We're interested in developmental dyslexia, as opposed to dyslexia following an accident, for example. In 1896, Dr. William Pringle-Morgan described a young patient with reading difficulties as congenital word-blindness (Pringle-Morgan, British Medical Journal, 1896). The WHO has proposed the following definition of dyslexia (2008): "specific, long-lasting and persistent disorders of written language acquisition appearing in a child of normal intelligence when intelligence is assessed by non-verbal tests, and without any other disorders". Dyslexia often leads to failure at school.
The aim of reading is to understand written information; it generally involves converting graphemes into phonemes. It is because of the phonological hypothesis proposed to explain this graphophonemic conversion disorder that dyslexia has been discussed in this lecture. The phonological hypothesis of dyslexia states that the learning of alphabetic, syllabic and word recognition involves establishing links between their mental representations and their perception within speech sounds; these phonological representations are said to be altered in dyslexics. The phonological hypothesis of dyslexia has become increasingly accepted over the last 30 years. As a result, the problem of phoneme perception in dyslexics was raised. It was then extended to a possible more general auditory deficit involving the temporal component of acoustic signal perception (Tallal P., Brain Lang, 1980). In the absence of sufficient temporal resolution, speech signals involving brief sounds or rapid transitions, for example between a consonant and a vowel, would not be perceived normally by dyslexic people. Yet, as we have seen, speech intelligibility relies primarily on deciphering the sound envelope of speech, itself characterized by the presence of rapid amplitude transitions. The three major symptoms of phonological deficit in dyslexia are poor phonological awareness, impaired verbal short-term memory, and difficulty in accessing the phonological lexicon. Dyslexia is the most common learning disability of childhood. Its prevalence depends, of course, on its definition. It is estimated to affect 3.6% of primary school children in the Netherlands, between 3% and 7% in France, and 10% in the United States. Boys are affected twice as often as girls. At the beginning of the 20th century, in 1939, Edith Norrie (Norrie E., 1939) reported that dyslexia runs in families. Then in 1950, Hallgren (Hallgren A., Acta Psychiatr. Neurol. Suppl., 1950) described 116 index cases, and found several affected family members. Studies of twins show heritability ranging from 50% to 70%. Remember that heritability is the share of phenotypic variance due to genotypic variance, or the share of genetic factors in the probability of occurrence of a given phenotypic trait within a given population.
