We live in the age of the fMRI machine, dazzled and bamboozled by pictures of brains “lighting up” in living Technicolor. Before these neuroscientific glory days, the mysteries of the mind had to be approached by rather less alluring methods: postmortem examination of the brains of psychiatric patients, animal experiments of legendary cruelty and intelligence testing after pioneering brain surgeries, to name but a few. During the knife-happy decades of the mid-twentieth century, surgical treatments for seizure disorders generated especially startling insights into human brain function. In Montreal, the neurosurgeon Wilder Penfield developed an exquisitely sensitive procedure for protecting the neurological integrity of his epileptic patients, keeping them conscious on the operating table and asking them to describe their sensations as he gently stimulated their exposed brains with electrodes while a stenographer in a little glass booth transcribed every word. A byproduct of this work was “Penfield’s Homunculus,” a cartoon character whose proportions corresponded to the area of cortex devoted to each body part: huge thumbs and outsize lips (parts under voluntary control) and a titchy little penis (not so voluntary). The most famous neurological patient in recent history, the amnesiac “H.M.,” was an epileptic who had areas of his temporal lobes removed in 1953. His seizures became less frequent, but he lost the ability to form long-term memories. Precise analysis of what he could and could not remember after his operation revolutionized psychology. Overall, the wave of experimental neurosurgeries between the 1930s and the ’70s established the principle of the division of the brain into specialized task areas. This scientific phrenology now goes under the name “modularity,” and the broad outlines have been confirmed (though the details are still hotly debated) by the evidence piling up from brain-scanning techniques.
Of all the surgeries to treat epilepsy, one of the most radical is the severing of the corpus callosum, the layer of nerve fibers that connect the two hemispheres of the brain. Pioneered by a surgeon who had observed that epileptic patients with tumors in the corpus callosum tended to suffer fewer seizures, the operation was first performed in the 1940s, leaving twenty-six people walking around with brains that had been split down the middle. Astonishingly, the patients reported no side effects from the surgery beyond blessed relief from their symptoms. Despite its success, the radicalism of the treatment rendered it controversial and almost two decades passed before a precocious Dartmouth undergraduate named Michael Gazzaniga applied for permission to test the patients. The animals in the experimental psychology lab where he had been working showed effects from the severing of the two hemispheres. Surely the same must be true of humans. Amazingly, permission was granted. Fizzing with excitement, Gazzaniga drove to Rochester, New York, in a car full of nifty new equipment from the Dartmouth psychology department. When he arrived, however, it turned out that someone had gotten cold feet at the prospect of this ambitious young man probing loss of function in the patients, and he was turned away.
The following summer, opportunity knocked again. A war veteran with intractable seizures was judged to be a good candidate for the split-brain procedure, and Gazzaniga—now enrolled in graduate school—was assigned to test him before and after surgery. He landed the job despite his tender years because he had devised a test that exploited the wiring of the human visual system. The nerves that run from our eyes to the backs of our brains divide at some point in their journey: one half stays in the same hemisphere; the other goes to the opposite hemisphere. This means that different sides of the visual field in the same eye are processed by different halves of the brain. Gazzaniga reasoned that if he showed the patient images in an appropriately restricted area of the visual field—if he showed the pictures to just one hemisphere, so to speak—he might be able to figure out if the two sides of the brain were acting independently of each other. Would the patient’s language center in the left hemisphere, for example, be able to speak about objects shown only to the other side of his brain? The answer turned out to be a resounding no.
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In Who’s in Charge?, the latest in a series of books about the brain aimed at a nonspecialist audience, Gazzaniga recalls his excitement at running the first of these tests, in the summer of 1962. Adrenaline pumping and heart pounding, he showed “W.J.” an image in the part of his visual field processed by the right hemisphere. When asked to describe the picture, the patient replied, “I didn’t see anything.” Gazzaniga immediately discerned the scientific importance of this response: “Not only could he no longer verbally describe, using his left hemisphere, an object presented to his freshly disconnected right hemisphere, but he did not know that it was there at all.” Just as the experimental neurosurgery on H.M. had opened up the problem of memory, W.J.’s operation promised to reveal fascinating aspects of the division of labor between the hemispheres and the nature of self-awareness. The famous psychologist Brenda Milner built her reputation on her testing of the amnesiac H.M., which she conducted when she was a graduate student. Likewise, Gazzaniga has built a distinguished career on his discovery of the split-brain phenomenon in humans. Such is the debilitating nature of epilepsy that sufferers willingly submit to radical experimental treatments. When they subsequently undergo hours of testing in the name of scientific curiosity, they become the unsung heroes of the history of the neurosciences.