How Can Theoretical Accounts of Developmental Dyslexia Be Informed by the Word Superiority Effect?

Research shows that there is evidence that the word superiority effect goes in some way with which to inform theoretical accounts of developmental dyslexia. Various research using different computational models of lexical access demonstrate this, with evaluation of the dual route model being discussed here. With regard to theories of developmental dyslexia, the phonological deficit hypothesis will be explored in this essay.

The DSM-V describes developmental dyslexia as a neurodevelopmental disorder characterized by a reading impairment in spite of normal educational opportunities and intellectual functioning (American Psychiatric Association, 2013), with lifelong prevalence. Developmental dyslexia describes those otherwise normal children who are shown to have difficulties in learning to read. Reading is a highly complex task that relies on the integration of visual, orthographic, phonological, and semantic information (Ziegler et al., 2007). Studies of the condition have discovered evidence of perceptual and sensory dysfunctions (Babcock, Hogben & Fletcher, 2011), such as slower reactions times to images of faces or objects. Cognitive psychologists propose several possible reasons for developmental dyslexia with one prevailing theory called the phonological deficit hypothesis. Evidence suggests impairments in phonological awareness are at the root causes of developmental dyslexia, specifically with regard to learning to read. In phonological processing tasks, dyslexic children performed more poorly compared to age matched normal readers (Snowling, 2001). Margaret Snowling (2000) states that the way in which dyslexic children’s brains code phonology is less efficient than that of normally developing children, despite some evidence of their relative strengths with other cognitive skills such in semantic processing.

Various research demonstrates that children with developmental dyslexia show a somewhat reduced or absent word superiority effect (WSE). WSE is a phenomenon where individuals recognize letters more easily if presented within words as compared to isolated letters, and to letters presented within non-word strings (orthographically illegal, unpronounceable letter array). The WSE was first described by psychologist James McKeen Cattell (1886). Cattell’s first study was of the time of exposure necessary to read letters, words, and colours. This divided into two parts in later investigations. One was a measure of the number of simple objects that could be seen with a short exposure, the other, the time required to name the object after it was briefly presented. These first series of experiments on the span of consciousness by Cattell were fundamental to later important and evolved studies of reading, with contributions coming from Reicher (1969) and Wheeler (1970).  Both Reicher and Wheeler discovered that it is easier for participants to recognise a single letter in a short real word, than on its own or in a non-word letter string. This has since become known as the Reicher-Wheeler Paradigm and goes some way to explaining the WSE. Reicher (1969; pp279) states that a possible explanation for the word superiority effect is that letters are more quickly forgotten than words are. In regard to the WSE, compared to orthographically illegal letter strings nonwords (consonant-consonant-consonant non-words, e.g., ptm) or orthographically legal letter string pseudowords (consonant-vowel-consonant nonwords, e.g., pym) there are several aspects of cognitive processing that go some way to explaining the encoding processes of real words in the mind. Cognition such as semantics, where meaning is attached to a word, orthographic regularity, the ability to pronounce a word or letter string, and word frequency where the more frequent a word is used the easier it is to recall, making real words much more memorable and most importantly, their letter components more recognisable.

 It has also been shown that that there is a Pseudoword Superiority Effect (PSE) where letters are identified within an orthographically legal and pronounceable nonword (Cole, Rudinsky, Zue & Reddy, 1980). Explanations for both the WSE and PSE can be described as top-down processing, which refers to perception skills that are driven by the brains cognitive processes, applying what it already knows and is expecting to see, and therefore filling in any blanks.

 There are several sub-types of developmental dyslexia with their own theories of aetiology. According to Ramus et al. (2003) there are three leading theories; the phonological deficit theory, the auditory and visual magnocellular theory and the cerebellar theory. Overall, the research data supports the phonological deficit theory as the leading theory of developmental dyslexia (Ramus et al., 2003). The phonological deficit theory proposes that those with developmental dyslexia have specific phonological difficulties impairment with sound manipulation, which affects their auditory memory, word recall, and sound association skills when processing speech. The theory explains that when learning to read an alphabetic writing system it requires the learning of grapheme/phoneme correspondence, which is learning the relationship between the letter symbols and speech sounds they represent, a key skill in learning to read.

 According to Margaret Snowling (2000) a major weakness of the phonological theory is its inability to explain the occurrence of sensory and motor disorders in dyslexic individuals. She goes on to say that those who support the phonological theory do not see them as playing a causal role in the aetiology of developmental dyslexia. This also does not preclude the existence of those rare dyslexic people whose problem is not phonological, such as with visual stress (Wilkins, 1995).

 Peterson, Pennington and Olson’s (2012) explored and evaluated several different computational models to investigate the phonological deficit theory of developmental dyslexia in light of competing predictions made each. Specifically by two computational models of single word reading, the Dual-Route Cascaded model (DRC) developed by Coltheart, Rastle, Perry, Langdon, & Ziegler (2001) and Harm and Seidenberg’s (1999) Connectionist model (HS model). For the purposes here the DRC model is explained.

 The DRC is a computational model of visual word recognition and reading aloud. The dual-route consists of three components: the semantic system, the orthographic lexicon, and the phonological lexicon. As the name suggests it has two routes from the printed word to speech; a direct lexical route (orthographical input/semantic system/phonological output) and an indirect non-lexical or sub-lexical route mediated by grapheme-to-phoneme (GPC) conversion rules (see Figure 1.) According to the DRC model phonological arises from differential damage to the GPC (non-lexical) route. This model was inspired by an earlier model developed by McClelland and Rumhart (1981) and Rumhart and McClelland (1982) called the Interactive Activation Model (IA model). The IA model is a model of visual word perception. The central features of this model assumes that the processing of information during reading consists of a series of levels or ‘traces’ corresponding to the bottom-up information gained from the visual features of the words and letters, and top-down existing semantic knowledge of the word and letters (Ellis & Humphries, 1999).

 The dual route model was also used in later research by Ziegler et al., (2008) to simulate individual differences in developmental dyslexia.  (Ziegler et al., 2008) using the WSE to measure children’s access to the orthographic lexicon.

Figure 1. The DRC model of visual word recognition and reading aloud

(Coltheart, Rastle, Perry, Langdon & Zeigler, 2001).

 As discussed above, a part of the DRC model is a generalization of the IA model, and one reason for making this choice was the successes of the IA model in accounting for human data in their Reicher-Wheeler experiments. Another reason for the choice was that the IA model is a cascaded model. The IA model applied only to four-letter words with the DRC version which has words from one to eight letters in length.

 In conclusion the phonological deficit theory of developmental dyslexia has had much research looking at causes, effect and treatments to aid those with it to be able to improve their reading skills. The word superiority effect and reicher-wheeler effect have been found to be significant in both the DRC and connectionist models which can both account for the existence of phonological developmental dyslexia. According to Peterson, Pennington and Olsen (2012), in their evaluation of the different models they reviewed, they state that in the vast majority of cases of developmental dyslexia it is caused by underlying impairments in phonological representations.

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