Aetiology of Dyslexia: A Visual Perspective on a Phonological Marker

Abstract

The very common association between developmental dyslexia and phonological deficits had led to the widespread suggestion that the core deficit in dyslexia is in phonological processing. However, causality has never been clearly established; indeed there are compelling reasons to believe that the aetiology may be primarily within the visual system. This stems from recent attempts to explain the various stages involved in decoding the letter stream in neuronal terms and anchoring the reading stages firmly in the known neuroanatomical and neurophysiological basis of the processing of visual stimuli. A central component of this framework is the role played by visuo-spatial attention in parsing the stream of letters at different spatial and temporal scales. It is proposed that a deficit in this process either from a lesion in the dorsal visual stream or in the inputs it receives from the magnocellular pathway can lead to poor reading abilities. Such impaired orthographic processing may even lead to some of the phonological impairments seen in dyslexics.