01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 57
amalgamation of phonological and orthographic identities; as indicating an impaired timing
mechanism affecting the quality of an orthographic code, or the amount of exposure and
practice needed to achieve good quality representations, or both (Bowers & Wolf, 1993). Now,
there are two broad conjectures. The first conjecture is that, if the magnocellular system is
aberrant in the mid-brain area known as the thalamus, then the processing of lower spatialfrequency
components will be slowed, potentially leading to slower visual discriminations,
slower letter-pattern identification, slower naming speed for serially presented visual stimuli
and delayed induction of orthographic patterns, so that slower naming speed is an index of
lower-level problems that disrupt the smooth development of fluency in word identification
and comprehension (Wolf & Bowers, 1999). The second conjecture is that naming speed might
both indicate dysfunction in lower level processes and also contribute to pervasive reading
failure as one manifestation of a cascading system of more general processing-speed deficits
affecting visual, auditory and possibly motoric domains, in addition to orthographic and
phonological processing systems (Wolf & Bowers, 1999).
Two broad explanations have been proposed for the naming speed deficit, but it is not yet
clear how or whether we should adjudicate between them.
The diagnostic significance of naming speed tasks
Although there is evidence that rapid automatised naming tasks differentiate dyslexics from
average readers, from ordinary poor readers and from readers with other learning disabilities
(Wolf et al., 2000a), there is also counter-evidence in the cases of both ordinary poor readers
(Swan & Goswami, 1997) and subjects with impaired executive functions (Rashid et al., 2001;
Tannock et al., 2000). In particular, it is unclear whether the only people to show impairments
in rapid naming are readers with persistent difficulty and additional impairments associated
with the constructs of attention-deficit/hyperactivity disorder (American Psychiatric
Association, 1994) or hyperkinetic disorder (World Health Organisation, 1992).
For most practitioners, the critical conflict of evidence is probably whether ‘ordinary’ readers
with difficulties do (Swan & Goswami, 1997; Waber et al., 2000) or do not (Marcus, 1997, in
Wolf & Bowers, 1999) have a naming-speed deficit. If they do not, then tests of rapid
automatised naming like that in the Dyslexia Adult Screening Test (Fawcett & Nicolson, 1998)
might be a useful way of differentiating dyslexic from ordinary poor readers. However,
resolution of this conflict could entail a reconceptualisation of dyslexia that denies the
assumption of specificity by incorporating deficits in both language and executive functions—
that is to say, dysfunctions in both the temporal lobe and the frontal lobe of the brain. As
recent research suggests that rapid automatised naming taps both visual-verbal (language
domain) and processing speed (executive domain) contributions to reading (Denckla &
Cutting, 1999), such a resolution is possible.
Performance on naming-speed tasks might distinguish dyslexic people from ‘ordinary’ poor
readers.
If dyslexia can be distinguished in this way, then it may be a less specific impairment than
it is sometimes believed to be.