2.2 Cognitive explanations of dyslexia
2.2.1 The phonological processing deficit
Recall Alexander Faludy's difficulties in learning to read and write, and the other behavioural characteristics associated with having dyslexia. You might have noticed that many features of dyslexia point to a difficulty with some aspects of memory. That is, people with dyslexia have difficulty with tasks that require short-term memory processing such as mental arithmetic, writing and learning new information. However, these tasks have an additional feature in common: they contain a phonological component. That is, they involve the processing of speech sounds in short-term memory. It is therefore possible to suggest that a deficit in phonological processing may provide an explanation of dyslexia. To understand why a phonological deficit would have an impact on reading and writing we need to understand how people typically learn to read (see Box 6).
Box 6: Learning to read (after Frith 1985)
It has been suggested that initially we adopt two strategies. One strategy, widely suggested to be the first to develop in beginning readers, is the whole word, or logographic strategy. This refers to the way children learn to associate a spoken word with its written form, without showing any awareness of the sounds that each of the individual letters make. This strategy is often encouraged in early years classrooms where objects are labelled with their names and teachers use ‘flashcards’ to teach children a core ‘sight vocabulary’ of common words. This technique is useful in enabling children to build a large sight vocabulary quickly, which will enable them to begin reading with some degree of fluency. However, this approach places huge demands on visual memory and does not provide children with a strategy for coping with unfamiliar words.
To address these limitations, children also need an alphabetic decoding strategy. This requires them to learn the sounds that each letter of the alphabet makes, and then learn how to blend those sounds together during reading to work out how to pronounce the word. Alphabetic decoding is also needed during spelling to analyse spoken words and break them down into their corresponding letter sounds.
A skilled reader is one who moves beyond letter by letter decoding and rapidly processes longer ‘strings’ of letters that recur across different words (an orthographic strategy).
Did you notice how these strategies reflect what we know about reading processes from the acquired dyslexias?
The alphabetic decoding strategy draws heavily on phonological processing – both in the learning of letter-sound correspondences, and in the manipulation of those sounds during reading and spelling. People with dyslexia often find it difficult to move beyond a logographic strategy and problems with spelling usually persist into adulthood.
The severity of the phonological deficit is best demonstrated by the awareness of rhyme by children with dyslexia. Recognising that two words rhyme is a skill that most children acquire at an early age. However, studies have repeatedly shown that children with reading difficulties have trouble identifying words that rhyme (e.g. Bradley and Bryant, 1978). This is just one finding from a large literature showing that children with reading difficulties find it difficult to isolate and manipulate sounds in words.
What is still not clear is whether the phonological deficit is related to the encoding or retrieval of phonological representations in memory. While there is evidence of difficulties in processing phonological information in short-term memory, there is also speculation that the way this information is represented and stored in long-term memory could further explain the poor performance of people with dyslexia on phonological tasks.
Much of the research into phonological awareness and reading disability has centred on English-speaking children. However, this presents a misleading picture, as letter-sound correspondences in English are complex. Often, the same sound can be spelt a number of different ways (e.g. /f/ can be spelt f and ph), and the same letter can make a variety of different sounds (consider the sound that ‘a’ makes in ‘bat’, ‘part’ and ‘apron ’). Furthermore, it is not a simple case of one letter per sound: mouse has five letters but only three sounds: /m/ /au/ /s/. The phonological awareness deficit that has been demonstrated with English speakers may not be a universal characteristic of reading disability, as many other European languages have much more predictable letter-sound correspondences. Research into phonological deficits in other languages is ongoing, but there does seem to be evidence of phonological deficits in people with dyslexia (and at risk of dyslexia) who learn to read in more regular languages (see Courcy, Béland and Pitchford, 2000; Müller, Saarenketo and Lyytinen, 2000). It has also been suggested that measuring the speed of performance on tests may be a more universal indicator of reading difficulties across languages.
Almost all types of reading difficulty appear to be characterised by a phonological processing deficit, not just dyslexia. However, this does not mean that because it appears to have the same underlying cognitive deficit as other reading difficulties, dyslexia is the same as other types of reading difficulty. While the phonological processing deficit may ‘explain’ the reading and writing difficulties associated with dyslexia, it cannot account for the full range of behavioural symptoms that are observed, and that make dyslexia a distinctive condition. Other cognitive accounts are needed to explain the origins of the other behavioural symptoms of dyslexia. We have already proposed in the previous section that dyslexia can be thought of as consisting of several overlapping dimensions – it seems likely that a phonological deficit may be just one of several cognitive components associated with the condition. Moreover the phonological deficit hypothesis is exactly that, a hypothesis. While it has a good deal of empirical support, it is a theoretical proposal – but not something that we know definitely exists.