Laterality and types of dyslexia.

The right shift theory of handedness and cerebral specialization suggests that there is an underlying substrate of random lateral asymmetries in all higher animals and a specific factor in humans which increases the probability of left hemisphere advantage. The specific factor displaces the random distribution along a continuum of asymmetry in favour of the left hemisphere and the right hand. The distribution of handedness in families can be explained if the shift to dextrality depends on a single gene, rs +, when the frequency of the gene is estimated from the proportion of dysphasics with unilateral lesions of the left versus the right hemisphere. The gentoype proportions suggested a genetic balanced polymorphism with heterozygote advantage for laterality and ability. Application of the model to developmental dyslexia led to the hypothesis that both homozygotes (rs -- and rs ++ genotypes) are at risk for reading. The critical question was whether the type of cognitive problem differs as expected, speech based processing for the rs -- and non-speech based for the rs ++. The former should include more left and mixed handers, while the latter should be more strongly dextral than controls. Evidence for this double dissociation between type of cognitive disability and type of handedness has been found in poor readers and in other samples.