Dyslexia

In this section we will summarise some of the main findings from the investigation of adult reading problems, focusing particularly on what these findings might tell us about models of word recognition and reading. This is in line with attempts that have been made to characterise types of dyslexia in terms of parts of a normal’ model of reading aloud that have been affected by damage (Caplan, 1992 Chapter 5; Marshall & Newcombe, 1981; Morton & Patterson, 1980; Patterson & Morton, 1985).

Dyslexia affects reading processes and is therefore different from, but will often co-exist with, dysgraphia, which affects writing processes. Dyslexia can be acquired or developmental. In developmental dyslexia (Snowling & Caravolas, 2007), there is no obvious single event such as head trauma that has resulted in the problems faced by the dyslexic, and it is usually the case that the full range of normal reading skills has never been present. Acquired dyslexia on the other hand is usually the result of brain damage, resulting from a blow to the head e.g. in a motor vehicle accident or from a stroke which affects the brain’s blood supply. In this form of dyslexia, there is disruption of previously intact reading processes. Most of the research relevant to this section has been on acquired adult dyslexia. Some types are referred to as peripheral’, affecting e.g. attention, visual processing, letter identification, and so on. Our focus here is on central’ types.

The study of acquired dyslexia, as is common with much study of impaired language processing, often involves the argument of double dissociation. If one patient A performs well in one area of processing Task 1, but not in another Task 2, while another subject B performs well in the second area but not in the first, then there is a dissociation between the skills involved in these two tasks – either can be impaired while the other remains intact. Often evidence is also sought that patients A and B differ in the type or location of physical damage. It can then be argued that certain areas of the brain are the loci of certain types of processing.

Attempts have been made to characterise different types of dyslexia in terms of which parts of a normal’ model of reading aloud might have been affected by brain injury. The normal model considered in this sec tion is an extension of the dual-route model of reading aloud discussed above Coltheart, Patterson Marshall, 1980 ; Coltheart e al , 2001 . It envisages that reading aloud can occur via the three routes shown in Figure 9.8. Routes a and b are the whole-word and grapheme–phoneme routes common to most dual-route models. Route a involves access of lexical forms, including meaning, via orthography; pronunciation is accessed through the output route from the lexicon to articulation. Route b proceeds from sub-word orthography letters to sub-word phonology phonemes via a grapheme–phoneme coding GPC route, and bypasses content-based aspects of lexical representations. It is the route that has to be used for nonword pronunciation. Note that the additional route in this model compared to the simpler dual-route model presented earlier is route c. This takes the reader from whole-word orthographic representations to whole-word phonology, without involving access to the meanings associated with a word.

A number of types of dyslexia have been described in terms of disruptions of the routes marked in this model. It is worth looking at a selection of these as illustrations of how the patterns of disrupted language processing are used to provide support for a particular model of processing, using the notion of double dissociation.

 Patients with surface dyslexia typically have good reading aloud of nonsense words, indicating that they are able to make use of the spelling to-sound conversion route marked as b in Figure 9.8. However, they show poor recognition and reading aloud of real words, particularly irregular words, i.e. words that do not follow the spelling–sound rules. The errors they make when reading such words aloud tend to involve over regularisation of the spelling–sound rules. So, for instance they might read <love> as though it were <loave>, <head< as heed, <steak> as <steek>. In some patients, this difficulty particularly affects low-frequency words.

This pattern of good reading of nonsense words and regularisation of irregular words suggests that such patients have to rely on the spelling sound route, and that there is therefore some impairment involving the lexical routes a and c. Note also that comprehension is good for the words these patients can accurately read aloud, i.e. regular words. Irregular words on the other hand are often understood on the basis of how they are pronounced, so that if there is a match between the forms that the patients produce and a known word, then the visual form will be misunderstood (e.g. <head> as heed), rather than rejected as a nonword which might be the case with reading <steak> as though it were steek. This suggests that semantic lexical representations are still available, so that meanings can still be accessed in these cases, but via the pronunciation, presumably by means of the route used in recognising spoken words.

 The opposite pattern to surface dyslexia is shown in phonological dyslexia, where patients demonstrate a good ability in reading real words, whether regular or irregular, but are poor at reading pronounceable non words. This seems to indicate an impairment of the spelling–sound route (b.) Surface and phonological dyslexics taken together provide the double dissociation required to provide a functional distinction between reading routes within the lexicon (i.e. (a) and (c) in Figure 9.8 and the external route (b).

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