The activity of one gene in particular, Lmo4, was markedly reduced in the tissue from the left-brain region. In the corresponding tissue from the right side, which isn't destined to have language functions, Lmo4 was expressed at normal levels. How this asymmetric gene activity is related to language functions remains unknown, but Walsh views these findings as an important clue to a still-obscure mechanism for generating asymmetry in the human brain. At the same time, it will send researchers on a search for even earlier molecular signals that, as in the roundworm and the zebrafish, regulate the necessary imbalance in gene expression to begin with.

Such painstaking research efforts in organisms of varying complexity could explain relationships between brain asymmetries and behavior. Walsh predicts that much will be learned about how the diverse specialization within our complex, asymmetric brains distinguishes humans from all other animals. Further, the true understanding of asymmetry may help illuminate brain function not just in relation to species but also on the individual level. There is, after all, a unique interplay of left- and right-brain skills that governs each person's perceptual tendencies and his or her way of being in the world.

		A chicken uses one eye to hunt for food grains while the other is tuned to detect predators. Such "asymmetrical behaviors," which impart specialized skills through a division of labor between the left and right hemispheres of the brain, have been found in species throughout the animal kingdom. Mice tend to turn more often in one direction than another when foraging, and bees show turning preferences in their ritual dances. Flying birds rely on one eye over the other for seeing at a distance. It has even been determined that one hemisphere of some songbirds' brains produces their melodies. Human beings have the most asymmetrical brains of all—and the most striking behavioral asymmetries. In the vast majority of people, language and speech are generated in the left hemisphere. Handedness—a preference for using one hand, which is usually more skilled—is the result of one hemisphere being dominant. Nonhuman animals show paw preferences, but in many animals they appear to be random; by contrast, 90 percent of people favor their right hand. In fact, notes HHMI investigator Christopher A. Walsh, ultrasound examinations show that at 15 weeks most fetuses tend to suck their right thumb, suggesting that handedness is present at birth. Genes must account for the strong right-hand preference in humans, but how this bias evolved is not known. With a few exceptions, such as hand and eye preference, people aren't aware of their asymmetrical behaviors because the two halves of the brain keep each other informed through a thick bundle of nerves connecting them. However, by studying "split-brain" patients, whose hemispheres were disconnected as a treatment for epilepsy, researchers demonstrated in the 1960s just how differently the brain's two sides function. For example, such a patient, if blindfolded and holding a toothbrush in his left hand, knew what it was for and how to use it but couldn't think of the word for it. When he took the brush in his right hand, he immediately named it. —R.S.

	PAGE 1 2 3 4 5	Go Back