Dyslexia is a complex, lifelong disorder involving difficulty in learning to read or interpret words, letters and other symbols. Dyslexia does not affect general intelligence, but is often co-diagnosed with ADHD. There are at least three sub-types of dyslexia that have been recognized by researchers: orthographic, or surface dyslexia, phonological dyslexia and mixed dyslexia where individuals exhibit symptoms of both orthographic and phonological dyslexia. Studies have shown that dyslexia is genetic and can be passed down through families. Although it is a genetic disorder, there is no specific locus in the brain for reading and writing. The human brain does have language centers (for spoken and gestural communication), but written language is a cultural artifact, and a very complex one requiring brain regions designed to recognize and interpret written symbols as representations of language in rapid synchronization. The complexity of the system and the lack of genetic predisposition for it is one possible explanation for the difficulty in acquiring and understanding written language. Furthermore, recent evidence has found that there are certain genes responsible for causing dyslexia. Research also suggests a clear genetic basis for developmental dyslexia with abnormalities in certain language areas of the brain. However, there is also evidence that orthography, the correspondence between the language's phonemes (sound units) and its graphemes (characters, symbols, letters), plays a significant role in the type and frequency of dyslexia's manifestations. Some psycholinguists believe that the complexity of a language's orthography (whether it has a high phoneme-grapheme correspondence or an irregular correspondence in which sounds do not clearly map to symbols) affects the severity and occurrence of dyslexia, postulating that a more regular system would reduce the number of cases of dyslexia and/or the severity of symptoms. Current psycholinguistic models of dyslexia are "largely developed on the basis of alphabetic writing systems such as English", but the amount of research on some logographic orthographies, Chinese in particular, is also fairly significant. Unfortunately, little research has been done on syllabic writing systems, and "cross-linguistic studies of the acquired dyslexia and dysgraphias are scarce."
Dyslexia and orthographic features
Orthographic Dyslexia Orthographic dyslexia, a subtype of dyslexia, results in difficulty decoding and encoding skills due to slow and inaccurate rates of storing word and letter formations into memory. Orthographic dyslexics have difficulty in storing mental representation of words, especially phonetically irregular words such as word spellings that end in -ight ("light" and "sight"). The problems underlying this type of dyslexia are related directly to memory and coding skills that allow representation of printed letters and words, not to poor phonological processing. This type of dyslexia is also termed surface dyslexia because people with this type have the inability to recognize words simply on a visual basis. Words that are misspelled cause the readers difficulty because they attempt to sound out the words by looking at each individual letter rather than the word as a whole. So a reader might read the word "cat" and pronounce the "c" as a hard "c" but then read the word "ice" and pronounce the "c" as a hard "c" as well because they are sounding out each individual phoneme rather than just recognizing the word "ice" in its entirety. Despite intervention, children with orthographic dyslexia continually have lower achievement reading levels when compared to their peers. Additionally, children show a greater difficulty throughout schooling when spelling words with irregular or unusual orthographies when compared to their other children. Research also shows that dyslexic children have primary difficulties in phonological processing and secondary difficulties in orthographic processing, aiding to the distinction of two subtypes.
The effects of orthographic depth on dyslexia The complexity of a language's orthography is directly related to the difficulty of learning to read in that language. Orthographic complexity also contributes to how dyslexia manifests in readers of different languages. Deep orthographies are writing systems, such as those of English and Arabic, that do not have a one-to-one correspondence between sounds (phonemes) and the letters (graphemes) that represent them. Shallow orthographies, such as Italian and Finnish, have a close relationship between graphemes and phonemes, and the spelling of words is very consistent. With shallow orthographies, new readers have few problems learning to decode words and as a result children learn to read relatively quickly. Most dyslexic readers of shallow orthographic systems learn to decode words with relative ease compared to dyslexics using deep orthographies, though they continue to have difficulty with reading fluency and comprehension. The hallmark system of dyslexia in a shallow orthography is a comparatively slow speed of rapid automatized naming.
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