The National Institute of Neurological Disorders and Stroke gives the following definition for dyslexia:
"Dyslexia
is a brain-based type of learning disability that specifically impairs a
person's ability to read. These individuals typically read at levels
significantly lower than expected despite having normal intelligence. Although
the disorder varies from person to person, common characteristics among people
with dyslexia are difficulty with spelling, phonological processing (the
manipulation of sounds), and/or rapid visual-verbal responding."
If you share the interest for this disorder, watch this video to find out more about it...
Also, take a look at this picture, comparing a nonimpaired brain with a dyslexic one...
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