Is Dyslexia a Visual or Phonological Deficit?

It's interesting how the public's impression of dyslexia differs from the impressions of researchers in the field. I recently read an article by Vidyasagar and Pammer arguing that dyslexia is a visual deficit. To the general public, this claim seems obvious because most people believe that people with dyslexia see things backwards.



Many dyslexia researchers, however, will find this claim unusual, if not controversial.  This is because the field concluded long ago that the "reading backwards" theory is a myth. As we touched on in a previous article, all children will write backwards to some extent, not just dyslexic children

The current prevailing theory of dyslexia is actually poor auditory or phonological processing. People with dyslexia score lower than controls on phonological awareness, and phonological awareness in children predicts reading skill later on. In addition, some phonological training studies have succeeded in improving reading performance in children with dyslexia.

Vidyasagar and Pammer propose a different theory.  They argue that dyslexia is due to a deficit in visual attention and the dorsal visual stream.

Human vision is processed in two pathways in the brain. The ventral stream processes object identity, and the dorsal stream processes object location. When we read, the dorsal stream helps us direct our attention smoothly from one word to the next. Vidyasagar argues that a deficit in the dorsal stream is the underlying cause for dyslexia.

They cite some evidence for causality. Studies have found that deficits in coherent motion detection and visual contrast sensitivity, both dorsal stream functions, predict future reading skill.

But what about all the phonological results? Vidyasagar argues that  both visual and auditory input are required to develop a good understanding and awareness of phonetics, and that the  phonological deficits in dyslexia arise from a lack of high quality visual input during the time period when phonological awareness is maturing.  It will be up to researchers to conduct comprehensive studies to test this.

So what do I think?  My favorite theory is neither purely phonological nor visual one, but rather another possibility that Vidyasagar touches on – a deficit in rapid sensory processing. Rapid auditory processing is important for phonological awareness, and rapid visual processing is important for visual attention and visual search.  There is evidence that people with dyslexia have deficits in both.

Genetically, the theory makes sense. Genes don’t just code for one function. More often, they code for proteins that show up in multiple systems. You could imagine a gene that codes for some rapid processing neuron that plays a roll in both visual and auditory processing, resulting in the complex disorder we know as dyslexia.

It’s an exiting time to be a brain researcher.

Vidyasagar, T., & Pammer, K. (2010). Dyslexia: a deficit in visuo-spatial attention, not in phonological processing Trends in Cognitive Sciences, 14 (2), 57-63 DOI: 10.1016/j.tics.2009.12.003

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