Saturday, July 15, 2006

Genes, cognition, and dyslexia.

Learning to read and write places unusual demands on the brain: explicit awareness of the structural elements of language and their relation to arbitrary visual symbols, rapid temporal processing, fine motor control, and visual acuity. Because sophisticated reading and writing systems appeared only a few thousand years ago, it is very unlikely that reading skills were shaped directly by Darwinian selection. Spoken or sign language, on the other hand, is acquired virtually effortlessly during the first few years of life, and is supported by brain specializations that evolved over hundreds of thousands of years.

It is interesting that studies on several European, Canadian, and American families have found genetic changes that correlate with reading disorders, or dyslexia, independent of general cognitive performance. Fisher and Francks, in Trends in Cognitive Sciences, 10:250 (2006), provide an overview of four prominent examples (genes DYX1C1, KIAA0319, DCDC2, and ROBO1). These are not "genes for reading". None are specific to reading-related neuronal circuits, or even to the human brain. They do have intriguing roles in neuronal migration or connectivity. Individual genes do not specify behavioral outputs, cognitive skill, or even particular neural circuits. They "influence brain development and function interactively by affecting processes such as proliferation and migration of neurons, programmed cell-death, axonal pathfinding, connectivity, levels of neurotransmitters/receptors, and so on."

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