On the relationship between language lateralization and handedness

Abstract

In this thesis, several aspects of the relation between hand-preference and language lateralization were investigated by employing meta-analytic, neuroimaging and genetic techniques. To achieve this, we tested whether there is a difference between left- and right-handers with regard to spatial ability, verbal ability and schizotypal thinking through meta-analysis of the available literature. The possible link between handedness and cognitive function assumes a difference in cerebral function between left- and right-handers. This functional difference may result from the slightly higher prevalence of atypical (right-hemispheric and bilateral) lateralization that left-handers show in comparison to right-handers. To investigate the phenotypic and genotypic relation between handedness and language lateralization, we collected data from large, multigenerational families with multiple left-handers from a town in the Netherlands that used to be an island. By selecting for left-handedness, we aimed to enrich the sample for atypical lateralization. In this sample, we investigated the relation between degree of hand-preference and degree of language lateralization. We further performed a genetic linkage analysis of left-handedness and atypical language lateralization to find loci related to the two traits in the sample. Language lateralization was measured with functional transcranial doppler (fTCD). Before applying this device in our studies, we estimated the correlation between language lateralization indexes measured with fTCD and those measured with fMRI. Meta-analysis of the available literature demonstrated that left-handers show slightly lower spatial skills than right-handers. In a second meta-analysis it was shown that non-right and especially mixed-handers show higher schizotypy than right-handers. We confirmed that fTCD is a reliable technique to measure language lateralization by comparing it to fMRI. In our study of the relationship between degree of language lateralization and degree of hand-preference, it was demonstrated that the prevalence of right hemispheric and bilateral lateralization peaks in strong left-handers. This indicates that strong left-handedness is associated with an increased randomness in the development of cerebral lateralization. It is argued to be unlikely that such contrasting phenotypes are determined by a single genetic mechanism. Indeed, our subsequent linkage analysis of left-handedness and atypical lateralization did not show evidence of a major locus. Instead, both traits should be considered complex genetic traits. The results in this thesis show that handedness and language lateralization are not as related as has been assumed thus far.