Authors: John Stein.
Article: Theories about Developmental Dyslexia.
Publication: Brain Sciences (MDPI). 13, no. 2: 208 2023 | DOI: 10.3390/brainsci13020208
Stein, John. 2023. “Theories about Developmental Dyslexia” Brain Sciences 13, no. 2: 208. https://doi.org/10.3390/brainsci13020208
For over a century, developmental dyslexia was recognised by neurologists and other
doctors as a useful diagnosis for which the key criteria were a significant discrepancy between a child’s backward reading, yet normal or high speech and oral comprehension and reasoning skills, together with a strong family history. However, since the turn of the millennium, this consensus has been thoroughly undermined.
To simplify, the essence of reading is
to learn to translate letters into the sounds they stand for, which are ‘phonemes’. However, phonemes do not actually exist as a consistent, standard acoustic signal. Their acoustic form varies according to the preceding and succeeding sounds in the spoken word; thus, children have to learn a semi-abstract concept, the ‘phonological principle’
This new emphasis on phonology meant that DD became attributed entirely to failure to learn the phonological principle properly, and the ‘phonological theory’ of dyslexia became dominant.
More than 90% of the new neurones born in the foetal brain in the last few months of pregnancy fail to thrive and are eliminated jn early infancy because they fail to make successful and useful connections . This means that the most numerous neurones, parvocells (parvus, Latin for small), may be able to flourish more than usual. Since parvocells are normally more extensively connected than magnocells, and their superabundance in dyslexia implies that their connectivity might be even greater. Thus, the magnocellular timing deficit could be matched by a superabundance of parvocells in DD.