Laterality And Dyslexia

It used to be thought that all right-handers speak with the left hemisphere, while left-handers speak with the right hemisphere. This is often referred to in the literature as Broca's rule though not in fact formulated by Broca himself (Harris, 1991). In any event, we now know that this is not true; most left-handers speak with the left hemisphere. However, it is the case that relatively few right-handers have language lateralized to the right hemisphere, while a greater proportion (though still a minority) of left-handers do. The figures often quoted are that fewer than 4% of right-handers have right-sided speech whereas in left-handers, approximately 15% have speech controlled from the right hemisphere (with another 15% having speech distributed across both hemispheres). For reasons that will not be discussed here, these figures are unlikely to be reliable. What is true is that right hemisphere speech in both left- and right-handers is more common than is usually supposed (see Annett, 1975; Annett & Alexander, 1996). Using the new technique of functional transcranial Doppler (fTCD) ultrasonography, it has been estimated that approximately one in 13 neurologically normal volunteers has speech represented in the right hemisphere (Knecht et al., 2000a), and that the proportion of right-hemisphere speakers varies systematically as a function of strength of handedness. Approximately 25% of strong left-handers appear to have right-hemisphere speech while for strong right-handers, the figure is about 10% (Knecht et al., 2000b).

During the 19th and early part of the 20th centuries, the idea was prevalent that one half of the brain, usually the left, is dominant for all psychological functions, not just speech. A dominant cerebral hemisphere seemed to follow naturally from the idea of a dominant hand. However, the notion of cerebral dominance was taken even further in that one half of the brain was said to inhibit or dominate the other. Lack of clear-cut handedness was regarded as showing a failure to establish dominance at the cerebral level.

Samuel Orton believed that the memory traces of letters presented to the brain were stored in mirror-image fashion in the left and right hemispheres. Normally, the "engrams" in the so-called minor (right) hemisphere were "suppressed" by the dominant hemisphere, but in cases of incomplete or weak cerebral dominance there was incomplete suppression. The result was confusion as to the correct orientation of letters. This "strephosymbolia" accounted for what Orton regarded as the relatively high number of letter reversals (e.g., b for d) shown by the poor readers referred to him (e.g., Orton, 1928). Orton's theory has logical as well as empirical shortcomings (Beaton, 1985, 2004) and is no longer taken seriously today. Like Freud, perhaps, his contribution to the field is valued not so much for the detail of his theories as for his recognition of the problems faced by dyslexic individuals.

Orton drew attention to what he believed was a raised frequency of left-handedness among children with dyslexia (and among those who stuttered). He also believed that children with severe reading difficulties showed crossed hand-eye dominance more often than expected. This refers to a dominant right hand in combination with a dominant left eye or a dominant left hand in combination with a dominant right eye. In actual fact, crossed-laterality is exceedingly common in the general population and its occurrence should no more be considered diagnostic of dyslexia than the presence of left-handedness (which is not to say that an association between crossed-laterality and dyslexia will not be found if one looks at a sufficiently large number of cases). There is certainly a genetic connection between language lateralisation and handedness although the exact nature of the relationship is controversial (see Beaton, 2003), and handedness is probably related in some way to reading and dyslexia (Eglinton & Annett, 1994), if not in precisely the way envisaged by Orton.

Handedness itself cannot be a cause of poor reading (any more than it can be a cause of language lateralisation, though presumably the two phenomena have mechanisms in common), but could there be a relationship between handedness and those language areas of the brain implicated in aphasia? Differences between left- and right-handers have been observed in part of Broca's area (Foundas et al., 1998). However, it is in relation to a region encroaching on Wernicke's area that most interest has centred. This region is known as the planum temporale (PT). It forms a roughly triangular area on the upper or superior surface of the temporal lobe and is exposed by a horizontal knife-cut through the Sylvian fissure. In 1968, a celebrated report by Geschwind and Levitsky appeared showing that the length of the PT on the left was larger than that on the right in 65 of 100 specimens studied postmortem. This appeared to confirm, in a large sample of brains, the similar observations made earlier in the century.

The borders of the PT have been defined somewhat differently by different researchers and the size of the PT has been variously measured in terms of length, area, and volume both postmortem and using in vivo neuroimaging techniques. The exact delineation and methods of measurement of the PT are technical matters involving some controversy (see Barta et al., 1995; Shapleske, Rossell, Woodruff, & David, 1999; Zetzche et al., 2001) but most authors seem to accept that there is an asymmetry in the size of the planum on the left and right sides of the brain. Given the location of the planum and reports that the asymmetry apparently varies with handedness (for review, see Beaton, 1997), a common assumption is that this asymmetry has something to do with language lateralisation. There are in fact some reasons to doubt this (Beaton, 1997, 2002) even though a correlation has been reported for 5-12-year-old children between asymmetry of the planum and scores on a test thought to require an appreciation of the phonemic structure of words (phonemic awareness) by Leonard et al. (1996).

The putative relevance of the PT for dyslexia is that it has been reported that PT asymmetry is reduced or reversed among dyslexic persons (for reviews, see Beaton, 1997; Hynd & Semrud-Clikeman, 1989). On the basis of postmortem studies of a small number of brains, Galaburda and his collaborators suggested that among dyslexics, the PT is symmetrical (Galaburda, Sherman, Rosen, Aboitiz, & Geschwind, 1985; Humphreys, Kaufmann, & Galaburda, 1990). Subsequently, reports of a number of neuroimaging studies appeared claiming to show that right-sided asymmetry or symmetry was more frequent among dyslexics than controls (Duara et al., 1991; Rumsey et al., 1986), although this has not always been found (Leonard et al., 1993; Schultz et al., 1994). One study in particular, that of Larsen, Hoien, Lundberg, and Odegaard (1990), has been much cited as it appeared to distinguish between dyslexics with phonological problems and one participant who had "purely orthographic dysfunction." The latter, in contrast to the majority of the former, was said to show asymmetry of the planum. However, the definition of "orthographic" problems was unsound and the data were evaluated purely by visual inspection rather than precise measurement.

If it is the case that dyslexia is characterized by reversed asymmetry or symmetry of the PT, how might this come about, and what are the implications? One theory concerns a link between genetic mechanisms under-lying speech lateralisation and handedness.

We are accustomed to thinking of left- and right-handedness in terms of a simple dichotomy. In fact, between those people who do everything with either one hand or the other, extremely right- or left-handed individuals, there are others who exhibit different preferences for different activities. The different preferences span the entire range between the two extremes. In short, it is helpful to consider hand preference as constituting a continuum rather than a dichotomy (Annett, 2002). This is especially so if one considers hand skill rather than preference (but see McManus, 1991; McManus, Murray, Doyle, & Baron-Cohen, 1992).

There are a number of competing theories of handedness (for review, see Beaton, 2003). Marian Annett has for many years championed what she terms the Right Shift Theory of cerebral lateralisation and handedness (Annett, 2002). She argues that, as for most mammals, human handedness is largely a matter of chance. If chance alone were the sole factor, then half the population would be left-handed and the other half right-handed (to an approximate extent). Similarly, half of us would speak with the left hemisphere and half with the right hemisphere. However, chance is not the only factor. In Annett's view, there is an additional genetic factor at work in the majority of the population (she calculates about 81.6%) which "forces" the left hemisphere to be the speech hemisphere by "impairing" the right hemisphere in some way. This in turn acts coincidentally in some way to make it more probable that the individual will be right-handed. This mechanism operates in addition to chance factors. Thus, a frequency distribution (as in a bell curve) of the relative skill of the two hands is symmetrical with its mean (average) value shifted somewhat to the right of the point at which the two hands are equal in skill.

Some people (approximately 18.4% of the population) lack the genetic factor which brings about left-sided speech in the majority (call them right shift minus, RS-). These people have speech distributed in the left or right hemisphere randomly (i.e., according to chance), and their handedness is also distributed randomly and independently of their speech lateralisation. The proportion of RS -individuals who are left-handed (approximately 50%, some "natural" left-handers become right-handed due to social or environmental pressure) is higher than the proportion of left-handers among those who have the right-shift factor (RS+). The latter proportion is very small, consisting of those in whom the right-shift factor was not sufficiently strong to counter chance factors which otherwise predisposed them to becoming left-handed. (Note that we are talking here of relative proportions and not of absolute numbers.)

With regard to the PT, Annett (1992) drew attention to interesting parallels between the distribution of handedness and the distribution of planum asymmetry. Without going into too much detail, it is possible to think of some proportion of the population as having planum asymmetry distributed by chance (half having a larger planum on the left and half on the right) and the remainder having a larger planum on the left. In short, Annett's theory might apply to the distribution of PT in a way which directly parallels the distribution of handedness (without implying that the one causes the other).

According to Annett's theory, those people who lack the right-shift factor also lack some subtle advantage in speech processing enjoyed by the majority (but have compensating advantages in other spheres). It is these people, on Annett's view, who are most at risk of a phonological form of developmental dyslexia (Annett, 1996, 2002; Annett, Eglinton, & Smythe, 1996).

If Annett is correct that (some proportion of) RS- individuals are at risk of phonological dyslexia, and if RS- individuals have hemispheric speech lateralisation, and perhaps therefore planum asymmetry, distributed at random, then one would expect to find that these individuals have rightward asymmetry of the planum in approximately 50% of cases (unlike controls who would be expected to show leftward asymmetry in the large majority of cases). Although no one has identified the relevant right-shift gene (rs+ and rs- alleles), the data on dyslexia and the planum are consistent with this expectation if it is considered that "symmetry" of the planum reported in some papers is in fact a statistical artifact of averaging over roughly equal numbers of rightward and leftward cases (leading to a mean asymmetry of zero).

Annett's theory is highly controversial. There are many who disagree with her, both in regard to her genetic theory of cerebral lateralisation and handedness (McManus, 1985; McManus, Shergill, & Bryden, 1993) and more particularly with regard to her extension of the theory to encompass cognitive abilities such as reading (Beaton, 1995). An equally controversial theory, as it applies to planum asymmetry and to dyslexia, is based not on genetic theory but on hormones (though the two approaches are not incompatible).

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