Embracing Neurodiversity: The Science Behind Dyslexia
What is developmental dyslexia?
Developmental dyslexia is a specific learning disorder that is neurobiological in origin and characterised by poor performance in reading and spelling despite typical intelligence, adequate motivation and sufficient education opportunities. It is a relatively prevalent condition, affecting 5-17% of children in English-speaking countries and 2-10% of children in Chinese societies.
What are the causes of developmental dyslexia?
Reading is a complicated skill that requires the integration and coordination of multiple component skills including visual, orthographic, phonological, semantic, morphological and syntactic skills. Theoretically, deficit in any one component should lead to poor reading, but some people think that visual skills should play a more important role in reading than other skills as reading is a visual task. However, scientific research based on alphabetic languages has rather consistently shown that the core deficit of developmental dyslexia lies in the phonological (sound) domain – specifically the ability to consciously aware of the sound structure of spoken words.
For alphabetic languages (e.g. English, Italian, Spanish, French, German), a grapheme (letter or letter cluster) typically represents a phoneme (the smallest unit of speech). For example, boy, bar, but all start with the same sound (or phoneme) /b/ and share the same letter b. This grapheme-to-phoneme correspondence (GPC) rules, s/he will be able to sound out a new printed word to link a new printed words with the already acquired listening vocabulary A grapheme (either a letter or letter cluster such as th, ch) b represents the phoneme /b/ (as in boy, bar, but) and the grapheme (or letter cluster) th represents the phoneme can be represented by many graphemes (e.g., i, e, ee, ea), and a grapheme (e.g., c) can map onto different phonemes (e.g., /s/ or /k/). which use letters to build up words, letters (graphemes) of a word are mapped onto respective sounds (phonemes), which are assembled to produce the pronunciation of a word. This process, called Grapheme-to-Phoneme Conversion (GPC), is the key to successful reading acquisition among children. This means that children must be aware of which sound(s) a letter will correspond to when they learn to read. Hence, children’s phonological awareness, i.e. the ability to distinguish between speech sounds and categorising sounds into units and segments, is crucial for reading development. Decades of research has shown that dyslexic children of alphabetic languages usually suffer from impairment in phonological processing.
In logographic languages like Chinese, the cognitive processes involved in reading and the cause of dyslexia are more obscure. Each Chinese character is mapped onto a monosyllabic meaning unit and each character is composed of strokes that make up different components in a character. While semantic radicals and phonetic components in a character can provide some information about its meaning and pronunciation, this correspondence is not consistent at large, so GPC is impossible in Chinese reading. Contrary to alphabetic languages, a number of cognitive factors have been found to correlate with Chinese reading, such as visual processing skills, phonological awareness, orthographic processing skills and morphological awareness. One of our research suggests that Chinese reading development also depends on rote memory through hand-copying. However, there is not any consensus in which cognitive processes uniquely predict successful reading acquisition in Chinese.
Apart from behaviour and cognitive deficits, dyslexia also has a neurobiological basis. To understand more about the relationship between reading and the brain, please click here.
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