Connectome by Sebastian Seung

pain. In the 1930s the Canadian neurosurgeon Wilder Penfield mapped both areas in his patients by using electrical stimulation. After opening the skull to expose the brain for epilepsy surgery, Penfield applied his electrode to different locations in area 4. Each stimulation caused some part of the patient’s body to move. Penfield drew the correspondence between area 4 locations and body parts (Figure 12, right), calling the map a “motor homunculus.” (
Homunculus
is from the Latin for “little human.”) Likewise, after each stimulation of area 3, the patient reported feeling a sensation in some part of the body. Penfield mapped the “sensory homunculus” in area 3 (left), and it looked similar to the motor one. Both ran in parallel along opposite banks of the central sulcus. (Roughly speaking, these maps represent vertical planes passing through the brain from ear to ear. The plane of the sensory map is just behind the central sulcus, and that of the motor map just in front. Only the outer border is cortex; the rest is the interior of the cerebrum.)
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Figure 12. Functional maps of cortical areas 3 and 4: the “sensory homunculus”
(left)
and the “motor homunculus”
(right)
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    The face and hands dominate the maps, even though they are small parts of the body. Their cortical magnification reflects their disproportionate importance in sensation and movement. Could the sizes of their territories be changed by amputation, which suddenly reduces the importance of a body part to zero? Using such reasoning, the neurologist V. S. Ramachandranand his collaborators have proposed that phantom limbs are caused by remapping of area 3. If the lower arm is amputated, its territory in the sensory homunculus loses its function. The surrounding territories, dedicated to the face and upper arm, encroach upon the nonfunctional one by advancing their borders. (You can see the adjacencies in Penfield’s drawing.) These two intruders start to represent the lower arm as well as their original body parts, giving the amputee the sensation of a phantom limb.
    According to the theory, the remapped face territory should represent the lower arm as well as the face. Therefore Ramachandran predicted that stimulation of the face would cause sensations in the phantom limb. Indeed, when he stroked the face of an amputeewith a Q-tip, the patient reported feeling sensations not only in his face but also in his phantom hand. The theory likewise predicts that the remapped upper-arm territory should represent the lower arm as well as the upper arm. When Ramachandran touched the stump, the patient felt sensations in both the stump and his phantom hand. These ingenious experiments strikingly confirmed the theory that amputation caused remapping of area 3.
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    Ramachandran and his collaborators used technology no more advanced than a Q-tip. In the 1990s an exciting new method of brain imaging was introduced. Functional MRIrevealed every region’s “activity,” or how much that part of the brain was being used. By now the images of functional MRI (fMRI) are familiar from their frequent appearance in the news media. They are usually shown superimposed on regular MRI images. The black-and-white MRI image shows the brain, and laid on top are the colored blotches of the fMRI image, which indicate the active regions. You can always recognize fMRI+MRI as “spots on brains,”while MRI is just brains.
    Researchers imaged volunteers while they performed mental tasks in the laboratory. If a task activated a region, causing it to “light up” in the image, that was a clue to the region’s function. Neurology had always been hampered by the accidental nature of brain lesions, but fMRI enabled precise and repeatable experiments on localization of function. Brodmann’s map became indispensable as researchers worked hard to assign functions to each of its areas. The boom in