Getting the words wrong
New research suggests visual problems have a role in dyslexia
The first record of dyslexia as an identifiable condition came to light in the late nineteenth century amongst the many other medical curiosities that seemed to fascinate the Victorian imagination. Of course modern scientists realise that dyslexia is in fact quite common, many people being affected to a greater or lesser degree. It's also now known that there is a strong genetic component putting children of dyslexic parents at heightened risk. However despite many years of research and several significant breakthroughs, a complete model of the processes that underlie dyslexia remains elusive.
Dr Kristen Pammer from the ANU School of Psychology is currently leading a team of scientists trying to unravel at least one piece of this complex puzzle. "Traditionally dyslexia is seen as a phonics based problem, a difficulty in associating particular sounds with letters on a page. But whilst there is clearly a large component of photonics in dyslexia, our research has lead us to believe that there may also be subtle underlying problems in the dyslexic brain's visual processing systems."
Within the visual cortex there are two distinct processing pathways. The mangocellular pathway (also known as the dorsal stream due to its physical placement) deals with conceptualising the movement of objects and their position in space. For this reason it's sometimes nicknamed the "where" pathway. Leading to a different part of the brain, the pavocellular or ventral pathway is concerned with detailed visual information used for the recognition of objects. And can be thought of as the "what" pathway.
In terms of reading, the pavocellular pathway is vital for recognising letters and words but interestingly enough, it's deficiencies in the mangocellular pathway that seem to be related to reading difficulties. Dr Pammer explains "Its been known for some time that adults with dyslexia often do poorly on visual tasks designed to test the functioning of the mangocellular pathway. What we were unsure of is wether this was a partial cause of dyslexia or a consequence of failing to learn to read?"
In order to investigate this, graduate student Alison Kevan undertook a study of kindergarten children who hadn't yet begun to learn to read. The kids were selected from those who had a first-degree adult relative with dyslexia and so formed a "high risk" group. The researchers adapted the visual tests into a series of simple computer games that the kids would find fun. The first was a coherent motion test taking the form of a series of sheep moving round a field. The task was to detect when some of the sheep are moving in the same direction. A non-dyslexic adult can usually detect coherent motion when about 20% of the sheep move synchronously, for a child it could be as high as 50%. However, in many children with a predisposition for dyslexia the number need to be much higher before they can recognise synchronised motion. This simple test was coupled with a number of other more complex games based on the visual frequency doubling illusion, to create a suite of specialised tests to probe the manogcellular pathway.
By following the progress of the children as they began to learn to read, the researchers found that those who did badly on the mangocellular pathway tests before they learned to read, also showed signs of dyslexia once they could read. This finding lends weight to the idea that inherent deficiencies in the visual cortex may be at least a partial cause for dyslexia rather than a consequence of it. But although the data correlation is excellent, Dr Pammer is cautious about jumping to conclusions. "Clearly phonemic problems are also a major factor, so I don't see our research producing a stand alone test for risk of developing dyslexia. They might however be useful as part of a broader series of visual and auditory tests used in the early diagnosis of dyslexia."
The team also hope that their work may help build a better understanding of the function and dysfunction of the brain. "Reading is a particularly interesting area of research because he brain has not evolved to read. It has specialised centres to deal with spoken language but in order to read, it has to recruit areas that have evolved for other purposes and build them into a network."