For most of the 40-plus years that 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 has been diagnosed with “dyslexia” if he or she is shown to have an IQ in the “normal” range but falls at or below the 10th percentile on standardized tests of reading for a specific age group. The cut-off has been arbitrary, often varying considerably from one setting to another. As a result, a child who falls at the 12th percentile might be considered a poor reader while a child falling at the 10th percentile would be diagnosed with dyslexia.
The technical term for that diagnostic approach is called “discrepancy criteria.” Stanislas Dehaene, in his book Reading in the Brain, succinctly explains that the diagnosis of dyslexia has thus depended “on the setting of an arbitrary criterion for ‘normality’ … [which] might lead to the erroneous conclusion that dyslexia is a purely social construction.”
Certainly, for those parents among us who have a child diagnosed with dyslexia, it is obvious quite early in the educational process that our bright child is not just behind in reading but dumbfounded by the written word. A child with dyslexia seems to struggle at every turn.
Psychologists, neurologists, and special educators have understood that as well and since the 1970s have assumed dyslexia has a neurological basis. In fact, the term “dyslexia” actually stems from the Greek alexia, which literally means “loss of the word” and was the diagnostic term used when adults lost the ability to read after suffering a brain injury. Dyslexia was a term adopted to confer a lesser (though still neurologically based) form of reading impairment seen in children. However, determining the neurological basis has been elusive until relatively recently.
The Search for a Neurological Basis
In the early attempts at researching the underlying causes of dyslexia in the 1970s there were no technological medical procedures available to study brain processes that might be involved in reading normally or abnormally. As a result, although the term implied that there was a neurological cause, the exact nature of the brain differences in children with dyslexia could not be determined.
Some of the early researchers believed that the cause was visual-spatial. Samuel Orton had originally thought that reading disorders in children were similar to “word blindness” in adults, caused not by a specific brain injury, but representing a maturational disorder based on delayed cerebral development of left hemisphere dominance. However, his theory could not be tested empirically and he and others became more aware over time that many children with reading problems seemed to have specific problems with other non-visual aspects of reading – specifically, sounding out of words.
Because of the inability to determine the neurological cause(s) of dyslexia, in some educational circles especially, it became synonymous with "developmental reading disorder" and the cause (neurological or perhaps otherwise) was deemed not important. Rather, the goal was to develop and test interventions and measure their outcomes without an effort to relate the interventions to underlying causation.
The problem with that approach, from a scientific standpoint, is that it is symptom based. Rather than getting at the root of the problem or distinguishing one child’s problem from another’s, the non-causative approach assumes that the solution to dyslexia depends on a specific teaching method. An analogy in medical science would be trying to treat all skin rashes with calamine lotion – it might make a person feel better no matter the cause, but it would be wholly inadequate for prevention of a virus like measles or treatment of a bacterial rash like impetigo.
Fortunately, just as medical science advanced our understanding of viral and bacterial causes of skin infections to allow for effective medical treatment, advances in neuroscience, buttressed by neuroimaging and brain electrophysiological technology starting in the late 1990s, have led to an emerging consensus about the causes of dyslexia and the most effective methods for remediating those causes. This neuroscience research has been accumulating from a variety of disciplines and is beginning to reveal a few underlying factors in brain development that can cause reading to be problematic. And the best news is that all of those processes are amenable to carefully designed training approaches.
What Happens in the Dyslexic Brain – and Why
In the early to mid-2000s, much of the available research on the underlying basis of dyslexia pointed to a primary problem with the phonological processing of speech sounds. The early research by Shaywitz (2003), Ramus (2003), and Vellutino, Fletcher, Snowling, & Scanlon (2004) – summarized in Stanislas Dehaene’s Reading in the Brain – identified problems with phonological awareness, the ability to segment words into their component speech sound components.
More resent research has delineated why that problem exists. For example, in 2012, Boets et al., using neuroimaging technology, found that in adults with dyslexia the brain connections between areas that represent speech sounds and a part of the left frontal lobe that is important for higher level processing of speech sounds is significantly hampered. In other words, they found that dyslexia is a problem accessing intact representations of speech sounds. Other recent neurophysiological research has indicated that disrupted timing of auditory processing, particularly in the range relevant to speech sounds, is a core deficit in dyslexia.
Retraining the Dyslexic Brain
These consistent findings have led to an emerging consensus, well summarized by Jane Hornickel and Nina Kraus in the Journal of Neuroscience in 2012: namely that dyslexia is primarily an auditory disorder that arises from an inability to respond to speech sounds in a consistent manner. This underlying problem with perception of speech sounds, in turn, causes problems relating a speech sound to the written letter. Therefore, reading interventions for dyslexia should be most effective if they combine auditory perceptual training of speech sounds with exercises that require relating speech sounds to the written letter. And, in fact, neuroscience research bears that out.
The Fast ForWord Language and Reading interventions contain neuroscience-based exercises. They have been empirically tested in independent neuroscience laboratories and shown to have a rapid and significant impact on children and adults with dyslexia. The exercises have been shown to have a positive effect on the neurological processes that support reading and language as well.
Our understanding of dyslexia has come very far in the past 40 years, with neurophysiological models developed in just the past five years explaining why letter-sound correspondence is so difficult for these children. 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.
Boets, B., Op de Beeck, H.P., Vandermosten, M., Scott, S.K., Gillebert, C.R., Mantini, D., Ghesquière, P. (2013). Intact but less accessible phonetic representations in adults with dyslexia, Science, 342, 1251-1254. doi: 10.1126/science.1244333
Burns, M.S. (2012). Application of Neuroscience to Remediation of Auditory Processing, Phonological, Language and Reading Disorders: The Fast ForWord® and BrainPro Programs. In D. Geffner & D. Swain (Eds.), Auditory processing disorders: Assessment, management and treatment (2nd ed.). San Diego, CA: Plural Publications.
Dehaene, S. (2009). Reading in the brain: The science and evolution of a human invention. New York, NY: Viking Press.
Gabrielli, J. (2009). Dyslexia: A new synergy between education and cognitive neuroscience. Science, 325, 280-283. doi: 10.1126/science.1171999
Hornickel, J. & Kraus, N. (2013), Unstable representation of sound: A biological marker of dyslexia. The Journal of Neuroscience, 33, 3500 –3504. doi: 10.1523/JNEUROSCI.4205-12.2013
 See Billet & Bellis (2011), Goswami (2011), and Lehongre, Ramus, Villermet, Schwartz, & Giraud (2011) summarized by Burns (2012).
 See Dehaene (2009) and Gabrielli (2009) for excellent summaries of the research on the Fast ForWord interventions for dyslexia.