01 NRDC Dyslexia 1-88 update - Texthelp

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Research Report
While the DDAT evidence might support the conjecture that a cerebellar deficit is causal, we
have yet to find out whether the finding can be replicated with a more rigorous research
design; the study has come under criticism from several quarters (e.g. Rack, 2003; Richards
et al., 2003; Singleton & Stuart, 2003; Snowling & Hulme, 2003; Stein, 2003) although its
methods have been defended (Nicolson & Reynolds, 2003a, 2003b).
If it should be established that the cerebellar deficit is not causal, it might nevertheless be
the case that the cerebellum plays a co-ordinating role in the processing of speech
production at the interface of cognitive and executive functions (Silveri & Misciagna, 2000), in
which case a causal direction would need to be established for any functional abnormality
(Beaton, 2002). Future research might also have to eliminate possible alternative explanations
such as attentional difficulties, general language impairment, or less specific
neurodevelopmental problems than the dyslexia construct implies (Bishop, 2002). Meanwhile,
there appears to be no support for the cerebellar deficit hypothesis when causal theories are
compared (Ramus et al., 2003a; Ramus et al., 2003b).
Importantly, we do not know yet how far cerebellar impairment might explain the problems of
every person with dyslexia (Fawcett et al., 2001). As Aristotle was among the earliest to point
out, with reference to the fallacy of division, what may be true of a group of people is not
necessarily true of every member of the group. Among the individuals in one study, the
dissociation between those with and without IQ-discrepancy on the static cerebellar tests was
not complete (Fawcett et al., 2001). This suggests that, as the ‘core’ deficits predicted by other
theories of dyslexia are not sufficient at group level to differentiate between dyslexics and
ordinary poor readers (Fawcett et al., 2001), so evidence of a cerebellar impairment may not
do so at the individual level.
It is unclear whether cerebellar abnormalities are causes or correlates of dyslexia.
Even if cerebellar abnormalities are found to cause reading difficulties for some people,
they might not explain every case of reading difficulty.
Dyslexia and processing speed: the temporal processing deficit theory
Introduction
There is a plausible link between hearing impairments and reading problems. Hearing
provides inputs to information-processing; this in turn yields phonemic categories, which are
important to speech perception; damage to the auditory pathway leads to degraded signals,
with a possibility of corresponding degradation in phonemic representation; and language
problems with their roots in speech perception and production will be the result (Farmer &
Klein, 1995).
In a hierarchy of temporal information-processing functions, timing may be critically
important (Farmer & Klein, 1995). At the lowest level of the processing hierarchy are target
detection and identification; at an intermediate level are individuation and temporal order
judgement in both auditory and visual domains; at the highest level is complex sequencing in
both perception and production. Deficits might be interpreted in terms of rate of processing,
‘smudging’ of encoded information over time so that mental representation loses crispness of
input, or to ‘temporal jitter’ where the times assigned to events or their properties are