Visual Contributions to Reading Difficulties: The Magnocellular Theory
© Oxford University Press, 2012. All rights reserved. About half of all children with reading problems complain that words and letters appear to blur and move around. This failure to achieve visual stability is probably due to impaired development of visual magnocellular (M-) neurones, which are responsible for the rapid and accurate direction of visual attention and binocular fixation. Although disputed, strong evidence for M- deficits has been found at retinal, LGN and V1 levels and throughout the dorsal 'where' M- dominated cortical visual processing stream. Furthermore M- sensitivity in infancy predicts reading ability 4 years later. Simple visual treatments, such as wearing blue filters, can often improve M- cell function and this is rapidly followed by improved reading. Analogous large cells specialised for rapid temporal processing are found throughout the nervous system; they appear to derive from a common developmental lineage since they all express similar surface signature molecules recognised by antibodies such as CAT 301. Thus, in dyslexics the development of auditory M- like cells is also impaired and this may contribute to their phonological difficulties. At least 3 genes that have been associated with dyslexia affect the migration of neurones during cortical development which helps to explain why the dyslexic brain shows mismigration in the cortex, LGN & MGN. M- cells are also especially vulnerable to lack of the omega-3 docosahexanoic and eicosapentanoic fatty acids. Supplementing the diet with these can often improve deployment of attention, reading and antisocial behaviour.