Understanding dyslexia
Understanding dyslexia

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2.3.4 Lateralisation

It has long been suspected that unusual patterns of cerebral lateralisation (i.e. the ‘division of labour’ between left and right hemispheres of the brain) may have some connection with dyslexia. Early researchers noticed an apparent excess of left-handedness in children with specific reading difficulties (and their relatives). However, most dyslexic people are in fact right-handed, and most left-handed people are not dyslexic. Nonetheless, large-scale analyses of the research findings have shown that mixed-handedness is more common than usual in dyslexic people (Eglington and Annett, 1994). You will recall how Alexander Faludy used his left and right hands interchangeably. Although the relationship between handedness and other aspects of cerebral lateralisation is far from clear-cut, these findings are consistent with Orton's original proposal that dyslexia may involve a relative lack of specialisation between the hemispheres.

Neuroanatomical studies, in which brain structure is studied either from postmortem samples or via brain imaging techniques such as magnetic resonance imaging (MRI) in live participants, also provide some support for this view. The typical human brain has a number of well-known structural asymmetries, which are more often reduced or reversed in dyslexic people (Hynd and Semrud-Clikeman, 1989). In particular, brains of people with dyslexia often show an unusual symmetry across hemispheres of a region called the planum temporale (see Figure 4), which is typically larger in the left hemisphere. This finding has attracted considerable attention because this area is involved in auditory and language processing. This planum symmetry appears to distinguish dyslexic from ADHD children (Hynd et al., 1990) and there is some evidence that it may relate to poor phonological skills (Larsen et al., 1990).

Figure 4 A cross-section of a ‘normal’ brain showing asymmetry of the planum temporale region across hemispheres

(Source: adapted from Kalat, 2001, Figure 14.8, p. 402)

So could this reduced brain lateralisation be a ‘biological marker’ for dyslexia? And could it be considered ‘abnormal’ in the ‘medical’ sense that we discussed in Section 1? It would seem not. Firstly, general population studies have shown that this ‘abnormal’ pattern of planum temporale symmetry is shared by up to one person in every four (Galaburda et al., 1987). This means that it cannot be a feature that is specific to dyslexia (but it could perhaps be one ‘risk factor’ among others). Secondly, the degree of planum temporale asymmetry in the general population is normally distributed on a continuum. If you recall our discussion in Section 1 about defining ‘abnormality’, you will remember that for dimensional traits, any cut-off chosen to distinguish ‘normal’ and ‘abnormal’ is essentially arbitrary. Another interesting point is that the ‘normal’ asymmetric pattern involves a large planum temporale on the left, and a smaller one on the right – in keeping with the usual left hemispheric specialisation for language processing. However, the symmetry associated with dyslexia appears to reflect two large regions (rather than two small ones): the total area of these brain regions may actually be greater in people with dyslexia (and others who share this symmetrical pattern). This might possibly help to explain the very superior language abilities shown by some people like Alexander Faludy.

Box 8: Definitions

  • 'Soft’ sign Any behaviour or response that may be linked to an underlying cause (like brain damage) but which is difficult to establish or open to interpretation.

  • Referral bias Any influence on the composition of a study sample that arises from the particular method or source by which participants are recruited.

  • Cerebral lateralisation Refers to the asymmetries found in many brain structures and functions, and/or the developmental processes by which these differences between the left and right sides of the brain usually emerge.

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