01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 27
Second, it is unclear whether the brains in the post-mortem studies are representative of
dyslexic brains in general (Beaton, 2002). Even if that were the case, it would be unsafe to
assume that the abnormalities in those brains are specific to dyslexia, because the full nature
and extent of the cognitive and behavioural problems experienced by their donors was not
identified before they died (Bishop, 2002). The specificity of the post-mortem brain
abnormalities is questionable with respect to planum temporale symmetry, since it is
possible that dyslexia (defined as a phonologically-based reading disability) might be
associated with asymmetrical brain structures, whereas symmetrical brain structures might
be associated with the linguistic deficit known as Specific Language Impairment (Leonard et
al., 1998).
Third, the evidence from brain imaging studies offers no support for the notion that there is a
causal relationship between dyslexia and an absence of left-greater-than-right asymmetry of
the planum temporale: one non-dyslexic brain in three shows similar symmetry or reversed
asymmetry; some researchers find abnormally large right plana, while others find abnormally
small left plana; patterns of symmetry or asymmetry vary from study to study; and it has not
been established that deviations from normal asymmetry are exclusive to dyslexia (Beaton,
2002; Cossu, 1999; Habib, 2000; Heiervang et al., 2000; Robichon et al., 2000b; Rumsey et al.,
1997a; Shapleske et al., 1999).
Whether ‘dyslexic’ brains differ in structure from ‘normal’ brains is not a question that can be
determined on the evidence of the post-mortem research. While there are undoubted
abnormalities in the autopsied brains, their relationship to dyslexia is open to question.
Although the post-mortem findings have suggested interesting new lines of investigation, lack
of convergence with findings from more recent in vivo studies is an obstacle to their
acceptance.
Evidence from in vivo studies: structural differences
Although no consistent morphological correlates have been associated with developmental
dyslexia in children or in adults (Cossu, 1999), there is broad agreement about the main
regions of interest. Thus, in vivo studies of ‘dyslexic’ and ‘normal’ brains examine two kinds of
structural anomaly. They consider both the patterns of symmetry in the temporal lobes and
the cerebellum and also the connectivity between left and right cerebral hemispheres. At the
time of writing, magnetic resonance imaging (MRI), the technique employed for this purpose,
cannot be used to resolve details as fine as the microscopic abnormalities reported in the
post-mortem studies.
The question of symmetry in the temporal lobes has typically focused on the planum
temporale, which is considered above. Additionally, dyslexia appears to be associated with
lack of normal asymmetry in the cerebellum, which may also be associated with mixed
handedness in ways that have yet to be explained (Rae et al., 2002).
Connectivity between the two sides of the brain is a function of the corpus callosum, the mass
of white-matter fibres that connects the brain’s two hemispheres. The issue of the link
between anatomical observations and clinical abnormalities in the corpus callosum remains
fundamentally unresolved (Habib & Robichon, 2002). Atypical development of the corpus
callosum may be characteristic of dyslexics (Hynd et al., 1995; Rumsey et al., 1996; von
Plessen et al., 2002), although the evidence is conflicting (Habib et al., 2000a). While it is
possible for qualitative differences in corpus callosum morphology to have biological origins
(Robichon et al., 2000a), it is also possible that they could be caused by atypical experience