Higher Visual Processing

There have been many attempts to classify dyslexia, although it should be noted that one large study failed to support the concept of clearly defined subgroups (Naidoo, 1972). Nevertheless, many authors have classified dyslexia into three subgroups and a good example is the classification of Boder (1971). Her system is based upon three reading-spelling patterns. Dysphonetic dyslexia is characterized by people who read words globally, as instantaneous visual wholes from the limited sight vocabulary, and as a result cannot cope with new or unusual words. The opposite is dyseidetic dyslexia, exemplified by the analytic reader who cannot perceive letters or words as visual wholes and who consequently reads laboriously and cannot deal with words that are irregularly spelled or pronounced. The final group, mixed dysphonetic-dyseidetic dyslexia-alexia, exhibits the combined deficits of both groups and are usually the most severely handicapped educationally.

The type of high-level cognitive visual deficit that, for example, Boder's dyseidetic subgroup manifest is very different to the visual deficits that have been described earlier in this chapter. The relationship between these two types of visual deficit has been studied very little and is far from clear. Lovegrove et al. (1986) argued that the visual disabilities shown by the dyseidetic (visuo-spatial) subgroup do not reflect visual processes of the type characterized by their research on a magno system deficit. There is even some evidence to suggest that the magno deficit is present in the dysphonetic, but not the dyseidetic, subgroup of dyslexia (Ridder, Borsting, Cooper, McNeel, & Huang, 1997). This has been linked to the theory described in the last section about magno visual and phonological deficits in dyslexia both representing a common impairment of fast systems in dyslexia.

A similar distinction needs to be drawn between the "visual IQ" assessed by tests such as the British Ability Scales and the type of visual deficits described in this chapter. It should not be assumed that children with lower "visual IQs" are particularly likely to have the visual anomalies described in this chapter. There is no good evidence to suggest that psychometric results obtained in a psychological evaluation can be used to decide which dyslexic children need to see an eye care practitioner.

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Adult Dyslexia

Adult Dyslexia

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