Heritability of language laterality assessed by functional transcranial Doppler ultrasound: a twin study

Abstract

Prior studies have estimated heritability of around 0.25 for Background: the trait of handedness, with studies of structural brain asymmetry giving estimates in a similar or lower range. Little is known about heritability of functional language lateralization. This report describes heritability estimates using functional language laterality and handedness phenotypes in a twin sample previously reported by Wilson and Bishop (2018).The total sample consisted of 194 twin pairs (49% monozygotic) Methods: aged from 6 to 11 years. A language laterality index was obtained for 141 twin pairs, who completed a protocol where relative blood flow through left and right middle cerebral arteries was measured using functional transcranial Doppler ultrasound (fTCD) while the child described animation sequences. Handedness data was available from the Edinburgh Handedness Inventory (EHI) and Quantification of Hand Preference (QHP) for all 194 pairs. Heritability was assessed using conventional structural equation modeling, assuming no effect of shared environment (AE model).For the two handedness measures, heritability estimates were Results: consistent with prior research: 0.23 and 0.22 respectively for the EHI and QHP. For the language laterality index, however, the twin-cotwin correlations were close to zero for both MZ and DZ twins, and the heritability estimate was zero.A single study cannot rule out a genetic effect on language Conclusions: lateralisation. It is possible that the low twin-cotwin correlations were affected by noisy data: although the split-half reliability of the fTCD-based laterality index was high (0.85), we did not have information on test-retest reliability in children, which is likely to be lower. We cannot reject the hypothesis that there is low but nonzero heritability for this trait, but our data suggest that individual variation in language lateralisation is predominantly due to stochastic variation in neurodevelopment.

Cerebral Polymorphisms for Lateralisation: Modelling the Genetic and Phenotypic Architectures of Multiple Functional Modules

“…In one study using whole hemispheres, the majority of participants showed left-hemisphere dominance (67/82; 81.7%), with a higher rate of left-hemisphere dominance amongst right-handers than lefthanders [210]. fTCD methods have mostly used the approach of Deppe and colleagues [211], but recent work has raised the question of whether searching for the peak blood flow difference artificially creates distributions that are more bimodal, resulting in a 'notch' at zero, so that it is better to use mean blood flow difference [212] (see https://osf.io/tfyk3/ accessed on 3 November 2021). Wada results have been interpreted in terms of the WLI, Wada Laterality Index, although there are several versions of the index [213].…”

“…However, as Ooki says in a review, "sample sizes are still not very large and hence the statistical power is insufficient" [310] (p. 4). The heritability of language dominance in twins has been assessed using fTCD, but the confidence interval is very wide, and is compatible with that of handedness in general, but the authors also emphasise its relatively low effect size with a confidence interval ranging upwards from zero [212].…”

The genetic architecture of structural left-right asymmetry of the human brain

“…While many brain asymmetries were strong and directional at the population level, their heritabilities were low. This suggests that genetic-developmental mechanisms for brain asymmetry are tightly constrained and largely genetically invariant in the population, and that environmental factors and/or developmental randomness are responsible for most variability 22,[82][83][84] . A cytoskeleton-based origin of brain asymmetry would fit this scenario, as the cytoskeleton is essential for fundamental functions in cellular biology, beyond axis formation 85,86 .…”

Development of laterality: Asymmetrical human behaviors and perceptual biases

“…In the case of handedness, developmental noise and randomness are suggested to play a major role in its ontogenesis, are supposed to be under the control of the C allele in the DC model . Stochastic variations are also discussed in Bishop and Bates (2020) regarding language lateralization. Mitchell (2018) provides an overview of the influence of randomness on handedness, but also on other phenotypes like intelligence, personality, and neurodevelopmental disorders.…”