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Phonological Deficit Hypothesis
The Phonological Deficit Hypothesis is a cognitive based causal theory of dyslexia, a reading disorder characterized by poor reading and spelling. The hypothesis states that the difficulties in reading found in dyslexia are the result of poor phonological processing. There is much convergent evidence from research showing that individuals with developmental dyslexia experience difficulties with phonological awareness, verbal short-term memory and naming. In addition, the hypothesis is supported by longitudinal studies investigating the predictive cognitive factors of reading ability.

There are criticisms of the phonological deficit hypothesis, with some stating it does not account for all the deficits found in dyslexia, for example visual , auditory  and motor control. Nor can it account for the presence of dyslexia without phonological processing difficulties. There are also criticisms of the phonological deficit being a circular argument, claiming that many of the studies supporting the phonological deficit hypothesis, first assess children's phonological awareness, then at a later time point assess their reading through measures that focus primarily on phonological processing.

Phonological Processing
The phonological deficit hypothesis, is defined as a difficulty in phonological processing, which is a broad category of cognitive processes used to interpret the sounds of a given language in both written and oral forms. The role of phonological processing in the acquisition of reading ability has been demonstrated through longitudinal  and response to intervention   studies. It can be accommodated within models of reading acquisition and been found to persist through life, concurrent with modern definitions of dyslexia .There are three key processes which define phonological processing: phonological awareness, verbal short-term Memory, and naming. For each of these factors of phonological processing there are different methods of measurement:

Phonological Awareness
Phonological Awareness is a persons ability to identify and manipulate the sound structure of their language. Liberman noted that reading ability is an independent, unnatural language skill unlike verbal language which she viewed as natural, and develops naturally over time. Reading, however, must be explicitly taught and requires an awareness of structure of the spoken language. As a child grows up, their phonological awareness develops from an ability to isolate larger sound units such as words or syllables, to onset and rimes, and finally phonemes, the smallest sound unit of a persons language. There are several methods of assessing a persons phonological awareness, for example:


 * Deletion: A child is required to repeat a word or non word with a particular phonological unit removed (syllable, onset or rime, phoneme). As an example, consider the non-word floop. In a phoneme deletion task, the child may be asked to repeat the non-word with the first phoneme missing. In order to do this the child must be able to identify the first phoneme of the non-word floop, which in the this case is /f/, remove it from the non-word and say loop.
 * Isolation: A child is required to repeat a particular phonological unit (syllable, onset or rime, phoneme) in a word or non-word. As an example consider the non-word tash. In a phoneme isolation task focusing on isolating the second phoneme the child would respond with /æ/.
 * Blending: A child is required to combine a number of phonological units (syllable, onset or rime, phoneme) together to form a word or non-word. Consider the word bat. In a phoneme blending task, the child would be presented with the three phonemes: /b/, /æ/, and /t/, and they would then combine these phonemes to make the word bat.
 * Spoonerisms

Verbal Short-term Memory
Children and adults with dyslexia have been found to have difficulties with verbal short-term memory (STM) which has been linked to the phonological deficit hypothesis. This has been demonstrated through word or digit list repetition, whereby the child will hear a list of words and then is expected to repeat them. Issues with verbal STM in dyslexia have been long established, but more recently questions as to the true nature of STM deficits in dyslexia have been raised. Commonly used methods of assessment, such as word list or digit span, require the individual to store both item information and order information. Effective item storage in STM is associated with language skills, specifically phonological awareness. However, order information is thought to be independent of language skills. Therefore, under the phonological deficit hypothesis it would be natural to assume that STM item storage would be effected by dyslexia but not order. This would therefore suggest that STM issues are the product of a phonological deficit and not a specific issue with STM. However several studies have found deficits in order processing which cannot be so readily explained by phonological deficit hypothesis.

Naming
Naming tasks are typically measures of verbal or word label retrieval from long-term memory at the presentation of a visual stimulus, for example saying ball when a picture of a ball is presented. This type of naming task is known as confrontation naming and it is thought to be a measure of the number of words a child can produce, otherwise known as expressive vocabulary. It is thought that good naming ability is associated with strong links between word meaning (semantics) and their phonological representations. In dyslexia, it is argued that naming ability is hindered by poor phonological representations of words but maintain adequate semantic knowledge. In an early comparison study between dyslexic children and chronologically age matched and (younger) reading age matched children, differences were found between confrontation naming and word/picture matching. Word/picture matching is an alternative naming task, whereby a child is told a word and they pick the most appropriate picture from a range available. It is thought to be a measure of the number of words a child can comprehend but possibly not produce, this is otherwise known as receptive vocabulary. In this study dyslexics were found to score poorly compared to both groups on confrontation naming, but not on word/picture matching, suggesting that the deficit is present for expressive vocabulary but not receptive. These findings have been repeated, comparing against chronologically age, reading age, and non-dyslexic poor reading groups. An example of a standardized confrontation naming task would be the Boston Naming Test initially developed for testing Alzheimer patients, however has since been adapted for use with children.

An alternative naming task is rapid automatized naming (RAN), which requires participants to name visual stimuli (objects, letters, digits and colours) arranged in a grid at speed. RAN has been consistently found to be highly predictive of children's later reading ability in English and other languages of simpler orthography. RAN can also distinguish between age matched dyslexic and non-dyslexic children. Although a measurement of naming ability, researchers are divided upon what cognitive skills RAN utilizes. Some believing it is another function of phonological awareness, others claiming it to be a measure of reading fluency.

Orthography
A key debate within phonological deficit literature is the role of a languages orthography on it's development, particularly in alphabetic languages of which there is varying complexity. Some researchers have noted that much of the research into phonological awareness focuses on the English language, which is considered as being one of the most complex alphabetic languages, sometimes referred to as having an opaque orthography. Researchers have compared the performance of children on phonological awareness between languages of different orthographies such as English and German, and have found that children who speak languages with more transparent orthographies are more adept at phonological awareness tasks. It is thought that differences in grapheme to phoneme mapping are not as consistent in English as they are in languages such as German and this results in poorer phonological awareness in both typically developing children and those with dyslexia. However, children with dyslexia in more transparent orthographies still performed poorly on reading skill assessments but not to the same extreme as those who spoke English. In addition research into the predictors of reading ability in more transparent orthographies found comparable results as those in English studies, with the exception of phonological awareness, which grows increasingly predictive the more opaque an orthography of a language.

Convergent evidence has also been found in Chinese, which differs in structure to alphabetic languages as it is logographic. In the Chinese language the smallest sound units are multi syllabic morphemes, rather than phonemes. However, Chinese does have some phonological components within it's written form and studies into skilled readers of Chinese show phonological effects on word recognition. Although orthography has some effect on the phonological processing in dyslexia, specifically within phonological awareness tasks, it seems that it does have some universal cognitive function in reading ability.

Criticisms
In recent decades numerous studies have been conducted that support the phonological deficit hypothesis, however a number of researchers have raised issues about it's claim that poor phonological processing causes dyslexia. There are several alternative causal theories of dyslexia. Auditory processing deficit, is one such alternative theory. It posits that the deficits found in dyslexia, such as those in phonological awareness, are in fact the product of underlying auditory perception problems. Many of the researchers who support the auditory deficit theory, claim that processing impairment in short sounds and fast transitions between sounds prevent adequate development of phonological awareness. However within these studies there are multiple methodological issues, as well as low prevalence rating of auditory deficits in individuals in dyslexia. Visual perception deficits have also been suggested as an alternative causal theory of dyslexia. One of the more prominent visual based deficits is the Magnocellular deficit. The Magnocelluar deficit theory suggests that difficulties arise in visual perception due to deficits in the magocelluar/parvocellular region of the visual pathway. However like the auditory hypothesis, prevalence of a visual deficit is much lower, and less predictive of poor reading ability when compared to phonological processing.

Despite methodological issues and low prevalence rates, the presence of other deficits within dyslexia raises important questions about the validity of the phonological deficit. Researchers who support alternative causal theories, do in the large accept the presence of the phonological deficit, however they do not acknowledge it as the sole causal deficit. Supporters of the phonological deficit therefore are adapting the existing theory into one of a more dimensional causal theory in which the phonological deficit is the most prolific and widespread difficulty, but is modulated in various degrees by other existing deficits, such as potential auditory and visual deficits.