Dyslexia: Why Precise Definitions Are Important and How We Have Achieved Them

Article excerpt

Of all the definitions of different types of learning disability (LD), dyslexia stands out as a type of LD for which clear criteria for identification have been specified (Lyon, 1995; Lyon, Shaywitz, & Shaywitz, 2003). The International Dyslexia Association (IDA) has taken the lead over the past decade in improving the definition of dyslexia by arguing successfully for anchoring the definition in evidence-based inclusionary characteristics that represent specific criteria for identification, such as a word reading problem. Specifically, dyslexia is more precisely identified and distinguished from other types of LD on the basis of difficulties with accurate and fluent reading of single words and spelling. When children with a well-defined form of LD can be reliably identified and differentiated from children who are typically developing or have other disabilities, studies can then be undertaken of the cognitive, linguistic, and neurobiological correlates of the disorder, along with interventions that are tied to that specific type of LD. The advances in dyslexia research and treatment in the past two decades are directly linked to studies that identify dyslexia on the basis of word-level reading skills (Fletcher, Lyon, Fuchs, & Barnes, 2007).

Historical Perspective

Although some would characterize IDAs definition cited by Louisa Moats in this issue's Theme Editor's Summary as narrow, the need for greater definitional precision is non-negotiable if we are to ever fully and effectively understand dyslexia. Historically, one of the most significant and persistent problems impeding progress in the field of LDs (including dyslexia) has been the difficulty in establishing a precise inclusionary definition that provides specific criteria for identifying dyslexia, such as a word reading problem, as part of a broader framework for 1) identification of different types of LDs and 2) recognizing distinctions and interrelationships between LD (including dyslexia) and other learning, sensory, social, and behavioral disorders. In the absence of this framework, the various definitions of LDs (including dyslexia) developed over the past two decades have been characterized by vague and ambiguous identification criteria based on little scientific research (Fletcher et al., 2007). The inability of historical definitions of LDs in general, and dyslexia in particular, to improve identification, distinguish different types of LDs from each other and from other disabilities, foster communication among professionals, and predict response to different instructional approaches, represents an overreliance on exclusionary criteria that only tell us what conditions aren't LD or dyslexia and the inappropriate use of an IQ-achievement discrepancy model in the identification process.

During the last 20 years, dyslexia has become an example of a form of LD that has seen rapid scientific advances in understanding the etiology, developmental course, and instructional response characteristic of the disorder. We now know that the principal cognitive impairments associated with dyslexia involve different aspects of phonological processing and we have developed sophisticated tools for measuring these skills. We know a great deal about neurobiological factors involved in dyslexia through structural and functional neuroimaging studies. Genetic investigations are flourishing and the array of data on both preventative and remedial interventions is extensive (Fletcher et al., 2007; Shaywitz, 2004). It is possible to construct an inclusionary definition that contains the characteristics noted above, so that we can reliably determine that a person has dyslexia; know a great deal about cognition and the brain; and have a good idea of the sort of intervention that should be considered depending on the age and reading/writing levels of the child. There is more to be learned, but we now know a great deal.

Definitions and Classifications

To understand the importance of definition, it is important first to understand the explicit linkages of classification, definition, and identification. …