Handedness in the brain

A nice article about the etiology and effects of left-handedness in the NY Times last week (“Left-Handedness Loses Its Stigma but Retains Its Mystery”).

In general, said Dr. Geschwind, left-handers have less asymmetric brains, with more even distribution over the two hemispheres. Perhaps a more accurate conceptual way to think about them is as non-right-handers, he said. Many of them are much more likely to be ambidextrous and have fine motor abilities with their right hands.
Because left-handedness has been seen as a key to the complex anatomy of the brain, researchers continue to look for and debate links to many other conditions, including immune disorders, learning disabilities and dyslexia, reduced life expectancy and schizophrenia.

It’s the sort of thing that we look at because it’s obvious – comparable between humans and chimpanzees, and even interpretable from some kinds of archaeological materials. Studying the associations between handedness and various other behavioral characters is sort of like running genetic association studies; there is generally a small signal and lots of noise, which doesn’t allow easy replication.

I’m pointing to this because I have recently been updating my knowledge about language localization in the brain, and handedness has given some important clues about the nature of localization of language function. Although language functions are typically localized in the left hemisphere, there are a good fraction of people for whom this is not the case. Knecht and colleagues Knecht:2000 assessed the frequency of right hemisphere localization of language in left-handed subjects, finding that 27% of strong lefties actually relied on their right hemisphere for language more than their left. The concept of extreme lateralization is itself too simplistic; for example, most people have some activity in Broca’s homolog in their non-dominant hemisphere, and some of them are roughly equal in the participation of both hemispheres.

Just following along my bootstrapping theme. Language doesn’t develop like an organ.