Dyslexia: Phonological Core Deficit or Multiple Subtypes?

At the heart of the current accepted definition of dyslexia is the phonological deficit, which is considered to be the cause of inaccurate and dysfluent word recognition. An inability to discern the individual phonemes within spoken language leads to great difficulty when it comes time for a child to learn how to read, for phonemic awareness is a necessary precursor to being able to grasp the alphabetic principal. Without solid phonemic awareness and an understanding of the alphabetic principal, children will not be able to effectively or efficiently unlock the code of print, which thus severely impacts a child’s ability to decode, comprehend, spell, and compose.

While the current definition of dyslexia highlights the fundamental deficit in the phonological component of language, some researchers have proposed other potential causes or subtypes of dyslexia.

Many in the field have observed deficits in naming speed within the dyslexic population, prompting much research into understanding the cognitive dynamics of naming speed. From researchers such as Maryanne Wolf, a double-deficit hypothesis has arisen in which phonology is but one possible, albeit primary, deficit underlying dyslexia. Through their studies, naming speed has been shown to be a separate cognitive process from phonology. Additional studies confirmed the existence of distinct subgroups among impaired readers: there were those with phonological deficits, those with naming-speed deficits, and those with both phonological and naming-speed deficits, thus the “double-deficit” subgroup (Wolf, 1999).

Wolf stresses that the double-deficit hypothesis is just a beginning; a multi-dimensional model including both deficits and strengths are needed. Therefore, the three subgroups proposed in the double-deficit hypothesis are only the beginning of a field of study that is intent on identifying and classifying multiple subgroups of dyslexia.

In a different conceptualization based on the simple view of reading (SVR), Tunmer and Greaney identify three subgroups of reading difficulties: one based on phonological deficits (this would be the group defined as dyslexic), another based on reading comprehension difficulties, and the third the mixed reading disability (2010).

The research into subtypes is redefining dyslexia as more than a reading disorder and instead as a combination of cognitive deficits. Pennington proposed that dyslexia must be viewed as the result of a complex combination of cognitive deficits that create an environment ripe for comorbid conditions such as ADHD, speech sound disorder, and autism, and that no single or even double-deficit theory can account for the cause or symptoms of dyslexia (2006).

In a recent study, researchers from the University of Amsterdam sought to identify specific subgroups of dyslexia. Although they were unable to clearly identify distinct subgroups, they isolated five describing factors of dyslexia: spelling, phonological, short-term memory, rhyme/confusion, and whole-word processing/complexity deficits (Tamboer, Vorst, and Oort, 2014), highlighting that there are multiple cognitive processes involved in defining dyslexia that extend beyond mere phonological deficits.  

We’re looking forward to #DyslexiaCon19’s Friday afternoon session, Consensus on the Dyslexia Definition: Then, Now & Next, chaired by Nancy Hennessy. What kind of modifications in the definition of dyslexia will be discussed? What kind of revisions would you propose? Come talk to us about these questions at the Moose Materials booth! 😊  

Pennington, B. (2006). From Single to Multiple Deficit Models of Developmental Disorders. Cognition, 101, 385-413.

Tamboer, P., Vorst, H., and Oort, F. (2014). Five Describing Factors of Dyslexia. Journal of Learning Disabilities, 49 (5), 466-483.

Tunmer, W. and Greaney, K. (2010). Defining Dyslexia. Journal of Learning Disabilities, 43(3), 229-243.

Wolf, M. (1999). What Time May Tell: Towards a New Conceptualization of Developmental Dyslexia, Annals of Dyslexia, 49, 3-28.             

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