The Second Division: Left vs. Right

Jonathan Haidt begins his book The Happiness Hypothesis: Finding Modern Truth in Ancient Wisdom by discussing the ways that the human mind conflicts with itself—that our conscious thoughts don’t always align with what we feel. He uses the metaphor of a man riding an elephant to illustrate this point.

The second is the often oversimplified division between the left and right hemisphere of the brain:

A second division was discovered by accident in the 1960s when a surgeon began cutting people’s brains in half. The surgeon, Joe Bogan, had a good reason for doing this: He was trying to help people whose lives were destroyed by frequent and massive epileptic seizures. The human brain has two separate hemispheres joined by a large bundle of nerves, the corpus callosum. Seizures always begin at one spot in the brain and spread to the surrounding brain tissue. If a seizure crosses over the corpus callosum, it can spread to the entire brain, causing the person to lose consciousness, fall down, and writhe uncontrollably. Just as a military leader might blow up a bridge to prevent an enemy from crossing it, Bogen wanted to sever the corpus callosum to prevent the seizures from spreading.

At first glance this was an insane tactic. The corpus callosum is the largest single bundle of nerves in the entire body, so it must be doing something important. Indeed it is: It allows the two halves of the brain to communicate and coordinate their activity. Yet research on animals found that, within a few weeks of surgery, the animals were pretty much back to normal. So Bogen took the chance with human patients, and it worked. The intensity of the seizures was greatly reduced.

But was there really no loss of ability? To find out, the surgical team brought in a young psychologist, Michael Gazzaniga, whose job was to look for the after-effects of this “split-brain” surgery. Gazzaniga took advantage of the fact that the brain divides its processing of the world into its two hemispheres—left and right. The left hemisphere takes in information from the right half of the world (that is, it receives nerve transmissions from the right arm and leg, the right ear, and the left half of each retina, which receives light from the right half of the visual field) and sends out commands to move the limbs on the right side of the body. The right hemisphere is in this respect the left’s mirror image, taking in information from the left half of the world and controlling movement on the left side of the body. Nobody knows why the signals cross over in this way in all vertebrates; they just do. But in other respects, the two hemispheres are specialized for different tasks. The left hemisphere is specialized for language processing and analytical tasks. In visual tasks, it is better at noticing details. The right hemisphere is better at processing patterns in space, including that all-important pattern, the face. (This is the origin of popular and simplified ideas about artists being “right-brained” and scientists being “left-brained”).

Gazzaniga used the brain’s division of labor to present information to each half of the brain separately. He asked patients to stare at a spot on a screen, and then flashed a word or a picture of an object to the right of the spot, or just to the left, so quickly that there was not enough time for the patient to move her gaze. If a picture of a hat was flashed just to the right of the spot, the image would register on the left half of each retina (after the image had passed through the cornea and been inverted), which then sent its neural information back to the visual processing areas in the left hemisphere. Gazzaniga would then ask, “What did you see?” Because the left hemisphere has full language capabilities, the patient would quickly and easily say, “A hat.” If the image of the hat was flashed to the left of the spot, however, the image was sent back only to the right hemisphere, which does not control speech. When Gazzaniga asked, “What did you see?”, the patient, responding from the left hemisphere, said, “Nothing.” But when Gazzaniga asked the patient to use her left hand to point to the correct image on a card showing several images, she would point to the hat. Although the right hemisphere had indeed seen the hat, it did not report verbally on what it bad seen because it did not have access to the language centers in the left hemisphere. It was as if a separate intelligence was trapped in the right hemisphere, its only output device the left hand.

When Gazzaniga flashed different pictures to the two hemispheres, things drew weirder. On one occasion he flashed a picture of a chicken claw on the right ,and a picture of a house and a car covered in snow on the left. The patient was then shown an array of pictures and asked to point to the one that “goes with” what he had seen. The patient’s right hand pointed to a picture of a chicken (which went with the chicken claw the left hemisphere had seen), but the left hand pointed to a picture of a shovel (which went with the snow scene presented to the right hemisphere). When the patient was asked to explain his two responses, he did not say, “I have no idea why my left hand is pointing to a shovel; it must be something you showed my right brain.” Instead, the left hemisphere instantly made up a plausible story. The patient said, without any hesitation, “Oh, that’s easy. The chicken claw goes with the chicken, and you need a shovel out lean out the chicken shed.”

This finding, that people will readily fabricate reasons to explain their own behavior, is called “confabulation.” Confabulation is so frequent in work with split-brain patients and other people suffering brain damage that Gazzaniga refers to the language centers on the left side of the brain as the interpreter module, whose job is to give a running commentary on whatever the self is doing, even though the interpreter module has no access to the real causes or motives of the self’s behavior. For example, if the work “walk” is flashed to the right hemisphere, the patient might stand up and walk away. When asked why he is getting up, he might say, “I’m going to get a Coke.” The interpreter module is good at making up explanations, but not at knowing that it has done so.

Science has made even stranger discoveries. In some split-brain patients, or in others who have suffered damage to the corpus callosum, the right hemisphere seems to be actively fighting with the left hemisphere in a condition know as alien hand syndrome. In these cases, one handed, usually the left, acts of its own accord and seems to have its own agenda. The alien hand may pick up a ringing phone, but then refuse to pass the phone to the other hand or bring it up to an ear. The hand rejects choices the person has just made, for example, by putting back on the rack a shirt that the other hand has just picked out. It grabs the wrist of the other hand and tries to stop it from executing the person’s conscious plans. Sometimes, the alien hand actually reaches for the person’s own neck and tries to strangle him.

These dramatic splits of the mind are caused by rare splits of the brain. Normal people are not split-brained. Yet the split-brain studies were important in psychology because they showed in such an eerie way that the mind is a confederation of modules capable of working independently and even, sometimes, at cross-purposes. Split-brain studies are important for this book because they show in such a dramatic way that one of these modules is good at inventing convincing explanations for your behavior, even when it has no knowledge of the causes of your behavior. Gazzaniga’s “interpreter module” is, essentially, the rider.

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