Thursday, March 23, 2006

The double deficit dyslexia hypothesis revisited

There is little doubt that the "double deficit" hypothesis is on of the most prominent models advanced to explain severe reading disabilities (dyslexia). It is refreshing to see a recent JLD review that casts fresh eyes on the validity of the DD hypothesis, and more importantly, a closer look at what the two core disabilities are, are not...or.....whether there is any agreement in the field re: what they represent in terms of basic underlying cognitive processes.

Below is the reference and abstract. I would encourage assessment professionals working with the education of students with reading problems to give this article a read. The authors conclude that the DD hypothesis may be given to much creedance, largely due to ubiqutous problems in research methodology across a diverse set of studies(e.g., use of different measures; different operational defintions of the core constructs; different theoretical explanations; etc.). The authors conclude that "the existing evidence does not support a persistent core deficit in naming speed for readers with dyslexia."

Although I need to reconcile this conclusion with the extant postive research that suggests that naming speed (Glr-NA: Naming Ability in the land of CHC theory)is significantly related to early reading success, such a review does give one pause. As with all highly visible established research lore, maybe the DD hypothesis, although true, may be over-stated. The article provides a nice overview of the various issues in the methodological problems in this research, competing theoretical explanations, and different attempts to define what measures of RAN (rapid automatic naming) actually measure.

Vukovic, R. K. Siegel, L. (2006). The Double-Deficit Hypothesis: A Comprehensive Analysis of the Evidence. Journal of Learning Disabilities, 39 (1), 25-47. (click here to view/download)

  • The double-deficit hypothesis of developmental dyslexia proposes that deficits in phonological processing and naming speed represent independent sources of dysfunction in dyslexia. The present article is a review of the evidence for the double-deficit hypothesis, including a discussion of recent findings related to the hypothesis. Studies in this area have been characterized by variability in methodology—how dyslexia is defined and identified, and how dyslexia subtypes are classified. Such variability sets limitations on the extent to which conclusions may be drawn with respect to the double-deficit hypothesis. Furthermore, the literature is complicated by the persistent finding that measures of phonological processing and naming speed are significantly correlated, resulting in a statistical artifact that makes it difficult to disentangle the influence of naming speed from that of phonological processing. Longitudinal and intervention studies of the double-deficit hypothesis are needed to accumulate evidence that investigates a naming speed deficit that is independent of a phonological deficit for readers with dyslexia. The existing evidence does not support a persistent core deficit in naming speed for readers with dyslexia.

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