A condition or syndrome, either acquired or developmental, which is manifested,
among other things, by a deficit in the ability to process language, particularly
in its written form.
Many studies have been undertaken to ascertain its aetiology. Identified as "Congenital Word-Blindness" by James Hinshelwood in his book of the same name in 1917, research and treatment focused on the likelihood that dyslexia's underlying cause lay in the visual system.
In the 1970s, research results showing that impairments in the processing of auditory stimuli contribute to language processing deficit, in particular dyslexia, were published. There followed a serious and unproductive dichotomy, with researchers adhering generally to either the visual or the auditory camp with little crossover and occasional vitriol as academics defended their respective and often entrenched positions. There has recently been a change of position as the result of two events: firstly, the greater availablitiy of MRE, PET scans and other non-invasive techniques and secondly: using these techniques to record neural activity during the reading process, many researchers have independently reported that the magnocellular routes in both hearing and vision were significantly underactive in reading tasks when performed by dyslexics, as compared to their activity when the same tasks were undertake by non-dyslexics.
There now appears to be a growing consensus by practitioners in research into dyslexia that deficits in neither visual nor auditory processing predominate as a cause of dyslexia, rather that both contribute to an individual's dyslexia in amounts which are unique to the individual and that the root of the problem is to do with the development or performance of the magnocellular pathways in these processing systems in the brain. The Magnocellular Theory of Developmental Dyslexia. John Stein, University Laboratory of Physiology, Oxford, UK
All of the brain's sensorimotor systems contain both magnocellular and parvocellular pathways. Magno and parvo derive from the Latin for "larger" and "smaller", respectively. The magno pathways consist of neurons with larger, compared with the parvo, cross-sections, which allows for more rapid transmission of impulses along the neuron's axon. It is this consensus which, perversely, has led to the recent controversy stirred up by Durham's Professor Julian Elliott, now referred to as "The Dyslexia Myth".