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

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.

Keywords: Laterality index; functional transcranial Doppler sonography (fTCD); handedness; hemispheric dominance; language.

Figure 1. Time-course of trials in word generation and semantic association tasks.

Figure 2

Pirate plot (Phillips, 2017) showing distribution of laterality indices (LI) in left handed and right handed participants, in the semantic association (yellow) and word generation (blue) tasks. Positive LI values indicate left lateralisation; negative LI values indicate right lateralisation. The solid bar represents the mean, and the box represents the 95% confidence interval. The shaded area represents the density.

Figure 3. Density histogram of laterality indices (lateralisation strength) in left handed (yellow) and right handed (blue) participants.

Figure 4

Scatterplot of lateralisation indices on the word generation and semantic association tasks. The colour coding indicates the participants’ handedness (yellow=left handed, blue=right handed) and triangles indicate which participants were outliers using Cook’s distance measure.

Figure 5

Scatterplots showing a weak positive relationships between the continuous measures of handedness (top, Edinburgh Handedness Index; bottom, Quantification of Hand Preference) and the strength of lateralisation on the two tasks (left, word generation; right, semantic association).