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

FIGURE 1.2 The Penfield map of the skin surface on the postcentral gyrus (see Figure Int.2). The drawing shows a coronal section (roughly, a cross section) going through the middle of the brain at the level of the postcentral gyrus. The artist’s whimsical depiction of a person draped on the brain surface shows the exaggerated representations of certain body parts (face and hand) and the fact that the hand map is above the face map.

Now, how to explain the bizarre tendency to attribute touch sensations arising from the face to the phantom hand? The orphaned brain map continues to represent the missing arm and hand in absentia, but it is not receiving any actual touch inputs. It is listening to a dead channel, so to speak, and is hungry for sensory signals. There are two possible explanations for what happens next. The first is that the sensory input flowing from the facial skin to the face map in the brain begins to actively invade the vacated territory corresponding the missing hand. The nerve fibers from the facial skin that normally project to the face cortex sprout thousands of neural tendrils that creep over into the arm map and establish strong, new synapses. As a result of this cross-wiring, touch signals applied to the face not only activate the face map, as they normally do, but also activate the hand map in the cortex, which shouts “hand!” to higher brain areas. The net result is that the patient feels that his phantom hand is being touched every time his face is touched.

A second possibility is that even prior to amputation, the sensory input from the face not only gets sent to the face area but partially encroaches into the hand region, almost as if they are reserve troops ready to be called into action. But these abnormal connections are ordinarily silent; perhaps they are continuously inhibited or damped down by the normal baseline activity from the hand itself. Amputation would then unmask these ordinarily silent synapses so that touching the face activates cells in the hand area of the brain. That in turn causes the patient to experience the sensations as arising from the missing hand.

Independent of which of these two theories—sprouting or unmasking—is correct, there is an important take-home message. Generations of medical students were told that the brain’s trillions of neural connections are laid down in the fetus and during early infancy and that adult brains lose their ability to form new connections. This lack of plasticity—this lack of ability to be reshaped or molded—was often used as an excuse to tell patients why they could expect to recover very little function after a stroke or traumatic brain injury. Our observations flatly contradicted this dogma by showing, for the first time, that even the basic sensory maps in the adult human brain can change over distances of several centimeters. We were then able to use brain-imaging techniques to show directly that our theory was correct: Victor’s brain maps had indeed changed as predicted (Figure 1.3).

FIGURE 1.3 A MEG (magnetoencephalograph) map of the body surface in a right-arm amputee. Hatched area, hand; black areas, face; white areas, upper arm. Notice that the region corresponding to the right hand (hatched area) is missing from the left hemisphere, but this region gets activated by touching the face or upper arm.

Soon after we published, evidence confirming and extending these findings started to come in from many groups. Two Italian researchers, Giovanni Berlucchi and Salvatore Aglioti, found that after amputation of a finger there was a “map” of a single finger draped neatly across the face as expected. In another patient the trigeminal nerve (the sensory nerve supplying the face) was severed and soon a map of the face appeared on the palm: the exact converse of what we had seen. Finally, after amputation of the foot of another patient, sensations from the penis were felt in the phantom foot. (Indeed, the patient claimed that his orgasm spread into his foot and was therefore “much bigger than it used to be.”) This occurs because of another of these odd discontinuities in the brain’s map of the body: The map of the genitals is right next to the map of the foot.