Article summary with Defining and understanding dyslexia: Past, present and future by Snowling a.o. - 2020

What is meant with the discrepancy definition of dyslexia?

For many years dyslexia was seen as a specific learning difficulty, meaning that people with dyslexia have difficulties with reading and spelling that are not in line with the expectations given their age and IQ. In order to diagnose dyslexia the aim was to find a discrepancy between expected and actual reading attainment. But no qualitative differences in reading were found between those with dyslexia and those with general learning problems, causing the discrepancy definition to lose credibility and the criteria for dyslexia to be loosened.

What criticism did Stanovich have against the discrepancy definition?

Stanovich was one of the earliest critics of the discrepancy definition. According to him, learning to read has a positive effect on verbal skills and vocabulary. The verbal skills of children who read less cannot keep up with their peers (the Matthew effect) and children who are good at reading read more and get better at it. He also showed that irrespective of IQ, poor readers have the same core phonological difficulties and that there are no clear qualitative or quantitative differences in reading as a function of discrepancy.

How does DSM-5 deal with the discrepancy definition?

Taking Stanovich´s criticism into account, viewing dyslexia simply as poor reading and spelling would fail to capture the fact that it has an early onset and is persistent over time. This is why the DSM-5 uses the term ¨specific learning disorder with impairment in reading¨. It has to involve a pattern of learning difficulties characterized by problems with accurate or fluent word reading, poor spelling, poor decoding that must have persisted for at least six months despite attempts to target these issues.

How does the skill of reading develop?

Reading development is a complex process that starts with learning mappings between print and sound, and phonemes and graphemes. Learning the alphabetic principle is a form of decoding process that provides a mechanism for acquiring detailed orthographic knowledge. With experience, there is less reliance on overt decoding because word recognition becomes increasingly efficient and automatized. 

How does the triangle model describe reading development?

The triangle model is a computational model that comprises three sets of representations (phonological, orthographic, semantic) that interact and connect together to constitute the word recognition system. Before being able to read, children connect phonological knowledge (sound) to their semantic system (meaning). When learning to read, children form mappings from orthography to phonology and thus to semantics. 

What is the phonological deficit hypothesis?

The phonological deficit hypothesis states that the nature of dyslexia is auditory and narrows the deficit to one affecting sound. Problems with phonology lead to difficulty in learning mappings between orthography and phonology, learning new spoken words, word retrieval, picture naming, and verbal short-term memory. The problem with this hypothesis is that a phonological deficit is often not the only observed deficit. Also, phonological skills are dimensional and can affect one or more aspects of reading.

Which disorders are comorbid with dyslexia?

Rates of comorbidity vary, but on average almost half of the children with dyslexia also have another disorder. Common comorbid disorders are developmental language disorder, mathematics disorder, and attentional and motor coordination problems. When a strict discrepancy definition is used there are definite cases of specific dyslexia. But when that definition is dropped, more difficulties are observed among children with reading disorders. For many children, reading difficulties sit within a constellation of difficulties.

How does the loosening of the discrepancy definition affect dyslexia diagnosis?

The way that the DSM-5 tries to deal with the discrepancy definition has led dyslexia to be labeled as a categorical impairment with no clear cut-off criteria behind the diagnosis. The increasing incidence of dyslexia doesn´t necessarily reflect an increased awareness of the condition, but could just be a case of over-diagnosis. 

What recommendation does this study have for dyslexia diagnosis?

This study suggests four criteria for diagnosing dyslexia:

• The term dyslexia should refer to a difficulty with decoding and spelling fluency which is evident from the early school years and persistent over time.
• It should affect academic functioning in a way that progress is less good in literacy-based areas than that of peers in a similar setting.
• Co-occurring features should be labeled but not considered core to the diagnosis.
• The diagnosis should be qualified as mild, moderate, or severe. Those with severe difficulties will more often suffer from comorbid conditions.