01 NRDC Dyslexia 1-88 update - Texthelp
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01 NRDC Dyslexia 1-88 update - Texthelp

01 NRDC Dyslexia 1-88 update - Texthelp 01 NRDC Dyslexia 1-88 update - Texthelp

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24 Research Report 4. Choosing the strategy The fourth issue concerns research strategies. Although the options are seldom discussed, the decision to study groups rather than individuals is another strategy choice for the researcher. It is a matter of serious debate whether group studies are the appropriate method for investigating mixed populations in either developmental or acquired disorders (Bates & Appelbaum, 1994; Caramazza & McCloskey, 1988; Martin, 1995). Case studies, which ‘address the problem of false generalisation from heterogeneous group means to individual group members’, are not without problems of their own, such as that of comparing the actual performance of an impaired subject with the theoretical performance of a ‘normal’ subject (Ellis & Large, 1987). However, in developmental dyslexia, by contrast with loss of reading ability through a stroke or head injury, single-case studies are both rare and untypical. Groups are often studied in a contrasting-groups research design, but this design is flawed whenever the groups are obtained by cutting a continuous distribution. Small-scale studies in which the groups differ on a single measure (‘univariate contrasting-groups studies’) commonly claim ‘significant’ between-group differences on the measure of interest, despite an appreciable overlap. However, only a minority of the target group may cause the differences. As researchers sometimes discover, controversy may be an inevitable consequence of applying a research strategy based on univariate contrasting-groups methods when the experimental group is not homogeneous and the basis for the disability is multivariate in nature (Fletcher & Satz, 1985). The most defensible position may be a pragmatic one: that group comparisons are justified only when the number of participants is large enough to permit statistically significant differentiation on all theoretically important measures, so that the most probable causes of group differences can be identified. Many studies of reading difficulties have a cross-sectional design; that is to say, they take place at a single point in time. This is not the most obviously appropriate design for research into development. Evidence from longitudinal research shows that differences between individuals are unstable over time, which implies that variations in development may invalidate some of the conclusions drawn from cross-sectional studies of developmental disorders. Longitudinal studies of reading development have shown that individuals’ nonheritable influences may change over time (de Jong & van der Leij, 1999; Snowling & Nation, 1997; Sprenger-Charolles et al., 2001; Wright et al., 1996), although the genetic influences shaping their development appear to be stable (Wadsworth et al., 2001). The choice of research method has significant implications for the research findings: different methods applied to the same study population can lead to markedly different conclusions, not all of which are likely to be valid.

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).

24<br />

Research Report<br />

4. Choosing the strategy<br />

The fourth issue concerns research strategies.<br />

Although the options are seldom discussed, the decision to study groups rather than<br />

individuals is another strategy choice for the researcher. It is a matter of serious debate<br />

whether group studies are the appropriate method for investigating mixed populations in<br />

either developmental or acquired disorders (Bates & Appelbaum, 1994; Caramazza &<br />

McCloskey, 19<strong>88</strong>; Martin, 1995). Case studies, which ‘address the problem of false<br />

generalisation from heterogeneous group means to individual group members’, are not<br />

without problems of their own, such as that of comparing the actual performance of an<br />

impaired subject with the theoretical performance of a ‘normal’ subject (Ellis & Large, 1987).<br />

However, in developmental dyslexia, by contrast with loss of reading ability through a stroke<br />

or head injury, single-case studies are both rare and untypical.<br />

Groups are often studied in a contrasting-groups research design, but this design is flawed<br />

whenever the groups are obtained by cutting a continuous distribution. Small-scale studies in<br />

which the groups differ on a single measure (‘univariate contrasting-groups studies’)<br />

commonly claim ‘significant’ between-group differences on the measure of interest, despite<br />

an appreciable overlap. However, only a minority of the target group may cause the<br />

differences. As researchers sometimes discover, controversy may be an inevitable<br />

consequence of applying a research strategy based on univariate contrasting-groups methods<br />

when the experimental group is not homogeneous and the basis for the disability is<br />

multivariate in nature (Fletcher & Satz, 1985). The most defensible position may be a<br />

pragmatic one: that group comparisons are justified only when the number of participants is<br />

large enough to permit statistically significant differentiation on all theoretically important<br />

measures, so that the most probable causes of group differences can be identified.<br />

Many studies of reading difficulties have a cross-sectional design; that is to say, they take<br />

place at a single point in time. This is not the most obviously appropriate design for research<br />

into development. Evidence from longitudinal research shows that differences between<br />

individuals are unstable over time, which implies that variations in development may<br />

invalidate some of the conclusions drawn from cross-sectional studies of developmental<br />

disorders. Longitudinal studies of reading development have shown that individuals’ nonheritable<br />

influences may change over time (de Jong & van der Leij, 1999; Snowling & Nation,<br />

1997; Sprenger-Charolles et al., 20<strong>01</strong>; Wright et al., 1996), although the genetic influences<br />

shaping their development appear to be stable (Wadsworth et al., 20<strong>01</strong>).<br />

The choice of research method has significant implications for the research findings:<br />

different methods applied to the same study population can lead to markedly different<br />

conclusions, not all of which are likely to be valid.

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