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

Cerebral lateralisation for language can vary from task to task, but it is unclear if this reflects error of measurement or independent lateralisation of different language systems. We used functional transcranial Doppler sonography to assess language lateralisation in 37 adults (7 left-handers) on six tasks, each given on two occasions. Tasks taxed different aspects of language function. A preregistered structural equation analysis was used to compare models of means and covariances. For most people, a single lateralised 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 lateralised. The results suggest that variation in strength of language lateralisation reflects true individual differences and not just error of measurement. Inclusion of several tasks in a laterality battery makes it easier to detect cases of atypical asymmetry.