1.7.2 Differentiating within dyslexia – acquired versus developmental dyslexia and the search for subtypes
There has also been continued debate regarding the variability within any dyslexic population, the apparent variety of forms that dyslexia can take. Given the complexity of the skills required to develop fluent reading and spelling perhaps this is not surprising. The variability within dyslexia may simply reflect the fact that this complex process can go wrong in different ways and for different reasons.
The term ‘dyslexia’ was originally used to refer to the acquired dyslexias – specific disorders of reading or writing that can follow from brain injury in adults. The study of people with such acquired difficulties has provided invaluable insights into how written language may be processed in the brain. These people often suffer selective impairments, where some abilities are lost while others are preserved. For example, people with acquired phonological dyslexia may have no problem reading familiar words by sight, but they can no longer ‘sound out’ unfamiliar words, or find a pronunciation for nonsense words like ‘flad’. Conversely, some people with surface dyslexia may have no trouble with regularly spelt words such as ‘bat’ or pronounceable nonsense words such as ‘pux’, but they seem to have lost the ability to recognise words purely by sight (i.e. without decoding each letter). They tend to misread even familiar words like ‘pint’ – which they would pronounce to rhyme with ‘flint’. Even more bizarre are the errors made by people with deep dyslexia, who may misread ‘lion’ as ‘tiger’, or ‘symphony’ as ‘orchestra’. They seem to have difficulties with both the visual and phonological components of reading, and yet their errors suggest that they still have some access to the meaning of words they cannot read.
The different patterns of impairment found in the acquired dyslexias indicate that skilled reading requires the interplay of many sub-processes. It is therefore not surprising that the search for subtypes in developmental dyslexia has been modelled on the different kinds of acquired dyslexia. However, there are good reasons why comparisons between acquired and developmental reading disorders may not be appropriate. For example, there is evidence that the development of literacy skills is not an additive process (where the earliest skills are extended and elaborated upon in later stages), but may involve substitution, with later stages or processes replacing earlier ones (Morton, 1989). If so, the development of reading and writing skills may be more akin to a metamorphosis, such that the mature written language processing system bears little relation to its early components. There are likely to be important differences between a system that developed ‘normally’ but was then damaged, and one where the process of development itself was abnormal.
The search for subtypes within developmental dyslexia has engendered much research, but no clear and consistent subgroups have stood the test of time and experimental investigation. The most frequent distinction has been between auditory problems (i.e. difficulties in identifying and manipulating letter sounds within words) and visual problems (difficulties in visually recognising and remembering words). These categories broadly resemble the phonological and surface forms of acquired dyslexia. However, many people with developmental dyslexia show both types of impairment. It has even been suggested that both kinds of difficulties have a common basis, as we shall see later. The truth is that no simple picture emerges from attempts to define subgroups, and it may be more appropriate to think about several distinct but overlapping components to dyslexic type difficulties. We shall return to the issue of subtypes in Section 2 of this course.