In the book The Proust and the Squid, Dr. Maryanne Wolf theorized that there are 3 subtypes of dyslexia. The most common of these subtypes is a deficit in phonological processing and this impacts the ability to decode/sound out words. This is the basis for the International Dyslexia Association’s definition of dyslexia:
Dyslexia is a specific learning disability that is neurological in origin. It is characterized by difficulties with accurate and / or fluent word recognition and by poor spelling and decoding abilities. These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge.
People with this subtype of dyslexia simply have a harder time processing the individual sounds that make up spoken words and they have a harder time mapping the sound (phoneme) to the written letter (grapheme).
The 2nd subtype of dyslexia is a deficit in rapid naming. People with this subtype of dyslexia will have normal phonological processing, but their fluency and comprehension will be affected by the retrieval of language based information. These are the people that will have problems with word recall, either by saying the wrong word or the word is “on the tip of their tongue”. This affects fluency because it takes longer to retrieve language information. This also affects comprehension because sometimes the retrieved language information is wrong, ie retrieving the wrong definition or wrong word can change meaning.
The 3rd subtype of dyslexia is double deficit: deficits in both phonological processing and rapid naming. This subtype is the least common and the hardest to remediate.
Research from neuroscientists at MIT and Stanford/UCSF now supports Maryanne Wolf’s theory on the 3 subtypes of dyslexia by showing different patterns of brain activation when reading and rhyming words. Children with a deficit in phonological awareness only, rapid naming only, or difficulty in both areas each showed different patterns of brain activation and connectivity as revealed by MRI.