobes, where they are compared to sensory feedback signals from the muscles, skin, joints, and eyes. If the parietal lobes detect any mismatches between the intended movements and the hand’s actual movements, they make corrective adjustments to the next round of motor signals. You use this servo-guided system all the time. This is what allows you, for instance, to maneuver a heavy juice pitcher into a vacant spot on the breakfast table without spilling or knocking over the surrounding tableware. Now imagine what happens if the arm is amputated. The motor command centers in the front of the brain don’t “know” the arm is gone—they are on autopilot—so they continue to send motor command signals to the missing arm. By the same token, they continue to CC these signals to the parietal lobes. These signals flow into the orphaned, input-hungry hand region of your body-image center in the parietal lobe. These CC’ed signals from motor commands are misinterpreted by the brain as actual movements of the phantom.

Now you may wonder why, if this is true, you don’t experience the same sort of vivid phantom movement when you imagine moving your hand while deliberately holding it still. Here is the explanation I proposed several years ago, which has been since confirmed by brain-imaging studies. When your arm is intact, the sensory feedback from the skin, muscles, and joint sensors in your arm, as well as the visual feedback from your eyes, are all testifying in unison that your arm is not in fact moving. Even though your motor cortex is sending “move” signals to your parietal lobe, the countervailing testimony of the sensory feedback acts as a powerful veto. As a result, you don’t experience the imagined movement as though it were real. If the arm is gone, however, your muscles, skin, joints, and eyes cannot provide this potent reality check. Without the feedback veto, the strongest signal entering your parietal lobe is the motor command to the hand. As a result, you experience actual movement sensations.

Moving phantom limbs is bizarre enough, but it gets even stranger. Many patients with phantom limbs report the exact opposite: Their phantoms are paralyzed. “It’s frozen, Doctor.” “It’s in a block of cement.” For some of these patients the phantom is twisted into an awkward, extremely painful position. “If only I could move it,” a patient once told me, “it might help alleviate the pain.”

When I first saw this, I was baffled. It made no sense. They had lost their limbs, but the sensory-motor connections in their brains were presumably the same as they had been before their amputations. Puzzled, I started examining some of these patients’ charts and quickly found the clue I was looking for. Prior to amputation, many of these patients had had real paralysis of their arm caused by a peripheral nerve injury: the nerve that used to innervate the arm had been ripped out of the spinal cord, like a phone cord being yanked out of its wall jack, by some violent accident. So the arm had lain intact but paralyzed for many months prior to amputation. I started to wonder if perhaps this period of real paralysis could lead to a state of learned paralysis, which I conjectured could come about in the following way.