For most of the 40-plus years the term “dyslexia” has been in existence — and although the diagnosis has long been considered a “learning disability” — it has been based on comparisons with average readers. Simply put, a child could be diagnosed with dyslexia if he or she shows an IQ in the “normal” range but falls at or below the 10th percentile on standardized reading tests. This cut-off has been arbitrary, often varying from district to district and based on Response to Intervention (RTI) criteria. As a result, a child who falls at the 12th percentile might be considered a poor reader while a child at the 10th percentile would be diagnosed with dyslexia.
The Search for a Neurological Basis:In early attempts at researching the underlying causes of dyslexia in the 1970s, there were no technological medical procedures to study brain processes that might be involved in reading normally or abnormally. Because of the inability to determine the neurological cause(s) of dyslexia, in some educational circles it became synonymous with “developmental reading disorder,” and the cause was deemed unimportant. Rather, the goal was to develop and test interventions and measure their outcomes, without an effort to relate the interventions to underlying causation.
The Dyslexic Brain:In the early to mid-2000s, research on the underlying basis of dyslexia pointed to a primary problem with the phonological processing of speech sounds. Early research, summarized in Stanislas Dehaene’s Reading in the Brain (2009), identified problems with phonological awareness, or the ability to segment words into their component speech sounds. More recent research has delineated why that problem exists.
These findings have led to an emerging consensus, well summarized by Jane Hornickel and Nina Kraus in theJournal of Neuroscience in 2012: dyslexia is primarily an auditory disorder that arises from an inability to respond to speech sounds in a consistent manner. And Finn and colleagues at Yale published research in August 2014(PDF, 4.7MB) indicating that this underlying problem with perception of speech sounds, in turn, affects the development of brain networks that enable a student to link a speech sound to the written letter.
The Potential to Retrain the Brain:Our understanding of dyslexia has come very far in the past 40 years, with neurophysiological models developed in just the past five years explaining the underlying capacities required for reading and the best methods for individualized adaptive interventions. Fortunately, treatment options have kept pace with the research, and children with dyslexia today have the potential to train their brains to overcome the learning difficulties that earlier generations were destined to carry with them for a lifetime.