Before we proceed any further into more technical aspects of this theory, it is worth reminding ourselves why individuals with dyslexia find reading to be far more difficult. The phonological model is central to the understanding of dyslexia since it’s a key element within the language system and it functions independently from hierarchical cognitive and linguistic functions (e.g. intelligence). The ability in learning to read is an age old process unlike speaking which is thought to be much more of a habitual process occurring at a pre-attentive state. Although reading fluency in children is improved with appropriate early age instruction, the biological pathway that governs this process needs special mention. English, being a universally spoken language, is phonemically complicated in that it is comprised with 26 letters of the alphabet which creates up to 44 potential sounds by incorporating roughly 70 combinatory letters. For instance, the word "cat" contains three phonemes, namely /k/, /æ/, and /t/, but in actual fact a person listening to this would hear it as one complete word, "cat", rather than three separate sounds. When this word (cat) is spoken, the three separate sounds get merged together to produce one word. But when it comes to reading, one has to be aware that there are three sounds which govern this one word. Therefore, learning to write requires the need to break down the constituent syllables of a particular word into unique smaller speech sound segments (phonemes)
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symbolised by a combination of few letters (grapheme). However, in learning to read, especially when it comes to nonwords, the reader should be first able to visually recognise letter shapes, the particular order in which they appear and the visual word form they are most likely to represent (orthography). The next most crucial process is to depict the meaning (semantics) of this particular word rapidly, automatically and accurately, and for this to happen, the graphemes have to be linked with its corresponding phonemes (grapheme-to-phoneme correspondence). Unlike with nonwords, for familiar words, the initial orthographic stage retrieves the meaning from previously stored representations in the lexicon within the semantic memory, the process which is backed by the phonology-to- semantics loop within the language system. Therefore, an individual’s working knowledge of the phonological sound structure of speech (phonemic/phonological awareness) is a prerequisite, as it then predicts how good the grapheme-to-phoneme correspondence is for the normal development of reading ability.
The phonological theory is said to be one of the most influential indicators of dyslexia which has gained a huge interest among many other areas of dyslexia research. Theorists have taken two stances when it comes to describing the nature of phonological deficits studied from dyslexic individuals (figure 2). At the neurological level, reports from both the anatomical (Galaburda, Sherman, Rosen, Aboitiz, & Geschwind, 1985) and functional imaging (Shaywitz et al., 2002) studies suggest that the foundation of the phonological problem is a resultant congenital dysfunction of the left hemisphere perisylvian brain areas, which is thought to hinder either the development of phonological representations and/or the connectivity linking both the phonological and orthographic representations. Meanwhile, others theorists proposed a deficit at the cognitive level, i.e. within the language system, which further manifests in a direct behavioural problem. The reading and spelling problems encountered by individuals with dyslexia was thought to be an underlying phonological impairment, i.e. an inability to break down a spoken word into constituent phonemes such that they are unable to develop the necessary grapheme-to- phoneme correspondence, all this despite an intact semantic system (Frith, 1997; Griffiths & Snowling, 2002; Snowling, 1981). Support in favour of the phonological theory comes from studies carried out largely in a population of normal readers, the findings which were then compared to individuals with dyslexia matched for age and IQ. In such a setting, the phonological awareness of an individual is thought to be a good predictor of reading ability. Studies in fact have shown poor performances demonstrated by dyslexic individuals in tasks requiring phonological awareness, e.g. naming speed (Rapid Automatised Naming – RAN, i.e. the retrieval and naming of digits, objects or colours in a rapid fashion: Torgesen, 1999) and nonword repetition (Pennington, Van Orden, Smith,
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Green, & Haith, 1990). Moreover, phonological deficits have been previously pointed out in individuals with dyslexia having demonstrated poor performances in slow automatic naming, verbal short term and working memory (temporary storage), these deficits possibly arising due to difficulties associated in the storage and retrieval of unique orthographic representations, along with shortcomings in the information processing pathway (Snowling, 2000). Studies which offered pre-training of phonological awareness skills to both the dyslexic and control groups has enabled the dyslexic group to improve their standards of reading significantly (Castles & Coltheart, 2004), which further goes to show the importance of early reading instruction.
Figure 2: Proposed cause of dyslexia according to the phonological deficit theory.
Any genetic abnormality during the brain maturation stage results in changes which affects both the asymmetricity of the planum temporale (which covers a part of the Wernicke’s speech area) and the regularity of the perisylvian region. This is then thought to affect the phonological language system at a more cognitive level, the effect which can certainly be minimized using appropriate remediation strategies. Any shortcoming in phonological processing therein hinders the conversion of grapheme-to-phonemes which leads to poor spelling and reading, in addition to a weak phonological awareness (inclusive of a phonological deficit). A phonological deficit alone is thought to result in an impaired short-term memory and naming skills at a behavioural level. [Source: Frith (1997)]
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While studies have demonstrated such strong links between reading ability and phonological skill, critics have argued whether such phonological deficits were actually a causal deficit or one that was influenced by an external third party. Supporters of the phonological deficit theory casted doubts over initial findings with respect to the phonological deficits observed in dyslexic populations thought to be a result of co- occurring visual difficulties. For instance, a study by Ellis (1981) carried out between controls and CwDys in a task required both groups to make appropriate judgements when matching letters that were ether similar or different. Findings from this study showed that, compared to the control group, the speed of matching judgements by the dyslexic group were far slower in tasks which tested for name encoding skills (e.g. Xz and Xz) unlike that for letter pairs which were visually similar (e.g. XX and zz). Another study by Elbro & Jensen (2005) tested a group of dyslexic and normal reading children on the ability to form phonological representations of lexical items, i.e. pseudo-names and known words, using a single word decoding task. Findings showed that, compared to the dyslexic group, the reading age controls had the better of nonword reading and phoneme awareness tasks. Although, both the groups showed equal ability to rapidly learn to associate familiar names with images of people, the time taken in learning to relate a list of pseudo-names with pictures were far too long in the dyslexic group. When the procedure was repeated using long familiar words, the dyslexic group demonstrated a shorter learning period compared to controls in the imitation task. All of these findings suggest the weakly specified phonological representations to be a fundamental problem in dyslexia independent of visual difficulties. Further support for an underspecified phonological representation, rather than an analytical problem, comes from a large array of studies (e.g. Boada & Pennington, 2006; Snowling & Hulme, 1989).
There have also been quite a few cases of dyslexic individuals reported to have demonstrated intact phonological skill (e.g. Curtin, Manis, & Seidenberg, 2001; Valdois et al., 2003). For instance, scholars have looked at potential speech perception deficits in dyslexia to be an area of concern when associating it with the weaker phonological awareness skills. Clinical observations in the past have mentioned possible word finding difficulties (i.e. despite being able to select correct word/s by the help of a reference/cue, it is the inability to construct a target word independently which then leads to behaviours including repetitions, omissions, substitutions, insertions and delays) within a child dyslexia population (e.g. Messer & Dockrell, 2006; Snowling, Wagtendonl, & Strafford, 1988). Similarly, studies providing a measure of pronunciation accuracy have demonstrated findings consistent with the argument that the phonological representations may have been omitted (Elbro, Borstrom, & Petersen, 1998). However, upon replication of
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Elbro and colleagues study, Fowler & Swainson (2004) claimed that the group differences were of a significantly smaller margin and this was the case only in a few of their experimental measures, suggesting that these results may have been due to a developmental co-morbidity between dyslexia, specific language impairment, speech sound disorder, or a resultant delay in speech. In response to this finding, a study carried out a few years earlier by Joanisse, Manis, Keating, & Seidenberg (2000) examined how three subgroups of CwDys (phonological dyslexics, developmentally language impaired and delayed-type dyslexics) performed in a task which demanded good language and reading skills. Both phonological dyslexics and developmentally language impaired individuals demonstrated identical below par reading patterns indicative of poor phonological skills. More importantly, a clear speech perception deficit was observed in both the language impaired and delayed-type dyslexics, whilst a phonological impairment was also evident in children whose speech perception was normal. Given the language skills of normal children showed a close resemblance to the delayed-type dyslexics, it was concluded that a related reading deficit may have been solely due to a general delay in reading than a phonological impairment.
Findings from several behavioural studies indicate dyslexic individuals with phonological deficits to also have poor verbal short-term memory (Baddeley, 1986) and working memory (Swanson, 1994) due to ineffective phonological coding (Snowling, 2001; Vellutino et al., 2004). Within the working memory, of interest was a particular impairment which was found to occur within the central executive (Beneventi, Tonnessen, Ersland, & Hugdahl, 2010; Reiter, Tucha, & Lange, 2005; Sela, Izzetoglu, Izzetoglu, & Onaral, 2012), the control centre thought to be in charge with modulation of the articulatory loop and visuo-spatial sketchpad (specialised for short-term storage and manipulation of spatial and visual information, Baddeley, 2003), information retrieval (from long-term memory) and integration of storage capacities, the latter two functions thought to be further supported by another component referred to as the episodic buffer (specialised for short- term storage with an ability to merge various types of information to form a given episode, Baddeley & Jarrold, 2007). Also, it assists in coordination of attention functions, namely attention focusing and exclusion of irrelevant stimuli (Baddeley, Gathercole, & Papagno, 1998), splitting of attention (Baddeley, Chincotta, & Adlam, 2001), and switching of attention between peripheral visual fields (Baddeley, Baddeley, Wilcock, & Bucks, 2001) towards incoming verbal and visual information. Therefore, any short comings with the central executive or associated components could result in an impairment of visuo-spatial attention, further hampering language comprehension and reading. For instance, it was reported that an impaired central execute function resulted in individuals with ADHD to
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perform poorly in tasks which required exclusion of irrelevant stimuli (Gathercole & Alloway, 2006). Individuals with lower span have also demonstrated inattentiveness with a greater tendency to be forgetful with regards to the study instructions given to the participants before conducting a certain task, which lead them into overcomplicating the task assigned to them (Gathercole, Alloway, Willis, & Adams, 2006; Gathercole et al., 2008). In essence, the difficulty experienced by such a group of individuals came down to a poor ability in focusing and switching attention, in addition to problems concerned with assimilating information coming from neighbouring components of the working memory. Moreover, some scholars suggested that reduced visual attention span in dyslexic individuals was connected to a limited processing resource effect, i.e. difficulty in maintaining information in the working memory, evident only when the experimental procedure requires a higher memory load, thus further affecting reading ability (Beneventi, Tonnessen, Ersland, & Hugdahl, 2010; Swanson & Sachse-Lee, 2001).
When considering the adult dyslexic population, studies claim that despite phonological skills being intact, the problem may potentially be in the ability to gain access to these phonological representations. This claim was based on a comprehensive sequence of studies carried out within AwDys in addition to in-depth review of the cognitive dyslexia literature (Ramus et al., 2003; Ramus & Szenkovits, 2008). Phonological short-term memory is considered to play a highly important role in forming sound based representations of written text, i.e. converging both the graphemes and phonemes of a word segment while holding on to the remaining segment within the memory. Ramus & Szenkovits (2008) favoured the use of simple rhyme judgement tasks to investigate the exact nature of short-term memory deficits between AwDys and control groups on a task requiring participants to differentiate two nonword sequences, one which rhymed but differing by just a single phonetic element (e.g. "jaz" and "jaʓ"), and the other with non- rhyming nonwords (e.g. "taz" and "bam"). Compared to controls, AwDys were significantly affected when differentiating between both word sequences. Although dyslexics demonstrated a phonological similarity effect just as the control group did, the performance for AwDys gradually improved parallel to that of controls as the phonological similarity reduced. This evidence was sufficiently enough for the authors to claim that phonological representations were perfectly intact in AwDys, and should that be not the case, they insisted that dyslexic individuals should have exhibited a higher phonological similarity effect than controls. Hence, it was proposed that the phonological difficulties in dyslexia were due to a defect in the short-term memory processes regulating phonological representations, and certainly not a direct phonological deficit per se. Furthermore, the study by Boets & colleagues (2013) also supported the view of Ramus & Szenkovits
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(2008) by presenting neural evidence linking dyslexia to intact phonological representations (similar to that of skilled readers) with poor accessibility for efficient higher-order phonological processing. The findings from both these studies clearly oppose the principal view held by proponents of the phonological theory, but it must be said that a majority of studies (especially electrophysiological and speech-perception types) have demonstrated degraded phonological representations within the child population, inclusive of both CwDys and pre-readers with a possibility of undetected dyslexia. Moreover, the controversy over the past decade or so concerning the exact source reflecting impaired speech processing in dyslexia have been circulating around both input (e.g. Ramus, 2001; Szenkovits & Ramus, 2005) and output (e.g. McCrory, Mechelli, Frith, & Price, 2005; Szenkovits & Ramus, 2005) processes, although a definite locus is yet to be identified on a consistent basis. Nonetheless, proponents of the phonological deficit theory remain firm in their stance claiming phonological deficits to be the sole cause of dyslexia.