If you browse through the dyslexia web-sites and forums you will notice people referring to Visual Dyslexia, Auditory Dyslexia, Phonological Dyslexia and Orthographic Dyslexia but do these terms mean anything? Before we can answer that question, we must define what we mean by dyslexia and there are two schools of thought exist about this.
Many academics give dyslexia a literal meaning based on the etymology. ‘Dys’ meaning wrong or problematic (e.g. dysfunctional) and ‘lexia’, pertaining to words and letters. So literally in means a problem with words. So anyone who has a problem reading, for whatever reason, has dyslexia.
On the other side of the issue are parents of dyslexics and dyslexic adults. They use dyslexia to refer to a range of symptoms that includes problems with reading, writing and spelling plus other problems such as poor memory and a lack of physical coordination.
In these studies mentioned in this article the focus is on the academic / literal meaning of dyslexia being just a reading problem. This is because most studies only tested reading ability and did not do a broad sweep of the child’s abilities. It also worth noting that this article focuses on development dyslexia and not acquired dyslexia. Development dyslexia is ‘normal’ dyslexia, where the problem manifests itself through the entire life of the individual. Acquired dyslexia happens when the individual has had normal reading abilities but because of brain damage caused by accidents, stokes or disease they have lost their normal abilities.
The first attempt to sub-divide dyslexia in to different types was the idea of ‘surface’, ‘phonological’ and ‘double-deficit’ dyslexia by Marshall and Newcombe in 1973. The symptoms of Surface Dyslexia relate to making mistakes where the rules of English are not consistent. For example, pretty is read as if it rhymed with “jetty”, and bowl is read as if it rhymed with “howl”. Phonological Dyslexia represents a failure to grasp the phonic nature of English. Individuals with it have great problems reading new or nonsense words because they cannot grasp the link between the individual sounds or phonemes and letters on the page. Where an individual has both types of dyslexia, it is called Double-Deficit Dyslexia.
Since these different types of dyslexia were proposed there has been a lot of research and debate on them. Various questions about the original research and how it was done have been raised. Some studies have shown that Double-Deficit Dyslexia is the most common type and only having one type of dyslexia, surface or phonological, is rare. Other studies have suggest that Surface Dyslexia only represents a child who is simply behind in their reading and will catch up with sufficient teaching whereas Phonological Dyslexia represent a deep seated neurological problem.
Auditory Dyslexia and Visual Dyslexia are both fruits of the magnocellular theory of dyslexia. This theory proposes that dyslexics have neurological have a weakness in the magnocellular cells of their thalamas. This area of the brain is used for rapid processing of visual and auditory information. Numerous studies have shown that dyslexics do have weaknesses in their visual and auditory processing but not all children have them to the same degree. So a child with poor hearing skills but with average visual skills might be diagnosed with Auditory Dyslexia where as poor visual but average hearing will be diagnosed as Visual Dyslexia.
The final type of dyslexia is Orthographic Dyslexia. Orthography is the set of symbols or letters that make up a language. In English this is the 26 letters of the alphabet whilst in Japanese it covers thousands of different symbols. Orthographic Dyslexia therefore relates to problems identifying and manipulating letters in reading, writing and spelling. This subtype of dyslexia has not been researched much and whilst most people in the field recognise that dyslexics have an orthographic problem there is too little evidence to say whether it constitutes a sub-type of dyslexia.
The usefulness of splitting dyslexia in to different types is debatable. First there is no strict definition of what each type is. Different researchers and educationalists use the same phrases to describe subtly different sets of symptoms. Secondly does it matter whether a child has Auditory Dyslexia or Visual Dyslexia if they are treated in the same way? Nine times out of ten, schools will teach all dyslexic children to read the same way, using phonics or multi-sensory methods.
Research: On the Bases of Two Subtypes of Developmental Dyslexia [ PDF ]; Varieties of developmental dyslexia ; Reliability of Phonological and Surface Subtypes in Developmental Dyslexia: A Review of Five Multiple Cases Studies; Theories of developmental dyslexia: insights from a multiple case study of dyslexic adults; To see but not to read;the magnocellular
theory of dyslexia [ PDF ]