Sometimes referred to as alexia and visual dysphasia, it is a disorder of reading in which oral language skill and non-verbal cognitive ability is normal. May be acquired or developmental in origin. In developmental dyslexia, reading and spelling are both impaired. Like dysgraphia, it is now considered to be a specific learning disorder that is neurological in origin. In a recent study, dyslexic children compared to typically developing children have been shown to have a larger regional distribution of metabolic activation in the left anterior quadrant as indicated by functional magnetic imaging during the performance of phonological tasks, characterised by the number of voxels with elevated lactate. Such a greater regional activation could not be directly attributed to the reading difficulties of dyslexics as the tasks were auditory and did not involve reading. It seems then that the dyslexic brain has a larger regional distribution of metabolic activation relative to the normal brain while attempting to accomplish the same linguistic task. Such a metabolic abnormality indicates a phonological processing problem located in the left anterior quadrant, which included the left frontal lobe and language areas surrounding the Sylvian fissure. While generally considered to be more prevalent among males, its exact male/female ratio is a question of debate, with figures varying from 3:5 to 5:1. First described by Adolph Kussmaul (1822-1902) in 1877, who termed it ‘word blindness’.
See Apraxia, Cell migration, Development coordination disorder, Dysgraphia, Dorsal visual pathway (or stream), Functional magnetic imaging (fMRI), Frontal quadrants, Orthography, Phonological dyslexia, Phonological deficit hypothesis/model/theory, Psycholinguistic guessing game, Voxel