Poster A51, Tuesday, August 20, 2019, 10:15 am – 12:00 pm, Restaurant Hall
Testing the unitary theory of language lateralisation using functional transcranial Doppler ultrasound in adults
Zoe Woodhead1, Abigail Bradshaw1, Alexander Wilson1, Paul Thompson1, Dorothy Bishop1;1University of Oxford
Introduction: Hemispheric dominance for language is often assumed to be unidimensional and consistent across language domains, but this assumption can be questioned. Discrepant laterality across different language tasks could be simply due to measurement error; alternatively, task differences may represent meaningful individual variation in the hemispheric organization of different language networks. It has been difficult to distinguish these possibilities, because relatively little is known about the reliability of lateralisation in individuals. In this study we used an ultrasound technique (functional transcranial Doppler sonography, fTCD) to assess strength and test-retest reliability of language lateralisation on a range of language tasks. We tested the hypotheses that laterality would vary across tasks, but be stable across sessions. Furthermore, we predicted that there would be more than one latent factor that predicted individual differences in laterality across tasks, which would provide evidence against the theory that language lateralisation is a unidimensional construct. Methods: Methods were preregistered prior to data collection (https://osf.io/tkpm2/). Laterality within the middle cerebral artery territory was tested using fTCD in 37 adults with typical language development (7 left-handers). Each participant was tested twice using six different language tasks including tests of speech production and comprehension, and phonological, semantic and syntactic decisions. We compared the strength and reliability of lateralisation for each task, and used Structural Equation Modelling to test whether individual differences in laterality across tasks could be explained by a unidimensional model (where laterality on all tasks covaried with each other), or whether a model with two factors (where covariances clustered into two independent factors) was a better fit to the data. Results: Significant left lateralisation was observed for all tasks except for a syntactic decision task. Lateralisation was strongest for production of meaningful speech (a sentence generation task). Test-retest reliability was good for all tasks (ranging from R=0.57 to 0.84) except for a task involving production of automatic speech sequences, e.g. days of the week (R=0.13). The Structural Equation Modelling showed that, for most people, a single lateralised factor explained most of the covariance between tasks. A minority, however, showed dissociation of asymmetry across tasks, giving a second factor. Interestingly, these participants were all left handed. The second factor had the strongest loadings from receptive tasks (sentence comprehension and syntactic decision). Conclusion: The results suggest that variation in strength of language lateralisation reflects true individual differences and not just error of measurement. In general, they support the idea of language lateralisation as a unidimensional construct – individuals and tasks may vary in their strength of lateralisation, but lateralisation measurements across all tasks tend to correlate together. However, this pattern may be broken in a minority of left handed individuals.
Themes: Language Production, Speech Perception
Method: Other