The nature of the visual deficits in developmental dyslexia

Vidyasagar, T.

Department of Optometry & Vision Sciences and Melbourne Neurosciences Institute, University of Melbourne, Australia

A neural framework for impairment in visual attention being a core deficit in developmental dyslexia
Reading is a relatively recent activity in the history of Homo sapiens. It almost certainly uses neural mechanisms, which had already evolved over many hundreds of millennia for a different purpose. What may these be? It is becoming apparent that a crucial stage in reading are visual attentional mechanisms that usually help us to select and process only that small fraction of sensory information which is relevant at a particular instant, from among a world of ?noisy? sensory inputs. A careful consideration of all the known neural pathways needed for reading a text opens up the possibility that the core deficit could be at any one of a number of stages in the visual system that apply the above process of attentional selection to reading. The cortical region that orchestrates such selective attention is the posterior parietal cortex, which seems to direct a spotlight of attention on to earlier visual areas. Such gating of sensory inputs rapidly and at a fine spatial scale by the feedback pathways from the dorsal stream to earlier visual areas is a fundamental process in reading and any deficit in this process can potentially lead to poor reading abilities. Such deficit could be either in the visual magnocellular inputs that project to the dorsal stream or in the dorsal stream areas themselves. It is also conceivable that even the severe phonological impairments commonly associated with dyslexia could at least partially, if not totally, be a simple downstream consequence of the deficit in visuo-spatial attention. However, there is also the possibility that a general impairment in spatio-temporal processing of sensory information could lead to both the visuo-spatial and phonological difficulties and the dyslexic symptoms may often have a multifactorial cause.