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 . 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 .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 .

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 Castles, A., & Coltheart, M. (1993). Varieties of developmental dyslexia. Cognition, 47(2), 149-180 </ref>.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 . 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 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 .

Variations and related conditions

Dyslexia is a learning disability. It has many underlying causes that are believed to be a brain-based condition that influences the ability to read written language. It is identified in individuals who fail to learn to read in the absence of a verbal or nonverbal intellectual impairment, sensory deficit (e.g., a visual deficit or hearing loss), pervasive developmental deficit or a frank neurological impairment.

The following conditions may also be contributory or overlapping factors, or underlying cause of the dyslexic symptoms as they can lead to difficulty reading:

• Auditory processing disorder is a condition that affects the ability to encode auditory information. It can lead to problems with auditory working memory and auditory sequencing. Many dyslexics have auditory processing problems including history of auditory reversals. Auditory processing disorder is recognized as one of the major causes of dyslexia.
• Cluttering is a speech fluency disorder involving both the rate and rhythm of speech, and resulting in impaired speech intelligibility. Speech is erratic and dysrhythmic, consisting of rapid and jerky spurts that usually involve faulty phrasing. The personality of the clutterer bears striking resemblance to the personalities of those with learning disabilities.
• Dyspraxia is a neurological condition characterized by a marked difficulty in carrying out routine tasks involving balance, fine-motor control, and kinesthetic coordination. Problems with short term memory and organization are typical of dyspraxics. This is most common in dyslexics who also have attention deficit disorder.
• Verbal dyspraxia is a neurological condition characterized by marked difficulty in the use of speech sounds, which is the result of an immaturity in the speech production area of the brain.
• Dysgraphia is a disorder which expresses itself primarily during writing or typing, although in some cases it may also affect eye-hand coordination in such direction or sequence oriented processes as tying knots or carrying out a repetitive task. Dysgraphia is distinct from Dyspraxia in that the person may have the word to be written or the proper order of steps in mind clearly, but carries the sequence out in the wrong order.
• Dyscalculia is a neurological condition characterized by a problem with learning fundamentals and one or more of the basic numerical skills. Often people with this condition can understand very complex mathematical concepts and principles but have difficulty processing formulas and even basic addition and subtraction.
• Scotopic sensitivity syndrome, also known as Irlen Syndrome, is a term used to describe sensitivity to certain wavelengths of light which interfere with proper visual processing. See also Orthoscopics and asfedia.