General Definition of Dyslexia
There are three disorders referred to as Dyslexia that result from damage to specific areas of the left hemisphere of the brain. These areas allow us to process the information needed to learn to code the relationships between written symbols (graphemes) and fundamental sound units (phonemes) that form the basis for reading and spelling.
Most commonly, Dyslexia is inherited as an autosomal dominant trait transmitted on Chromosome-6. Some forms of Dyslexia are influenced by other factors such as chronic ear infections during infancy.
Three Types of Dyslexia
1. Dyseidetic Dyslexia: Poor sight recognition of words
Dysfunction of the angular gyrus in the left hemisphere of the brain causes Dyseidetic Dyslexia. This type of Dyslexia is also sometimes called “Surface Dyslexia” or “Visual Dyslexia”. The Dyseidetic Dyslexic generally has a good grasp of phonetic concepts, but great difficulty with whole word recognition and spelling.
Children with this form of Dyslexia have trouble analyzing and remembering written symbols, and confuse the orientation. For example, they will write numbers and letters backwards long after other children have mastered these skills. They often confuse letter sequences in reading, and in spelling often get all the letters but in the wrong sequence. This is demonstrated by transpositions (spelling “dose” for “does”), and even complete reversals in spelling (such as the word “need” being written “deen”), but the letters - that correspond to the right sounds - are all there.
Their visual memory for words is poor, and after learning a new word they may fail to recognize that same new word later in the sentence. They have trouble learning to read and spell phonetically irregular words. For example, they may read “ laugh” as “log” and spell it as “laff”, both of which are phonetically consistent. Their spelling will have many mistakes, but will be phonetically consistent and one can usually tell what the word was they were trying to spell. When they are attempting to read an unknown word they will usually attempt to sound it out and do so very slowly.
Unfortunately, phonics-based remedial programs will not help the purely “Dyseidetic” Dyslexic at all. Rather, they will only increase confusion, because the student is being drilled on something he already knows, without being given a means to develop whole-word recognition skills or learn to recognize words that do not sound exactly the way they are spelled.
2. Dysphonetic Dyslexia: Inability to “sound out” words
Dysphonetic Dyslexia is caused by a dysfunction of the Wernicke's Area in the left hemisphere of the brain. Children with this form of Dyslexia have difficulty sequentially analyzing and remembering what and where the sounds are in words. The resulting phonemic processing problems make it difficult to sound out new words, learn phonics, and make them dependent on their sight vocabulary. When they come to an unknown word they will often substitute a word using context clues. For example, “pony” for “horse”, even though the substituted word doesn't look or sound anything like the original word.
When spelling unknown words it is often difficult to determine what the original word is. For example, they may write “fmlue” for “familiar” or “lap” for “lamp”. They cannot learn phonics because they cannot process where the sounds are. Their short term sequential auditory memory can be poor and result in repeating “8167” as “8671”, or remember to go to their room but forgetting to get the item requested.
The Dysphonetic has difficulty connecting sounds to symbols, and might have a hard time sounding out words, and spelling mistakes would show a very poor grasp of phonics. This is also sometimes called “auditory” Dyslexia, because it relates to the way the person processes the sounds of language.
Most remedial programs tend to emphasize phonics. This can help the “Dysphonetic” Dyslexic somewhat, but does not address all underlying problems associated with Dyslexia. Often, instruction in phonics will help the person learn to read, but the student will still find reading very difficult and will not read for pleasure or progress beyond reading primary-grade level material.
3. Dysphoneidetic (Mixed) Dyslexia: Characteristics of both types
Dysfunction of both the angular gyrus and Wernicke's area causes the mixed form of Dyslexia. The inability to process both the sound (phoneme) and the symbol (grapheme) is particularly devastating for learning to code language. They exhibit the problems of both the other types of Dyslexia. Unlike people with only one type of Dyslexia, people with mixed Dyslexia are not strong in either processing sounds or symbols. Therefore they have no effective way to encode (spell) or decode (read) words. A marked mixed Dyslexic will typically not learn to code language past a first grade level and will remain a virtual nonreader for the rest of their life without appropriate intervention. Often they are misidentified as learning disabled, placed into special education classes for the duration of their academic career without ever learning to properly read and spell.
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