4.13.2014

Faulty Access and Not Representation of Phonemes in Dyslexia? New Scientific Evidence Sheds Light on the Debate





One of the best metaphors to describe the neurological processing in dyslexia is the one of trying to use a fork (the right hemisphere) to eat soup (language processing in the left hemisphere). Most of the times, it doesn't work very well, that being the reason why people still use spoons to eat soup. It just makes sense.

Individuals with dyslexia are often said to have their brains wired differently: they use the right hemisphere to compensate for the processing deficits in the left hemisphere.

Interestingly, in children with dyslexia, the right hemisphere equivalent of this connection has been shown to predict reading outcome, consistent with the idea that the right hemisphere is used to compensate for faulty left hemisphere reading networks (Hoeft et all., 2011).

The study here presented attempted to analyse the difference in language processing between adults with and without dyslexia, using machine-learning algorithms to decode handwritten characters combined with brain imaging solutions. What they found out was that individuals with dyslexia had significant impairments in the connections in the left hemisphere, which proved to be a sign of problems of access to phonemes.

In this study, using neuroimaging technology, researchers investigated 23 adults severely affected by dyslexia and 22 adults without dyslexia. They found that the connection between left hemisphere regions that are important for processing speech sounds and speech output was impaired in people with dyslexia. They also applied machine-learning algorithms often used to optimize online search engines and to decode handwritten characters. Using these algorithms, they observed brain activation patterns in response to different types of speech sounds in adults with and without dyslexia. The researchers did not find differences in brain activation patterns in the accuracy of the algorithm between adults with and without dyslexia. This was in striking contrast to the connection impairment found in these adults with dyslexia. Based on these results, they concluded that it could be the access to phonemes that is impaired in dyslexia (based on connection impairment) and not the phoneme representation itself (based on no differences in brain activation).

Or to put it in different words, this means that individuals with dyslexia have the same brain activation as individuals without dyslexia in the representation of phonemes but, contrary to the latter, the former have their brains confused on the routes to access the right phonemes to the visual cues that their brains receive. 

This is important because it adds a new note to the discussion: it isn't just the left-right hemisphere connection that is impaired but also the ability to access the phonemes. 

....researchers found no indication that the neural representation of phonemes was abnormal in people with dyslexia.

Contrary to many studies, the researchers found no differences in the neural representation of phonemes with individuals with dyslexia. However, others have found that phonetic feature representations are impaired in those with dyslexia.

The debate between access and/or representation problems in individuals with dyslexia seems to be unanswered by this new data. However, this makes the debate far more interesting, as it seems that the brain has more complex ways to process language in individuals with dyslexia than previously believed.

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