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

<p>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.</p>