Dyslexia refers specifically to decoding difficulty, but includes difficulty caused by two distinct disorders in brain processing: phonological and orthographic processing disorders (see definition below). It is estimated that 85% of children who experience reading difficulty have a phonological processing disorder.
Phonological processing disorder and orthographic processing disorder refer to the particular brain processes at work in people who experience difficulty when they read. An individual who has a phonological processing disorder will have difficulty perceiving and manipulating the phonemes that would enable them to “hear” the sounds of the words they read.*
Orthographic processing involves recognizing and remembering the spatial orientation and sequence of language symbols. When individuals with orthographic processing disorders attempt to read, their brains have trouble perceiving and/or processing the direction and sequence of written language.
* Shaywitz, S. (2003) Overcoming Dyslexia: A new and complete science-based program for reading problems at any level. New York: Knopf.