An updated investigation of the multidimensional structure of language lateralization in left- and right-handed adults: a test–retest functional transcranial Doppler sonography study with six language tasks

A previous study we reported in this journal suggested that left and right-handers may differ in their patterns of lateralization for different language tasks (Woodhead et al. 2019 R. Soc. Open Sci. 6 , 181801. ( doi:10.1098/rsos.181801 )). However, it had too few left-handers ( N = 7) to reach firm conclusions. For this update paper, further participants were added to the sample to create separate groups of left- ( N = 31) and right-handers ( N = 43). Two hypotheses were tested: (1) that lateralization would be weaker at the group level in left-than right-handers; and (2) that left-handers would show weaker covariance in lateralization between tasks, supporting a two-factor model. All participants performed the same protocol as in our previous paper: lateralization was measured using functional transcranial Doppler sonography during six different language tasks, on two separate testing sessions. The results supported hypothesis 1, with significant differences in laterality between groups for four out of six tasks. For hypothesis 2, structural equation modelling showed that there was stronger evidence for a two-factor model in left than right-handers; furthermore, examination of the factor loadings suggested that the pattern of laterality across tasks may also differ between handedness groups. These results expand on what is known about the differences in laterality between left- and right-handers.

Investigating multidimensionality of language lateralisation in left and right handed adults: an update on Woodhead et al. 2019

A previous study we reported in this journal suggested that left and right handers may differ in their patterns of lateralisation for different language tasks. However, it had too few left handers (N=7) to reach any conclusions. For this update paper, further participants were added to the sample to create separate groups of left (N=31) and right handers (N=43). Two hypotheses were tested: 1) that lateralisation would be weaker at the group level in left than right handers; and 2) that left handers would show weaker covariance in lateralisation between tasks, supporting a two factor model.All participants performed the same protocol as in our previous paper: lateralisation was measured using functional transcranial Doppler sonography during six different language tasks, on two separate testing sessions. The results supported hypothesis 1, with significant differences in laterality between groups for four out of six tasks. For hypothesis 2, structural equation modelling showed that there was stronger evidence for a two factor model in left than right handers; furthermore, examination of the factor loadings suggested that the pattern of laterality across tasks may also differ between handedness groups. These results expand on what is known about the differences in laterality between left and right handers.

Investigating the effects of handedness on the consistency of lateralisation for speech production and semantic processing tasks using functional transcranial Doppler sonography

The left hemisphere is dominant for language in most people, but lateralisation strength varies between different tasks and individuals. A large body of literature has shown that handedness is associated with lateralisation: left handers have weaker language lateralisation on average, and a greater incidence of atypical (right hemisphere) lateralisation; but typically these studies have relied on a single measure of language lateralisation. Here we consider the relationships between lateralisation for two different language tasks. We investigated the influence of handedness on lateralisation using functional transcranial Doppler sonography (fTCD), using an existing dataset (N=151 adults, 21 left handed). We compared a speech production task (word generation) and a semantic association task. We demonstrated stronger left-lateralisation for word generation than semantic association; and a moderate correlation between laterality indices for the two tasks (r=0.59). Laterality indices were stronger for right than left handers, and left handers were more likely than right handers to have atypical (right hemisphere) lateralisation or inconsistent lateralisation between the two tasks. These results add to our knowledge of individual differences in lateralisation, and support the view that language lateralisation is multifactorial rather than unitary.

Inconsistent lateralisation of language functions: a risk factor for language impairment?

Disruption to language lateralisation has been proposed as a cause of developmental language impairments. In this study we tested the idea that consistency of lateralisation across different language functions is associated with language ability. A large sample of adults with variable language abilities (N=67 with a developmental disorder affecting language and N=37 controls) were recruited. Lateralisation was measured using functional transcranial Doppler sonography (fTCD) for three language tasks that engage different language sub-processes (phonological decision, semantic decision and sentence generation). The whole sample was divided into those with consistent versus inconsistent lateralisation across the three tasks. Language ability (using a battery of standardised tests) was compared between the consistent and inconsistent groups. The results did not show a significant effect of lateralisation consistency on language skills. However, 26 of 31 (84%) of individuals with inconsistent lateralisation were in the disorder group, compared to 41 of 73 (56%) of those with consistent lateralisation, a difference that was higher than would be expected by chance. The developmental disorder group also demonstrated weaker correlations between laterality indices across pairs of tasks. In summary, although the data did not support the hypothesis that inconsistent language lateralisation is a major cause of poor language skills, the results suggested that some subtypes of language disorder are associated with inefficient distribution of language functions between hemispheres. Inconsistent lateralisation could be a causal factor in the aetiology of language disorder, or may arise in some cases as the consequence of developmental disorder, possibly reflective of compensatory reorganisation.

Testing the unitary theory of language lateralization using functional transcranial Doppler sonography in adults

Hemispheric dominance for language can vary from task to task, but it is unclear if this reflects error of measurement or independent lateralization of different language systems. We used functional transcranial Doppler sonography to assess language lateralization within the middle cerebral artery territory in 37 adults (seven left-handers) on six tasks, each given on two occasions. Tasks taxed different aspects of language function. A pre-registered structural equation analysis was used to compare models of means and covariances. For most people, a single lateralized factor explained most of the covariance between tasks. A minority, however, showed dissociation of asymmetry, giving a second factor. This was mostly derived from a receptive task, which was highly reliable but not lateralized. The results suggest that variation in the strength of language lateralization reflects true individual differences and not just error of measurement. The inclusion of several tasks in a laterality battery makes it easier to detect cases of atypical asymmetry.