Three diffusion tensor images of the brain's white matter.

Left Brain, Right Brain?

Where are the boundaries in forming our parts? We are each a collection of trillions of cells in a society of millions and billions of other people, yet—barring abnormalities–we feel like whole units with single identities. It’s not hard to poke holes in this identity illusion, but many of us keep searching for the essential person.

However, if human nature is thought of as an inner essence, it doesn’t exist. We’re free to think of human nature as a set of statistical averages, but perish the thought that there is a human nature that is superior to animal nature. We are part of the animal kingdom.

As we readily admit for other animals, we ourselves must be assemblages of parts, and we can manage this uncomfortable thought by identifying with the controller, the mind. Nowadays it has become customary to equate the mind with the brain and perhaps the entire body and beyond.

This perception of unity is somehow satisfying. The most potent threat to this impression is probably our bilateral symmetry, which evolved long before our human propensities, exhibiting asymmetries within the basic plan. The asymmetries, such as lodging language mostly on one side of the brain and spatial cognition on the other, prevents duplication of effort. At the same time the asymmetry raises the question of which of our mostly symmetrical halves is us.

From a logical point of view, the answer might come from a procedure that took one cerebral hemisphere out of commission while keep the other functional. This was possible with the Wada test, a procedure by which an injection of amobartital puts one hemisphere temporarily to sleep. This procedure reveals asymmetries such as left-hemisphere dominance for speech, though it has the drawback that patients are not free to behave normally in the clinic and usually possess abnormalities like epilepsy that would affect normality of behavior in some tests. But this is not the only strategy possible.

Seventy-five years ago, the practice of split-brain surgery (commissurotomies) started with efforts to prevent the spread of epilepsy within a patient’s brain. Epileptic seizures are triggered by a region in the cerebral cortex called a seizure focus, where neurons fire in synchrony every so often. A seizure consists of waves of synchronized activity among neurons, usually in the cerebral cortex.  A few neurons in the seizure focus that emit activity in bursts may trigger a seizure by recruiting larger numbers of neurons in the synchronized activity.  In larger seizures, high-amplitude EEG waves may be seen to pass across the surface of the cortex in a progression called the Jacksonian march, which is revealed outwardly in the spread of convulsive jerks across the body.

Since one active seizure focus can train other areas of the brain to develop seizure foci, it was logical to try cutting the nerve fibers (or axons) that connected the cerebral cortex in one hemisphere with the corresponding area in the other hemisphere. (These fiber bundles are called commissures, and the largest is the corpus callosum.) This procedure may have been logical, but back in the 1940s, Van Wagenen and Akelaitis (ak-uh-LYE-tis) couldn’t know that the surgery would do more than limit the spread of epilepsy, as work by Sperry, Gazzaniga and Bogen subsequently revealed, and as this account and that one relate. The corpus callosum looked like a good candidate for surgery. It’s the largest commissure in humans, and the surgeon can see it by spreading apart the cerebral hemispheres. It’s a nearly bloodless procedure, since there is almost no blood supply for the corpus callosum on the midline, where the transection was made.

Many people have taken the surprising results of split-brain surgery to mean that different minds reside inside the two hemispheres, which is not true. The evidence of lateralization of functions suggested that the two minds might be different, and that the self might inhabit only one of them. Thus the notion of hemisphericity or right-brain/left-brain personalities sprang up.

It’s true that the cerebral hemispheres are specialized in function; we see this in speech and emotion, for example. But the deficits in split-brain patients that made it so compelling to believe in a bipartite mind can be explained in other ways.

The two-consciousness view is often called hemisphericity. It is hard to prove. The educational doctrine of teaching according to learning styles that was based on hemisphericity has not held up, either. It does not require transection of the corpus callosum to turn off one hemisphere; an injection of sodium amobarbital (Amytal) into the appropriate carotid artery will suffice. The bulk of the evidence seems to show that splitting the brain does not split consciousness.

The two hemispheres are not as independent and distinct as many popular books would have us believe. That means that most higher mental processes rely on both hemispheres to some extent and that no one is a right-brain or a left-brain thinker.

I suppose it’s a promising sign that some folks are trying to rethink the hemispheres even if it sometimes means just replacing old overgeneralizations with new overgeneralizations. It’s definitely a good sign when the death of right-brain vs. left-brain thinking becomes part of neuropop.

A black and white image of the two cerebral hemispheres in superior aspect.

BIO: Hemisphericity accords each function a place in the brain and makes localization the key to understanding, somewhat as cities became the key to prominence in the middle ages.

But in the way that electronic communications have robbed locations of some of their isolated importance, by making innovation and dominance moveable, our realization that the connections among neurons are plastic and sensitive to use have changed the spotlight from localization to patterns of connection.

So we have come to see brain injury not so much as the destruction of centers as the interruption of brain communication, of information processing.

The conventional way to interpret the changes in behavior of the three men in the “Three Lesions, Three Lives” video–Phineas Gage, Tan/Leborgne, and H.M.–has been to say that their brain lesions removed islands of function responsible for emotional regulation, speech production, and memory formation. 

Another tack is to investigate the connections that were interrupted. A penetrating wound, stroke, or surgical ablation severs the connections that integrate different brain regions. 

If we think of brain functions as localized to pathways–connections–rather than centers, we can avoid the exaggerations of reductionism, or explaining major behavioral events as no more than what goes on in a few million neurons.

PSYCHO: Neuroscience is often interpreted by the public as a series of metaphors that are not tied to operational definitions as scientific constructs must be. The brain is seen as reflecting popular ideas about intelligence, disease, heredity, and collective or individual thinking. Its two cerebral hemispheres have provoked sophisticated or thoughtless versions of the old notion that there are two kinds of people. The split-brain research of Sperry and Gazzaniga was parent to the idea, first, that we have changed from bicameral minds to a unified consciousness, and second, that everyone’s behavior is either dominated by the right hemisphere or the left. Of course there are also popular tests to diagnose yourself.

SOCIAL: The social impact of brain research has had a number of unintended consequences and it’s hard to predict what the next new finding will mean to everyone. People can’t see minds and they can see brains—or imagine them pretty concretely—so if there’s a right brain and a left brain, the idea that we have right-brained minds and left-brained minds can be persuasive, and still is!

Psychologists sometimes make it worse, using “extreme male brains” to characterize autism and “neuroeducation” to advance questionable policies for the classroom. When an idea can’t be tied to an unambiguous measurement, confusion results even among professionals.

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