01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 25
Interpretational issues
Do ‘dyslexic’ brains differ from ‘normal’ brains?
Introduction
By now it will be clear, both from the preceding sections on conceptual and methodological
issues and also from Appendixes 1, 2 and 3 (dyslexia definitions and their operationalisation
in research), that the interpretation of evidence for a qualitative distinction between
developmental dyslexia and ‘ordinary’ problems with literacy learning is anything but
straightforward. Could it be true that researchers have ‘misconstrued their object of study—
unexplained underachievement—interpreting it neurologically and ignoring classroom
practices and events’ (Carrier, 1983), so that the theory masks societal forces as they affect
academic performance? Are people justified in believing that the neurological studies validate
dyslexia as a qualitatively distinct condition? How should the neurological evidence be
interpreted? Also, can we afford to forget that ‘the diagnosis of dyslexia is itself a theory,
distinguishing reading failure arising ultimately from internal rather than solely external
reasons, but a rather unspecified one’ (Frith, 2001)?
For a long time, most of our knowledge about the workings of the brain was gained in the
course of autopsies on people whose previously normal abilities had been compromised by
head injuries or strokes. These findings suggested that people are born with a brain rather
like a Swiss Army knife, with a modular component for every purpose—an analogy that now
seems false (Karmiloff-Smith, 1992; Quartz & Sejnowski, 1997). Accordingly, loss of function
was not a good guide to what happens in developmental disorders (Alarcón et al., 1999;
Karmiloff-Smith, 1998; Thomas & Karmiloff-Smith, 2002). In any case, there have been very
few autopsy studies of people who had experienced difficulty in learning to read, write and
spell.
Over the past 15 years, a number of brain imaging (or ‘scanning’) techniques have been
introduced. So there are now two sources of evidence for differences between ‘dyslexic’ and
‘normal’ brains. From post-mortem studies, there is evidence of differences in brain
structure; from brain imaging studies of living people (or in vivo studies), there is evidence of
differences in brain structure and function. To date, the in vivo studies are almost without
exception cross-sectional. However, in future, prospective longitudinal imaging studies may
show how brains change as people acquire complex skills.
The table and figures in Appendix 10 may help readers to locate the brain areas named in the
following sub-sections.
Evidence from post-mortem studies
Structurally, the brains in the autopsy studies reveal anomalies at two levels of analysis
(Galaburda et al., 1989). At the microscopic (or neuronal) level, researchers have found
abnormal outgrowths known as ‘ectopias’ and abnormal infoldings known as ‘microgyria’,
visible on the surface of the brain (Galaburda et al., 1989). In addition to these abnormalities
in the outer layer of the brain, or cortex, researchers have found a magnocellular defect
within the inner chamber of the brain, in a part of the thalamus known as the lateral
geniculate nucleus, which is activated during visual processing (Livingstone et al., 1991). At
the macroscopic level, researchers have found an unusual symmetry in that part of the
temporal lobe known as the planum temporale (Galaburda et al., 1989), which is normally
larger in the left hemisphere, where it is activated in tasks related to language, than in the
right hemisphere (Shapleske et al., 1999).