01 NRDC Dyslexia 1-88 update - Texthelp

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Research Report
comprehension. Only when the basic skill of decoding print is so fluent as to be automatic does
comprehension become possible—that is to say, we cannot properly understand what we read
unless the act of decoding is involuntary, free from any demands on processing capacity and
resistant to interruption from competing activity in the same domain (Yap & van der Leij, 1994).
For reading comprehension, it is necessary that word-recognition is automatic.
The dyslexia automatisation deficit hypothesis
The dyslexia automatisation deficit hypothesis attributes the reading deficits of dyslexics to a
general failure to automatise skills (Nicolson & Fawcett, 1990). Since memory span is partly
determined by reading rate and reading rate is determined by processing speed, which is in
its turn partly determined by the degree of automatisation, the evidence from reading appears
compatible with an automatisation deficit explanation (Nicolson & Fawcett, 1990). Where
dyslexic people have problems in other motor and cognitive domains, as the hypothesis would
predict, an associated ‘conscious compensation’ hypothesis suggests that these problems are
masked by coping strategies and by active allocation of extra attentional resources (Nicolson
& Fawcett, 1990). The resulting threefold prediction that dyslexic performance breaks down
primarily for resource-intensive tasks, is particularly susceptible to stress and can be
maintained only for relatively short periods, is consistent with the available evidence (Nicolson
& Fawcett, 1990).
Taken together, the dyslexia automatisation deficit hypothesis (Nicolson & Fawcett, 1990) and
the phonological deficit hypothesis can be viewed as twin components of the double-deficit
hypothesis, in that they offer cognitive-level explanations of the two main difficulties of
persistently poor readers—those involving fluency and accuracy. However, the dyslexia
automatisation deficit hypothesis reaches beyond reading skills to offer a cognitive
explanation of dyslexics’ difficulties in automatising skills unrelated to reading (Nicolson &
Fawcett, 1994), thus breaking new ground that the phonological deficit hypothesis could not
cover and which the double-deficit hypothesis does not cover.
The dyslexia automatisation deficit hypothesis proposes an explanation of failure to
automatise skills in word-recognition and other domains.
The cerebellar deficit hypothesis
Although, initially, the dyslexia automatisation deficit hypothesis proposed only a proximal,
cognitive explanation of the dyslexics’ difficulties in skill mastery, it was soon conjectured that
a distal, biological explanation might be found in ‘noisy neural networks’ (Fawcett & Nicolson,
1991). Previously, there had been speculation that cerebellar function might be mildly
impaired in dyslexia (Frank & Levinson, 1973). However, it was only with confirmation that the
cerebellum—‘the brain’s autopilot’ (Stein, 2001)—was involved in linguistic as well as motor
skill acquisition (Ito, 1993) that there was reason to investigate cerebellar dysfunction as a
possible cause of dyslexia (Nicolson et al., 1995). Brain imaging studies have since shown, for
example, that the normal cerebellum is activated during speech perception (Dogil et al., 2002;
Mathiak et al., 2002) and in reading (Fulbright et al., 1999) and that the patterns of activation
are different for phonological and semantic tasks.
The cerebellar deficit hypothesis can be stated as a series of propositions. First, the
behavioural symptoms of dyslexia can be characterised as difficulties in skill automatisation;
this pattern of difficulties in cognitive, information-processing and motor skills would be