Revisiting the double deficit dyslexia hypothesis (again) - guest post by J. Evans

The following is a guest post by Jeff Evans (SLP and Assessment Project Director with Pearson Assessments ), a member of the IQs Corner Virtual Community of Scholars project. Evans provides a much more detailed set of comments than were originally made by the blogmaster in a prior post re: this article (visit prior post to view pdf copy of article)

• Vukovic, R. K., & Siegel, L. S. (2006). The double-deficit hypothesis: a comprehensive analysis of the evidence. Journal of Learning Disabilities, 39(1), 25-47.

• Abstract: 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.

It is interesting how quickly we accept "good ideas" and begin to incorporate them into our work. Or not. I had almost come to accept the double-deficit hypothesis as fundamental to my understanding of dyslexia. This article was a reminder that good science takes time, and a community effort. Below are a few comments/conclusions from my reading of this article.

• There is limited evidence that naming speed should be characterized separately from the phonological skills family, as naming speed and phonological tasks tend to be positively correlated, and phonological interventions have been found to reduce the occurrence of naming speed deficits.

• The results from previous studies have suggested that a naming speed deficit is not necessarily a core deficit of dyslexia and that naming speed deficits characterize dyslexia only in the presence of concurrent phonological deficits.
  

• The results of this review shed light on the current evidence for the double-deficit hypothesis, identified gaps in our understanding of the hypothesis, and identified outstanding issues yet to be resolved.

• Consistent with the conclusions of a prior review (McCardle et al., 2001), the findings from this review suggest that evidence in support of the double-deficit hypothesis of developmental dyslexia remains limited.

• One could make the argument that low naming speed is characteristic of some readers with dyslexia, who are also characterized by phonological deficits, but the evidence does not support a persistent core deficit in naming speed across individuals with dyslexia.

• There is a lack of evidence to support the hypothesis that deficits in naming speed skills are independently related to reading impairment. Research that has examined the independence of naming speed from phonological awareness has demonstrated inconsistent findings, with most studies supporting naming speed as a phonological variable.

• It was difficult to explicate common theoretical, methodological, and psychometric criteria and standards used in double-deficit and naming speed studies. There was tremendous variability surrounding sample characteristics and the identification of reader types. And there is 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 independently of phonological processing.

• Much of the research cited in support of the double-deficit hypothesis of dyslexia was not actually designed with this hypothesis in mind, and there appear to be just as many theoretical as empirical studies of the double-deficit hypothesis.

• A significant gap in the double-deficit literature is the lack of properly designed intervention studies.

• As Wagner and Torgesen (1987) outlined, to evaluate the relations between a variable and a process, converging evidence must be found from cross-sectional, longitudinal, and intervention studies. With respect to the double-deficit hypothesis, the majority of the research has employed cross-sectional designs, and evidence from longitudinal studies remains sparse.
  

• Intervention studies designed to remediate dyslexia by targeting naming speed processes are particularly important to properly investigate the double-deficit hypothesis.

• A study that would contribute to our understanding of the double-deficit hypothesis would match groups of children with dyslexia who have similar phonemic awareness skills but differ significantly in RAN performance. These groups would then be randomly assigned to a phonologically based reading program and a control group, and a subtype-by-treatment interaction could be examined, also referred to as at-tribute-by-treatment interaction (ATI).

• At this time, the variation in reading ability and definitions of dyslexia employed in the double-deficit literature and the yet undefined relationship of RAN to a particular aspect of the reading process have resulted in a lack of clarity about slow naming and its specific role in reading ability.

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