Hemispheric Disturbances: On Michael Gazzaniga
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.
It has been half a century since Gazzaniga discovered that W.J.’s left brain did not know what his right brain was doing, and this slim, accessible volume sets out some of what Gazzaniga takes to be the philosophical fruits of this revelation. The first question that work with these patients promised to illuminate was nothing less than the nature of consciousness. If W.J. and others like him could function perfectly well with what were, in effect, two separate brains, what happened to their sense of unified purpose and coherent identity? Did they have two consciousnesses? It certainly seemed like it. If a picture of a bicycle was presented to the appropriate part of W.J.’s visual field, his mouth would deny that he had seen anything but his left hand would draw a bicycle. If the right hemisphere was shown the word “key” and the left side was shown the word “ring,” his mouth would say “ring” but his left hand would choose a key from an array of objects in front of him. The implications were profound, and Gazzaniga seems to have lost none of his youthful excitement at having a crack at one of the oldest conundrums in the philosophical book: “WHY WHY WHY was there this apparent feeling of unity?”
The question, of course, doesn’t apply only to split-brain patients. Gazzaniga’s testing of the two hemispheres separately revealed that humans have the most asymmetrical brains in the animal kingdom. Neural task specialization is a feature of all primate brains, but we appear to take it to an extreme. One of the many rewards of Who’s in Charge? is a compelling account of the evolution of our hyper-modularity. Gazzaniga explains that nervous systems run on connections between cells. As brains became larger over the course of mammalian evolution, connectivity had to become more specialized, in order not to become self-defeatingly complex. (Think of a social-networking dystopia in which everybody is compelled to “friend” everybody else, with no blocking mechanisms allowed.) To compensate for their increasing scale, primate brains—like social networks—are built according to “small world” architectural principles: dense local connections, linked by a few long-distance fibers, allowing for fast processing in specialized areas, alongside economical communication to the global network. “The end result,” Gazzaniga says, “is thousands of modules, each doing their own thing.” So how do any of us, let alone split-brain patients, extract a coherent sense of self from all this atomized complexity?
More than a decade later, Gazzaniga began to probe this question deliberately with a new cohort of split-brain patients on the East Coast. In one experiment, the subject was asked to perform an association test. He was shown a picture on a screen and had to choose an accompanying image from a set of cards in front of him. The left hemisphere was shown an image of a chicken claw, and with his right hand he chose the card with a picture of a chicken. The right hemisphere was shown a house in the snow, and his left hand picked out the card with a shovel on it (a natural association for an East Coaster used to having to shovel his driveway). Then Gazzaniga asked him to say why he had picked out those two cards (speaking being a left hemisphere task). The split-brain patient looked down at his choices and answered, “Oh, that’s simple. The chicken claw goes with the chicken…and you need a shovel to clean out the chicken shed.” Without missing a beat, he had come up with a perfectly fluent rationalization for the unlinked associations made by the two halves of his brain. Hence one reason that the original cohort of split-brain patients reported no side effects of the operation: using the dreamer’s capacity to make a story out of random stuff, they were able to weave any right-brain anomalies seamlessly into the conscious left-brain fabric of their lives.
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