Dyslexia classification
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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Classification
Castles and Coltheart prove through their study that developmental dyslexia includes at least two prevalent and distinct varieties or subtypes of dyslexia. Subtypes include surface dyslexia and phonological dyslexia. Understanding these subtypes is useful in diagnosing learning patterns and developing approaches for overcoming impairments that may be visual perception impairments or speech discrimination deficits. These subtypes are based on differing patterns of underlying symptoms, as supported by a finding using large-scale data from comparative studies of reading patterns in dyslexic and normal readers [1]. In the study by Castles and Coltheart, 56 dyslexic boys and 56 non-dyslexic boys as a control group were tested. During the test, the boys read aloud words and non words that were presented to them. The researchers found that surface dyslexics (subjects who have poor lexical skills, or can’t make out irregular words well) had a mean difference of 14.4 words between reading regular words versus irregular words, however, the mean difference in subjects with phonological dyslexia (subjects who can’t use sub lexical skills, or can’t make out non words) was only 7.75 words which was comparable to the control group .The majority of their subjects showed signs of phonological dyslexia. Twenty-nine subjects showed that their non word reading skills were poorer than their irregular word reading skills. However, sixteen subjects showed the opposite where their irregular word reading skills were poorer than their non word reading skills and were called surface dyslexics.
Surface Dyslexia
Surface dyslexia is characterized by subjects who can read non words but who have trouble reading words that are irregular [2].Surface dyslexia is the outcome of an individual who cannot function using the lexical procedure for reading out loud. The lexical procedure includes sounding out a word though the use of a past word already known . In Castles and Coltheart’s study, the researchers matched 30 regular words with 30 irregular words and asked their surface dyslexic subjects to read the word out loud. As hypothesized in their research, the difference between the surface dyslexic subjects and control subjects was about 4.6 words which was 2.55 words higher than the difference between the phonological dyslexics to the control group. In Castles and Coltheart’s study, both control and dyslexic subjects were shown a card with a word that is irregular or that isn’t pronounced as it looks. Fifteen of the 51 dyslexics were below the confidence limit set by the control subjects on ability to read irregular words. These subjects were then called surface dyslexics .
Phonological Dyslexia
Phonological dyslexia is characterized by subjects who can read aloud both regular and irregular words but have difficulties with non words and with connecting sounds to symbols, or with sounding out words . Phonological processing tasks predict reading accuracy and comprehension. This subtype is the most predominant form of dyslexia.In Castles and Coltheart’s study, they had 56 dyslexic boys and 56 non dyslexic boys read words and non words given to them. The majority of the boys, 55%, showed a phonological dyslexic pattern.In Castles and Coltheart’s study, dyslexic subjects and control subjects were asked to read non words listed on a card, 17 out of 51 cases of dyslexics were below the confidence limit in non word reading, which was derived by the control group of subjects their own age. These phonological dyslexics have a lower non word reading level than expected by reviewing their irregular word reading level [3]. Phonological dyslexia is the outcome of a subject who cannot function using the sub lexical (pronunciations are constructed from smaller orthographic components) procedure for reading out loud .In Castles and Coltheart’s study, dyslexic and control subjects read words off a note card; the researchers found that while reading irregular words, the dyslexic subjects scored comparable to the control subjects because sub lexical skills weren’t involved in this test.
Double Deficit Dyslexia
Other researchers have identified a deficit related to “naming speed”, which relates to the ability of students to rapidly verbalize the names of symbols such as letters and numbers when tested [4]. In their study, Wolf and Bowers tested out naming speed by having their subjects name a symbol as quickly as possible when shown on a flash card. Difficulties in naming speed exist in conjunction with a phonological deficit, is characterized as double deficit dyslexia [4].
References
- ↑ Castles, A., & Coltheart, M. (1993). Varieties of developmental dyslexia. Cognition, 47(2), 149-180
- ↑ Castles, A., & Coltheart, M. (1993). Varieties of developmental dyslexia. Cognition, 47(2), 149-180
- ↑ Manis, F., Seidenberg, M., Doi, L., McBride-Chang, C., Petersen, A., (1996). On the basis of two subtypes of developmental dyslexia. Cognition, 59, 157-195
- ↑ 4.0 4.1 Wolf, M., Bowers, P. G., (1999). The double-deficit hypothesis for the developmental dyslexias. Journal of educational psychology, 91, 415-438