The
structural differences appear to be in the cerebral neocortex of the brain. According to
Albert Galaburda and
Thomas Kemper, pioneers in the study of brain structure and its relation to dyslexia, the problem stems from "
ectopias," neurons that migrate past where they are supposed to and into the outermost layer of the
cerebral neocortex. This area usually has no
nerve cell bodies, and the presence of these seems to
disrupt the neural organization of the brain. The damage
reverberates and the changes in organization of
neural networks spread to other areas. In the end, the brain appears to reorganize its thinking
patterns in such a way that
language is much more difficult to process.
The damage appears to be different in females and males, with males having more ectopias (these are also called nodules in some books) and women less so, although they showed other evidence of "neuronal loss" in the cerebral cortex.
These nodules occur during embryonic development, and their cause is unknown, although many believe it to be genetic, perhaps having to do with chromosome 6.
Much of this was adapted from an International Dyslexia Association article. Most books discuss the fact that structure plays into it, but few have any specifics. http://www.interdys.org/servlet/compose?section_id=5&page_id=47