01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 55
environmental factors can explain the observed differences in dyslexic behaviour better than
differences in cognitive profiles might explain them (Sprenger-Charolles et al., 2001). Does
this mean that, strictly speaking, the problems with phonological awareness represent a
knowledge deficit rather than a learning disability? Or does it mean that one type of learning
disability is activated only by ‘non-transparent’ orthographies and a second type of learning
disability is activated by all orthographies? Genetic evidence appears to support the latter
conjecture (Grigorenko et al., 2001) and thus to support the hypothesis that there are two
independent sources of reading dysfunction, one related to phonological processing only and
one related to lexical access, or naming speed only (Lovett, 1984)—the ‘double deficit’
hypothesis (Bowers & Wolf, 1993; Wolf & Bowers, 1999; Wolf et al., 2000a). If this is so, then it
might suggest an alternative to the phonological-core variable-difference model.
The double-deficit model of reading disability is a further alternative to the phonological
deficit model.
Dyslexia and lexical access: the double-deficit hypothesis
Introduction
A combination of deficits in accuracy (or phonological deficits) and fluency (or naming-speed
deficits) appears to characterise students with the most serious and pervasive impairments in
reading across various languages (Wolf & Bowers, 1999). The double deficit hypothesis
represents an evolving, alternative conceptualisation of reading disabilities in which, in
addition to a core deficit in accuracy, there is a second core deficit in fluency (Wolf & Bowers,
1999). Ultimately, the hypothesis could lead to a more differentiated view of reading failure
and a more comprehensive approach to reading intervention (Wolf et al., 2000a).
The double-deficit model conceptualises dyslexia as entailing core deficits in both accuracy
and fluency.
Is fluency distinct from phonological processing?
Fluency is the most salient characteristic of skilled readers (Fuchs et al., 2001). It can be
viewed as a performance indicator of overall reading competence, achieved when efficient
low-level word recognition frees up memory capacity for higher-level comprehension (Fuchs
et al., 2001). However, there is some uncertainty as to whether rapid automatised naming, the
conventional test of lexical access, is simply a phonological subprocess or whether it is a way
of ‘stressing’ the phonological system through the demand for retrieval of phonological codes
(Cutting & Denckla, 2001).
Evidence for two independent sources of reading difficulty might be provided by a double
dissociation of accuracy and fluency, but not without the risk of circular argument: there is no
theory-independent way to determine whether a given case is ‘pure’, since it requires in the
first place a reliable theory of cognitive modules to guarantee that a ‘pure’ dissociation has
been observed (Van Orden et al., 2001). From a behavioural perspective, oral reading fluency
is a direct measure of phonological segmentation and phonological recoding (Fuchs et al.,
2001). On the other hand, independent effects on reading achievement have been found for
phonological awareness and rapid naming (de Jong & van der Leij, 1999). For separable
effects, the evidence from psychometric research is supplemented by evidence from
molecular genetics (Grigorenko et al., 2001).