Dyslexia is often thought as a visual disorder since children with dyslexia are often described as having reading and spelling problems. However, recent physiological findings indicate that the auditory processing system may be the primary cause of or major contributor to dyslexia.
Dr. Glenn Rosen at Beth Israel Hospital in Boston and doctors. Albert Galaburda and Matthew Menard at Harvard Medical School conducted post-mortem examinations of the brains of five dyslexic subjects and seven non-dyslexic, control subjects. Those with dyslexia had a decrease in neurons on the left-side of the medial geniculate nucleus, an auditory relay station which receives sound signals from the ear, encodes them, and then sends them to the auditory cortex. Interestingly, the left side of the medial geniculate nucleus processes fast-changing sounds, such as "stop consonant" sounds. Such sounds include: ba, da, ka, and ta. The duration of stop consonant sounds is about 1/25 of a second whereas the duration of vowel sounds is approximately 1/10 of a second. It is argued that children with dyslexia do not appear to perceive stop consonant sounds appropriately and have difficulty distinguishing among various sounds. As a result, they will not be able to form a mental lexicon which would allow them to associate letters and words to specific sounds.
Dr. Guy Berard, a French physician and the developer of auditory integration training, has treated hundreds of children with dyslexia; and according to Dr. Berard, most of them improved dramatically from his treatment. Similar to the findings reported above, Dr. Berard has always argued that the underlying cause of dyslexia and other disorders, such as autism, is a dysfunctional auditory processing system.
To me, it just reaffirms how important a multidisciplinary evaluation can be.