On the Trail of Stuttering Genes
Stuttering is very much in the public eye, thanks to the Academy Award winning movie, "The King's Speech." Though his therapist concluded that King George's stuttering was environmentally produced, twin studies indicate that the malady is 50% to 70% heritable. By studying highly intermarried families in Pakistan, Dennis Drayna was able to identify 10 mutations in three genes that are linked to stuttering. Though the mutations account for only 5%-10% of cases, this is the first success at pinning down responsible genes. New England Journal of Medicine, Vol 362, 677-685. While mutations can occur spontaneously in an individual, Drayna was able to trace one of the 10 mutations back to a common ancestor who lived 14,000 years ago. Journal of Human Genetics, Vol 56, 80-82. He is now experimenting with inserting a human mutation into mice; if it produces stuttering in their high-frequency vocalizations, mice could be a very useful research model. The three genes are all involved in the function of lysosomes—cellular sacs where debris is recycled—and Drayna speculates that neurons involved in speech production are particularly vulnerable to the metabolic effects of lysosome disruption. Researcher Luc De Nil, in a presentation at the meeting of the 2011 American Association for the Advancement of Science, reported that the brains of stutterers have anomalies in both white and gray matter, as well as overactivation of speech areas and underactivation of the auditory area. AAAS EurekaAlert, February 20, 2011.
Language Lateralization is Related to Gray Matter
In Chapter 9 you learn that there are several anatomical correlates of language lateralization, such as a larger Broca's area and a longer lateral fissure and planum temporale in the left hemisphere. Now more precise research has shown that the degree of language lateralization, as determined by fMRI monitoring during language tasks, is related to the degree of gray matter lateralization. The relationship was strongest in areas known to be involved in semantic and phonological processing of auditory and visual words and, in most comparisons, could be demonstrated at the level of individual voxels (the smallest area resolvable by the fMRI procedure). Journal of Neuroscience, Vol 29, 13516-13523.
Learning to Read Changes the Brain
Spoken language apparently is supported by brain characteristics that are present at birth and appear to have evolutionary roots. Reading, on the other hand, is a skill that has developed much more recently in human history and requires extensive instruction. It has long been presumed that learning to read must be accompanied by significant modifications in neural circuitry, but this has been difficult to demonstrate because of developmental changes that are going on simultaneously. Now a unique situation has provided the opportunity as Columbian guerillas have re-integrated into society and many have learned to read as adults. Comparing their brains with those of non-literate individuals and individuals who learned language early shows that the brain modifications include increased gray matter in several language-related areas. In addition, white matter increased in the posterior portion of the corpus callosum (splenium), and this was accompanied by increased connections between the left and right angular gyri and between the left and right occipital areas involved in higher-level visual processing. A surprising finding was that, during reading aloud, information flowed from the angular gyrus to the supramarginal gyrus (which is involved in semantic processing) rather than to the superior temporal gyrus, as the classic theory indicates. Nature, Vol 461, 983-986.