Dyslexia is a complicated process that involves many different procedures and cognitive faculties. Before people are discussing the specific syndromes of acquired dyslexia, the processes mediating word recognition and pronunciation are briefly reviewed. The visual system efficiently processes a complicated stimulus
that is, at least for like alphabet based on languages, is composed of smaller meaningful unit letters. In part the number of words, there is often a considerable visual similarity between words. In addition, the position of letters within the letter string is also critical to word identification. The light of theses factors, it is perhaps not surprising that reading places a substantial burden on the visual system and those disorders of visual processing or visual attention may substantially disrupt reading.
The fact that normal readers are so adept at word recognition has led some investigators to suggest that words are not processed as a series of distinct letters but rather as a single entity in a “process akin” to the recognition of objects. At least for normal readers under standard conditions, it seems that this does not appear to be the case. Rather, normal reading appears to require the identification of letters as alphabetic symbols. Support for this claim comes from demonstrating that presenting words in an unfamiliar form for example, by alternating the case of the letters or introducing spaces between words would not mat substantially influences reading speed or accuracy. These data argue for a stage of letter identification in which the graphic form is transformed into a string of alphabetic characters that are sometimes called “abstract letter identities.”
The word identification requires not only that the constituent letters be identified but also that the letter sequence be processed. There are mechanisms by which the position of letters within the stimulus is determined and maintained is not clear, but a number accounts have been proposed. One possibility is that each letter is linked to a position in a world “frame” or envelope. It should be noted that under normal circumstances letters are not processed in a strictly serial fashion, but may be analyzed by the visual system in parallel. Disorders of reading resulting from impairment in the processing of the visual stimulus or the failure of this visual information to access stored knowledge appropriate to a letter string are designated “peripheral dyslexics” and are discussed in the future.