The split brain: A tale of two halves : Nature News & Comment
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Nature | News Feature
The split brain: A tale of two halves
Since the 1960s, researchers have been scrutinizing a handful of patients who underwent a radical kind of brain surgery. The cohort has been a boon to neuroscience — but soon it will be gone.
In the first months after her surgery, shopping for groceries was infuriating. Standing in the supermarket aisle, Vicki would look at an item on the shelf and know that she wanted to place it in her trolley — but she couldn’t. “I’d reach with my right for the thing I wanted, but the left would come in and they’d kind of fight,” she says. “Almost like repelling magnets.” Picking out food for the week was a two-, sometimes three-hour ordeal. Getting dressed posed a similar challenge: Vicki couldn’t reconcile what she wanted to put on with what her hands were doing. Sometimes she ended up wearing three outfits at once. “I’d have to dump all the clothes on the bed, catch my breath and start again.”
In one crucial way, however, Vicki was better than her pre-surgery self. She was no longer racked by epileptic seizures that were so severe they had made her life close to unbearable. She once collapsed onto the bar of an old-fashioned oven, burning and scarring her back. “I really just couldn’t function,” she says. When, in 1978, her neurologist told her about a radical but dangerous surgery that might help, she barely hesitated. If the worst were to happen, she knew that her parents would take care of her young daughter. “But of course I worried,” she says. “When you get your brain split, it doesn’t grow back together.”
In June 1979, in a procedure that lasted nearly 10 hours, doctors created a firebreak to contain Vicki’s seizures by slicing through her corpus callosum, the bundle of neuronal fibres connecting the two sides of her brain. This drastic procedure, called a corpus callosotomy, disconnects the two sides of the neocortex, the home of language, conscious thought and movement control. Vicki’s supermarket predicament was the consequence of a brain that behaved in some ways as if it were two separate minds.
After about a year, Vicki’s difficulties abated. “I could get things together,” she says. For the most part she was herself: slicing vegetables, tying her shoe laces, playing cards, even waterskiing.
But what Vicki could never have known was that her surgery would turn her into an accidental superstar of neuroscience. She is one of fewer than a dozen ‘split-brain’ patients, whose brains and behaviours have been subject to countless hours of experiments, hundreds of scientific papers, and references in just about every psychology textbook of the past generation. And now their numbers are dwindling.
Through studies of this group, neuroscientists now know that the healthy brain can look like two markedly different machines, cabled together and exchanging a torrent of data. But when the primary cable is severed, information — a word, an object, a picture — presented to one hemisphere goes unnoticed in the other. Michael Gazzaniga, a cognitive neuroscientist at the University of California, Santa Barbara, and the godfather of modern split-brain science, says that even after working with these patients for five decades, he still finds it thrilling to observe the disconnection effects first-hand. “You see a split-brain patient just doing a standard thing — you show him an image and he can’t say what it is. But he can pull that same object out of a grab-bag,” Gazzaniga says. “Your heart just races!”
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Michael Gazzaniga reflects on five decades of split-brain research
Work with the patients has teased out differences between the two hemispheres, revealing, for instance, that the left side usually leads the way for speech and language computation, and the right specializes in visual-spatial processing and facial recognition. “The split work really showed that the two hemispheres are both very competent at most things, but provide us with two different snapshots of the world,” says Richard Ivry, director of the Institute of Cognitive and Brain Sciences at the University of California, Berkeley. The idea of dichotomous consciousness captivated the public, and was greatly exaggerated in the notion of the ‘creative right brain’. But further testing with split-brain patients gave a more-nuanced picture. The brain isn’t like a computer, with specific sections of hardware charged with specific tasks. It’s more like a network of computers connected by very big, busy broadband cables. The connectivity between active brain regions is turning out to be just as important, if not more so, than the operation of the distinct parts. “With split-brain patients, you can see the impact of disconnecting a huge portion of that network, but without damage to any particular modules,” says Michael Miller, a psychologist at the University of California, Santa Barbara.
David Roberts, head of neurosurgery at Dartmouth-Hitchcock Medical Center in Lebanon, New Hampshire, sees an important lesson in split-brain research. He operated on some of the cohort members, and has worked closely with Gazzaniga. “In medical school, and science in general, there is so much emphasis on large numbers, labs, diagnostics and statistical significance,” Roberts says — all crucial when, say, evaluating a new drug. But the split-brain cohort brought home to him how much can be gleaned from a single case. “I came to learn that one individual, studied well, and thoughtfully, might enable you to draw conclusions that apply to the entire human species,” he says.
Today, the split-brain patients are getting on in years; a few have died, one has had a stroke and age in general has made them all less fit for what can be taxing research sessions of sitting, staring and concentrating. The surgery, already quite rare, has been replaced by drug treatments and less drastic surgical procedures. Meanwhile, imaging technologies have become the preferred way to look at brain function, as scientists can simply watch which areas of the brain are active during a task.
Michael Gazzaniga has worked with split-brain patients for 50 years.
PHOTO BY MIKE MCGREG OR/CONTOUR BY GETTY
But to Miller, Ivry, Gazzaniga and others, split-brain patients remain an invaluable resource. Imaging tools can confirm, for example, that the left hemisphere is more active than the right when processing language. But this is dramatically embodied in a split-brain patient, who may not be able to read aloud a word such as ‘pan’ when it’s presented to the right hemisphere, but can point to the appropriate drawing. “That gives you a sense of the right hemisphere’s ability to read, even if it can’t access the motor system to produce speech,” Ivry says. “Imaging is very good for telling you where something happens,” he adds, “whereas patient work can tell you how something happens.”
A cable, cut
Severing the corpus callosum was first used as a treatment for severe epilepsy in the 1940s, on a group of 26 people in Rochester, New York. The aim was to limit the electrical storm of the seizure to one side of the brain. At first, it didn’t seem to work. But in 1962, one patient showed significant improvement. Although the procedure never became a favoured treatment strategy — it’s invasive, risky, and drugs can ease symptoms in many people — in the decades since it nevertheless became a technique of last resort for treating intractable epilepsy.
To Roger Sperry, then a neurobiologist and neuropsychologist at the California Institute of Technology, and Gazzaniga, a graduate student in Sperry’s lab, split-brain patients presented a unique opportunity to explore the lateralized nature of the human brain. At the time, opinion on the matter was itself divided. Researchers who studied the first split-brain patients in the 1940s had concluded that the separation didn’t noticeably affect thought or behaviour. (Gazzaniga and others suspect that these early sections were incomplete, which might also explain why they didn’t help the seizures.) Conversely, studies conducted by Sperry and colleagues in the 1950s revealed greatly altered brain function in animals that had undergone callosal sections. Sperry and Gazzaniga became obsessed with this inconsistency, and saw in the split-brain patients a way to find answers.
The duo’s first patient was a man known as W. J., a former Second World War paratrooper who had started having seizures after a German soldier clocked him in the head with the butt of a rifle. In 1962, after W.J.’s operation, Gazzaniga ran an experiment in which he asked W.J. to press a button whenever he saw an image. Researchers would then flash images of letters, light bursts and other stimuli to his left or right field of view. Because the left field of view is processed by the right hemisphere and vice versa, flashing images quickly to one side or the other delivers the information solely to the intended hemisphere (see ‘Of two minds’).
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