Today Corballis readily admits he was wrong. Lateralized brains are not unique to humans. Parrots prefer picking up things with their left foot. Toads tend to attack other toads from the right but go after prey from the left. Zebra fish are likely to look at new things with their right eye and familiar things with their left. Even invertebrates are biased. Pinar Letzkus, a vision researcher at Australian National University, rewarded bees with sugar whenever they extended their tongue at the sight of a yellow rectangle on a computer screen. He then fashioned tiny eye patches and put them on a new set of subjects. Bees with their left eye covered learned almost as quickly as did bees without a patch. But bees with their right eye covered did far worse.
The broken symmetry of the nervous system may thus be as old as the symmetry itself. If so, it is an ancient puzzle. Being biased to one side would seem like a serious handicap: A toad that hopped to the left whenever it was startled by a predator, for instance, would be easy prey for an attacker that could anticipate which way it would go; the same holds for any other kind of ingrained behavioral imbalance. A number of scientists have run experiments to find the benefits that might offset such costs.
One hypothesis is that a lateralized brain is more powerful than one that works like a mirror image. Instead of two matching parts of the brain performing an identical task, one can take charge, leaving the other free to do something else. Lesley Rogers, a biologist at the University of New England in Australia, tested this hypothesis on chickens. The birds use their left hemisphere to peck for seeds and their right hemisphere to detect predators. Some chickens have more lateralized brains than others, and there is a simple way to make any chicken more lateralized: Just shine a light on it while it is still in the egg. Chick embryos usually develop with the left eye tucked inward and the right eye facing out. The stimulation of light on the right eye alters the developing left-brain hemisphere but not the right.
Rogers and her colleagues reared 27 chicks that had been exposed to light and 24 that had not. Each day the researchers put the chicks in a special box with grain and pebbles scattered on the floor; at the same time, they distracted the birds by moving a hawk-shaped cutout overhead. They then observed how well the chicks were able to distinguish between pebbles and grain. Light-exposed chicks learned to do a much better job. Rogers concludes that lateralized brains allowed the chicks to multitask more effectively, with each eye handling a separate job.
The pop psychology notion of a left brain and a right brain doesn’t capture their intimate relationship. The left specializes in the sounds that form words; the right is more sensitive to the emotions of language.
David Stark of Harvard Medical School recently found additional clues about lateralization in his studies of 112 different regions in the brains of volunteers. He and his collaborators discovered that the front portions of the brain are generally less tightly synchronized across the hemispheres than are the ones in the back. It may be no coincidence that the highly synchronized back regions handle basic functions like seeing. To observe the world, it helps to have unified vision. At the front of the hemispheres, in contrast, we weave together streams of thought to produce complex, long-term plans for the future. It makes sense that these areas of the brain would be more free to drift apart from their mirror-image partners.
No matter how lateralized the brain can get, though, the two sides still work together. The pop psychology notion of a left brain and a right brain doesn’t capture their intimate working relationship. The left hemisphere specializes in picking out the sounds that form words and working out the syntax of the words, for example, but it does not have a monopoly on language processing. The right hemisphere is actually more sensitive to the emotional features of language, tuning in to the slow rhythms of speech that carry intonation and stress.
Neuroscientists know that the hemispheres work together and that they do so by communicating through the corpus callosum. But exactly how the hemispheres cooperate is not so clear. Perhaps paired regions take turns being dominant. That is known to happen in some animals. For instance, dolphins use this strategy to sleep and swim at the same time: One hemisphere remains active for hours, then fades while the other takes over. Bird brains switch as well. In order to sing, a songbird makes the two sides of its lungs open and close. The two hemispheres of the bird’s brain take turns controlling the song, each dominating for a hundredth of a second.
The intimate cooperation between the two hemispheres makes it all the more remarkable that a person can survive with just one—a sign that the brain is far more malleable than we once thought. After a hemisphere is forced to manage on its own, it can rewire itself to handle all the tasks of a full brain. In fact, two hemispheres can cause more trouble than one if they cannot talk clearly to each other. Neuroscientists have linked some mental disorders, including dyslexia and Alzheimer’s, with a breakdown in left-right communication.
The two sides of the brain may be a legacy that we inherited from our wormlike ancestors. But their delicate balance of symmetry and specialization is now woven into the very essence of human nature.
