Abstract
Human language is dominantly processed in the left cerebral hemisphere in most of the population. While several studies have suggested that there are higher rates of atypical right-hemispheric language lateralization in left-/mixed-handers, an accurate estimate of this association from a large sample is still missing. In this study, we comprised data from 1,554 individuals sampled in three previous studies in which language lateralization measured via dichotic listening, handedness and footedness were assessed. Overall, we found a right ear advantage indicating typical left-hemispheric language lateralization in 82.1% of the participants. While we found significantly more left-handed individuals with atypical language lateralization on the categorical level, we only detected a very weak positive correlation between dichotic listening lateralization quotients (LQs) and handedness LQs using continuous measures. Here, only 0.4% of the variance in language lateralization were explained by handedness. We complemented these analyses with Bayesian statistics and found no evidence in favor of the hypothesis that language lateralization and handedness are related. Footedness LQs were not correlated with dichotic listening LQs, but individuals with atypical language lateralization also exhibited higher rates of atypical footedness on the categorical level. We also found differences in the extent of language lateralization between males and females with males exhibiting higher dichotic listening LQs indicating more left-hemispheric language processing. Overall, these findings indicate that the direct associations between language lateralization and motor asymmetries are much weaker than previously assumed with Bayesian correlation analyses even suggesting that they do not exist at all. Furthermore, sex differences seem to be present in language lateralization when the power of the study is adequate suggesting that endocrinological processes might influence this phenotype.
Highlights
Human language is dominantly processed in the left cerebral hemisphere in most of the population
Since speech impairment syndromes such as aphasias in patients with unilateral brain damage were found to be less severe in left-handers[7], it was assumed that these two asymmetries are to some extent associated and may even share a common genetic foundation[8,9]
We found a pronounced right ear advantage in the entire sample indicating that there was a significantly stronger bias towards lefthemispheric language processing (mean dichotic listening lateralization quotients (LQs) = 20.86, SD = 30.88; one sample t test: t(1553) = 26.63, p < 0.001, Fig. 1A)
Summary
Human language is dominantly processed in the left cerebral hemisphere in most of the population. We found differences in the extent of language lateralization between males and females with males exhibiting higher dichotic listening LQs indicating more left-hemispheric language processing Overall, these findings indicate that the direct associations between language lateralization and motor asymmetries are much weaker than previously assumed with Bayesian correlation analyses even suggesting that they do not exist at all. Badzakova-Trajkov et al.[15] for example used fMRI to measure asymmetrical frontal activation during a word generation task in 155 adult participants They found a moderate correlation (r = 0.357) between frontal asymmetries and handedness LQs. only 13% of the variance in hand preference could be explained by language asymmetry. In line with this result, Somers et al.[20] found no predictive value of handedness for language lateralization in a study investigating 310 left-handers calling into question whether there truly is a link between these phenotypes
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