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

During the preamputation period, every time the motor cortex sent a movement command to the arm, the sensory cortex in the parietal lobe would receive negative feedback from the muscles, skin, joints, and eyes. The entire feedback loop had gone dead. Now, it is well established that experience modifies the brain by strengthening or weakening the synapses that link neurons together. This modification process is known as learning. When patterns are constantly reinforced—when the brain sees that event B invariably follows event A, for instance—the synapses between the neurons that represent A and the neurons that represent B are strengthened. On the other hand, if A and B stop having any apparent relationship to each other, the neurons that represent A and B will shut down their mutual connections to reflect this new reality.

So here we have a situation where the motor cortex was continually sending out movement commands to the arm, which the parietal lobe continually saw as having absolutely zero muscular or sensory effect. The synapses that used to support the strong correlations between motor commands and the sensory feedback they should generate were shown to be liars. Every new, impotent motor signal reinforced this trend, so the synapses grew weaker and weaker and eventually became moribund. In other words, the paralysis was learned by the brain, stamped into the circuitry where the patient’s body image was constructed. Later, when the arm was amputated, the learned paralysis got carried over into the phantom so the phantom felt paralyzed.

How could one test such an outlandish theory? I hit on the idea of constructing a mirror box (Figure 1.4). I placed an upright mirror in the center of a cardboard box whose top and front had been removed. If you stood in front of the box, held your hands on either side of the mirror and looked down at them from an angle, you would see the reflection of one hand precisely superimposed on the felt location of your other hand. In other words, you would get the vivid but false impression that you were looking at both of your hands; in fact, you would only be looking at one actual hand and one reflection of a hand.

If you have two normal, intact hands, it can be entertaining to play around with this illusion in the mirror box. For example, you can move your hands synchronously and symmetrically for a few moments—pretending to conduct an orchestra works well—and then suddenly move them in different ways. Even though you know it’s an illusion, a jolt of mild surprise invariably shoots through your mind when you do this. The surprise comes from the sudden mismatch between two streams of feedback: The skin-and-muscle feedback you get from the hand behind the mirror says one thing, but the visual feedback you get from the reflected hand—which your parietal lobe had become convinced is the hidden hand itself—reports some other movement.

FIGURE 1.4 The mirror arrangement for animating the phantom limb. The patient “puts” his paralyzed and painful phantom left arm behind the mirror and his intact right hand in front of the mirror. If he then views the mirror reflection of the right hand by looking into the right side of the mirror, he gets the illusion that the phantom has been resurrected. Moving the real hand causes the phantom to appear to move, and it then feels like it is moving—sometimes for the first time in years. In many patients this exercise relieves the phantom cramp and associated pain. In clinical trials, mirror visual feedback has also been shown to be more effective than conventional treatments for chronic regional pain syndrome and paralysis resulting from stroke.