Abstract
ABSTRACT The left hemisphere is dominant for language in most people, but lateralization strength varies between different tasks and individuals. A large body of literature has shown that handedness is associated with lateralization: left handers have weaker language lateralization on average, and a greater incidence of atypical (right hemisphere) lateralization; but typically, these studies have relied on a single measure of language lateralization. Here we consider the relationships between lateralization for two different language tasks. We investigated the influence of handedness on lateralization 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-lateralization 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) lateralization or inconsistent lateralization between the two tasks. These results add to our knowledge of individual differences in lateralization and support the view that language lateralization is multifactorial rather than unitary.
Highlights
Left cerebral hemisphere dominance for language is one of the most robust findings in neuropsychology, but it is not a universal trait
Three more participants were rejected because the standard error of their Laterality Indices (LIs) values across trials was outside of the acceptable limits on one or both tasks
It is worth noting that the LI values observed in this study were lower than would be expected from comparable studies in the literature
Summary
Left cerebral hemisphere dominance for language is one of the most robust findings in neuropsychology, but it is not a universal trait. Functional imaging studies have shown this effect of handedness with various language tasks, including word generation ( known as phonemic or semantic fluency, depending on the task) (Badzakova-Trajkov, Haeberling, Roberts, & Corballis, 2010; Flöel, Buyx, Breitenstein, Lohmann, & Knecht, 2005; Häberling et al, 2011; Knecht, 2000; Krach, Chen, & Hartje, 2006; Powell, Kemp, & GarciaFinana, 2012; Pujol, Deus, Losilla, & Capdevila, 1999; Westerhausen et al, 2006; Whitehouse & Bishop, 2009), verb generation (Vernooij et al, 2007), animation description (Groen, Whitehouse, Badcock, & Bishop, 2013) and semantic association with spoken words (Szaflarski et al, 2002). A large fTCD study (N = 310) demonstrated that the prevalence of right or bilateral hemispheric dominance increased with the strength of left handedness (Somers et al, 2015)
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