Although many assumed that the asymmetry-producing genes, when found, would be more highly expressed on the left side of the brain than the right, Sun Tao, Christopher A. Walsh, and their colleagues found otherwise. One gene, LMO4, that showed the most consistent difference was much more highly expressed on the right.

The researchers found that mouse brains also express the LMO4 gene asymmetrically. But the asymmetry was unpredictable. Some mice expressed LMO4 at higher levels on the right, others on the left.

Inspired by the fact that a small percentage of humans also have a reversed asymmetry, Sun, an HMS research fellow in neurology at Beth Israel Deaconess, and Walsh hypothesized that slight differences between the hemispheres could be used to establish dominance in language.

Sun started by collecting samples of the right and left perisylvian cortices of human brains. He then compared their gene expression patterns. One gene, LMO4, exhibited consistent left- right variation in expression at two important stages, 12 and 14 weeks. It was only when the researchers approached the perisylvian cortex that the right-left difference started appearing.

Although LMO4’s role is unclear, researchers believe it may be involved in selecting one type of connection over another. If so, it could be receiving instructions from earlier genes. “We want to look at these earlier stages to find out what the genes are that guide the patterning,” said Sun.