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The human brain is split into two hemispheres that to the naked eye look strikingly like the two halves of a walnut. In her engrossing account of her stroke, Dr. Jill Bolte Taylor proposes that these two halves of the brain house two distinct identities -- that we are all masters of two distinct minds, one housed in a logical, linguistic, analytic half-brain on the left, and the other in a qualitative, imagistic, intuitive half-brain on the right. If true, this would be an elegant physiological explanation of how we're able to use our minds in these different modes.
But modern neuroscience tells a different story. The two halves of the brain are tightly integrated to the point where pretty much anything you want to do with the brain uses both hemispheres in a delicately choreographed dance.
This isn't to deny that our minds have different modes or that we shift among them as circumstances require. We all experience times when we're aware of being analytical, linguistic, and logical: perhaps when taking a multiple choice test or calculating how much tip to add to a restaurant bill. This contrasts with the feeling of simply being in the flow, experiencing and perceiving the present moment. For some people, dancing, singing, or painting will induce this. For others, it's either sports or hallucinogens (or in the case of frisbee golf, both simultaneously).
And this idea that we have multiple minds in one is not new. Aristotle and Plato believed that the mind was composed of three separate souls, corresponding to the capacities of plants, animals, and humans, respectively . So our animal soul, according to these ancient Greeks, was perceptive, and could feel pleasure, pain, desire, and so on. But our uniquely human soul had the ability to reason, as only humans ostensibly can. Freudian psychoanalysis also divides the mind into a society of forces -- where the Id, Ego, and Superego lock horns in an eternal tussle .
It wasn't until the 1970s that neuroscientists started to believe that they had evidence that the mind's different functions were housed in the two distinct halves of the brain. The most compelling evidence came from people whose had the connection between their two hemispheres severed. These were severely epileptic patients, in whose brains lightening storms of electrical activity spread from one hemisphere to the other, causing massive and dangerous seizures. The radical treatment of last resort was to sever the corpus collosum, the bundle of neuronal fibers that connects left and right, to corral seizures in just one hemisphere . Work with these patients revealed, remarkably, that the two halves of the brain have different abilities. If you were to show a picture of, say, a giraffe or a truck to the left hemisphere, the patient could name it readily. The severed right hemisphere, however, at least in the few patients who were studied, was unable to find words for the pictures.
At this point, it was easy to take an intuition most people already shared -- the old and compelling idea that our minds are fractured into a linguistic, analytical, and logical self and a separate feeling subjective self, and map it onto the largest obvious anatomical schism in the brain. Maybe the left hemisphere is where our analytical, linguistic self lives, and the right hemisphere is where our intuitive self lives. This has proved one of the catchiest ideas in pop science -- the idea that people might be right-brained or left-brained in the same way that they're right- or left-handed was quickly popularized in the 1980s and has since been extended to everything from management advice to individualized teaching strategies.
But like a lot of science that makes its way into self-help books, this has turned out to be an oversimplification, based on an element of truth. What has stood the test of time is the finding that some functions the brain performs seem to rely slightly more on one hemisphere than the other. Language for instance is often held up as the paragon of lateralization of function -- when people suffer damage to their left cerebral hemisphere, as Dr. Taylor did during her stroke, they often show acute language impairments, while people whose right hemispheres are damaged are less likely to show language impairments of the same magnitude.
But the two hemispheres of the brain are extremely similar from their global organization down to the minute details like the distribution of specific neurotransmitters used in particular corresponding regions across the two hemispheres. And even language -- this strongest case for lateralization -- is processed in both halves of the brain. Understanding something as simple as a word, let alone something as complex as the sentence you're reading right now, takes the intense coordination of massive swaths of both cerebral hemispheres. For an example, take a look at the dynamic images of the brain in action shown in these videos of what happens when people see words in their native language. Certainly, there's a little more activity in the left hemisphere than the right, but both hemispheres are active throughout the process.
What's more, the language-on-the-left story turns out to be rife with exceptions. Some people, about one fifth of left-handers, use their right hemisphere more for language than their left. And for many people, language is not lateralized at all -- another fifth of left-handers, for instance, show wholly bilateral language processing. And add to the mix the fact that the brain is remarkably flexible. People who suffer brain damage, especially early on (even children who have an entire hemisphere ablated ) still recover brain functions -- including language -- which demonstrates that while the two halves of the brain may often tend to be slightly better or slightly more often used for specific purposes, this is more a matter of convenience, efficiency, and practice than a hard-and-fast rule.
None of this is to bring into question Dr. Taylor's account of her terrifying and enlightening experience. Nor does is detract from the laudable effort she's making to encourage people to recognize that our mental lives can be not merely rational and analytical, but also present and grounded. But trying to squeeze these different aspects of mental experience into a rough and ultimately inaccurate simplification of how the two halves of the brain work doesn't help us understand any better how our minds coordinate these different facets of our experience. The brain, with its billions of neurons and trillions of connections, performing operations every millisecond, is the paragon of a complex system. It would be surprising if high-level aspects of our minds, like "analytical" versus "intuitive" were so easily boiled down to gross anatomy. Instead, the answer to how we're able to coordinate and shift between different modes of thought will have to wait on serious studies of the dynamics and function of the myriad overlapping and interconnected circuits housed in the brain. Anything less detracts from the real, human message that we should be hearing from Dr. Taylor: that with practice and patience, we can consciously toggle between our different modes of cognition. That's a message anyone can understand, whether they're using both halves of their brain or only one.
 Woods, M. (1987). "Plato's Division of the Soul", Proceedings of the British Academy, 73: 23-47.
 Freud, S. (1949) "The Ego and the Id". The Hogarth Press Ltd. London, 1949.
 Gazzaniga, M. S. (2005). Forty-five years of split-brain research and still going strong. Nature Reviews Neuroscience, 6(8), 653-U651.
 Bayard, Sophie; Lassonde, Maryse (2001). "Cognitive Sensory and Motor Adjustment to Hemispherectomy". In Jambaqué, Isabelle; Lassonde, Maryse; Dulac, Olivier. Neuropsychology of Childhood Epilepsy. Advances in Behavioral Biology. 50. pp. 229-44.
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