01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 37
have shown that over time the two subtypes are inconsistent in their relative proportions
(Sprenger-Charolles et al., 2001).
Diagnoses of dyslexia are unstable over time.
Are there phonological and surface subtypes of dyslexia?
Before the term ‘dyslexia’ was used in a developmental context, it had been applied to alexia,
or traumatic loss of the ability to read (Anderson & Meier-Hedde, 2001). Implicit in the new
application was a belief that the difficulties of developmental dyslexics could be conceptualised
in the same way as the difficulties of previously-competent readers whose abilities had been
impaired by a stroke or other brain injury. Since traumatic loss of reading ability was
associated with focal damage (Pickle, 1998), the dysfunction in developmental dyslexia was
attributed ‘most probably to defective development of that region of the brain, disease of which
in adults produces practically the same symptoms’ (Morgan, 1896).
The appearance of two distinct kinds of selective impairment in stroke patients meanwhile
suggested a ’dual route’ model of normal reading in which words are recognised either as
wholes (the ‘semantic’ or ‘lexical’ route) or as letter-strings in which the graphemes have to be
converted into phonemes before recognition is possible (the ‘phonological’ or ‘nonlexical’
route). At best, the subtypes represent trends; even among stroke patients, no ‘pure’ cases of
selective impairment have been reported (Coltheart & Davies, 2003).
Just as selective impairments in either the ability to read ‘irregular’ or ‘exception’ words with
an atypical spelling-sound relationship (‘surface’ alexia) or the ability to read ‘regular’ but
unfamiliar letter-strings (‘phonological’ alexia), or both, may be caused by traumatic loss of
function, it has been argued that a similar pattern of dysfunction, while lacking such specific
impairments, might be found in developmental dyslexia (Castles & Coltheart, 1993). However,
although ‘surface’ and ‘phonological’ subtypes are consistent with the ‘dual route’ model of
reading, they can also be explained within connectionist models (Harm & Seidenberg, 1999;
Manis et al., 1996; Plaut et al., 1996). A ‘connectionist model’ is a computational, neural
network model of parallel distributed processing by which a great deal of information latent in
the environment can be derived, using simple but powerful learning rules (Elman et al., 1996).
If, in addition, connectionist models can account for different kinds of alexia, then the notion of
selective disorders of reading among stroke patients also comes under challenge (Patterson &
Lambon Ralph, 1999).
Does this then represent a challenge to the concept of selectivity in developmental dyslexia and
with it a challenge to the idea of ‘surface’ and ‘phonological’ subtypes? How persuasive is the
evidence for a widespread belief that visual (or orthographic) and phonological mechanisms
represent equivalent alternatives for acquiring skilled word recognition (Share, 1995)? What is
the role of phonic and whole-word methods of teaching word identification (Snowling, 1996)?
How should we understand the distinctions drawn between ‘dyseidetic’ and ‘dysphonetic’
subtypes of disabled readers (Boder, 1973), or between ‘Phonecian’ and ‘Chinese’ readers
(Baron & Strawson, 1976)? How valid is the analogy between developmental difficulty and later
loss of function? Could it be true that ‘the whole question of subgrouping of dyslexics has
arisen because we have not been working empirically and inductively, but rather deductively
and intuitively’ (Tønnessen, 1997)?
Theoretical arguments against the analogy between developmental difficulty and later loss of