There are at least three subtypes of dyslexia that have been recognized by researchers. Orthographic, or Surface Dylexia, and Phonological Dyslexia are described below. Mixed dyslexia has also been diagnosed when individuals present with symtoms of both Orthographic and Phonological.
Orthographic Dyslexia
Orthographic dyslexia, according to Roberts and Mather (1997: 237), refers to “a problem with the acquisition of decoding or encoding skills that is caused by difficulty with rapid and accurate formation of word images in memory.” Orthographic dyslexics have difficulty in storing mental representations of words, especially phonetically irregular words. The problems underlying this type of dyslexia are related directly to memory and coding skills that allow representation of printed letters and words and not to poor phonological processing.
This type of dyslexia is also called Surface Dyslexia and is defined as an inability “to recognize written words on a purely visual basis. They have trouble reading aloud words that are irregularly spelled. Rather than recognizing words visually, these patients apparently sound out the words on the basis of correspondences between letters and sounds” (Caplan 1987:225).
Given these definitions of dyslexia, the affected person would have trouble distinguishing between the hard and soft pronunciations of ‘c’:
Hard: C = [k] cat
Soft: C = [s] ice
Most often, the dyslexic will use the most common pronunciation of the grapheme, which inevitably leads to errors in reading and writing. For example, an orthographic dyslexic might read “cat” as [kFt] correctly, but misread “ice” as [Ik]. In this example, the dyslexic has identified /k/ as the phoneme corresponding to the grapheme ‘c’ and will produce [k] all or most of the time.
Phonological Dyslexia
Phonological dyslexia means that the individual’s dyslexia is rooted in their difficulty manipulating and integrating the sounds of a language effectively. Strong language-sound processing skills are needed to learn to read and write successfully and a deficit in this phonological skill is the most common characteristic of individuals with dyslexia. The phonological dyslexic is unable to segment, analyze, and synthesize speech sounds and is identifiable by their phonetically inaccurate misspellings.
Snowling (1981:225) defines phonological dyslexia similarly as an inability to produce novel words due to poor grapheme-phoneme knowledge; the patient exhibits poor performance on phonological awareness tasks and deficits in verbal working memory. Phonological dyslexics are frequently unable to segment words into individual sounds most likely because of an impaired representation and use of phonology.
Based on these definitions of phonological dyslexia, the affected person would have difficulty distinguishing between similar-sounding phonemes, for example b/p or d/t. Given the word “bat”, for example, the dyslexic might read or write “pat.” Likewise, given the word “till” the dyslexic might read or write “dill.”
It is important to note that in the example provided for the misreading of “ice” the orthographic dyslexic can hear, or perceive, the difference between the phoneme [k] and the phoneme [s]. Their difficulty lies in identifying the correct grapheme, in this case ‘c,’ so that they can read or spell the given words. In the examples provided for the phonological dyslexic, this person would be unable to perceive the difference between [t] and [d] or between [p] and [b], therefore their chances of reading or spelling correctly are compromised as well as their ability to discern the proper semantics of spoken language.
References
Caplan, David. 1987. Disturbances of the sound system. In D. Caplan. Neurolinguistics and Linguistic Aphasiology: An Introduction, 201-232. New York: Cambridge University Press.
Roberts, Rhia and Nancy Mather. 1997. Orthographic Dyslexia: The neglected subtype. Learning Disabilities Research & Practice, 12(4):236-250.
Snowling, Margaret J. 1981. Phonemic deficits in developmental dyslexia. Psychological Research 43:219-234.
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