Answer: Hypermyelination is observed in the left auditory cortex in patients with developmental dyslexia.
Dyslexia is a remarkably common developmental disorder. It is estimated that 1 in 10 adults meet the diagnostic criteria, but most people with dyslexia are not aware of it. Dyslexia is not correlated with a low IQ. Dyslexia is characterized by difficulty in understanding the written word or the spoken word.
Myelin is a fatty sheath that surrounds axons. Myelin helps the cell conduct signals faster by increasing membrane resistance along the axon. In the peripheral nervous system, many nerves are myelinated, which allows motor command signals to reach the muscles quickly. Also, the nerve fibers from the somatosensory system are also myelinated to allow a rapid processing of sensations detected at the skin.
Researchers discovered a link between myelination and developmental dyslexia in a 2018 study called “Hypermyelination of the left auditory cortex in developmental dyslexia” published in the journal Neurology.
In this study, the experimenters were searching for a noninvasive marker to identify developmental dyslexia using a brain imaging technique called functional magnetic resonance imaging (fMRI). They focused on the perisylvian cortex.
There were 12 subjects in this study - 6 with developmental dyslexia and 6 neurotypical control subjects. Both groups had comparable cortical thickness. But the myelinated cortex ratio was increased in the people with dyslexia.
They also observed enhanced T1 relaxation in layer IV of the left auditory cortex.
The perisylvian cortex is implicated in short term verbal memory and language processing (Areas of left perisylvian cortex mediate auditory-verbal short-term memory). Because dyslexia is a deficit of language, differences in perisylvian cortex are not surprising.
Even though these imaging techniques shed some knowledge in brain regions that are affected in dyslexia, most likely, multiple parts of the brain are needed for reading comprehension. Reading and comprehension of the written word is a highly developed system that formed long after the basics of nervous system function.