Handedness and midsagittal corpus callosum morphology: a meta-analytic evaluation

Abstract

Following a series of seminal studies in the 1980s, left or mixed hand preference is widely thought to be associated with a larger corpus callosum than right handedness, influencing the interpretation of findings and various theories related to interhemispheric processing, brain lateralisation, and hand preference. Recent reviews, however, find inconsistencies in the literature and cast doubt on the existence of such an association. The present study was conducted to clarify the relationship between hand preference and callosal morphology in a series of meta-analyses. For this purpose, articles were identified via a search in PubMed and Web Of Science databases. Studies reporting findings relating to handedness (assessed as hand preference) and corpus-callosum morphology in healthy participants were considered eligible. On the basis of a total of k = 24 identified studies and databases, random-effects meta-analyses were conducted considering four different group comparisons: (a) dominantly right- (dRH) and left-hand preference (dLH), (b) consistent right (cRH) and non-cRH preference, (c) cRH with mixed-hand preference (MH), and (d) cRH with consistent left-hand hand preference (cLH). For none of these meta-analyses did we find a significant effect of hand preference, and narrow confidence intervals suggest that the existence of population effects larger than 1% explained variance could be excluded. For example, considering the comparison of dRH and dLH (k = 14 studies; 1910 dRH and 646 dLH participants) the mean effect size was Hedge's g = 0.016 (95% confidence interval: - 0.12 to 0.15; explained variance: < 0.001%). Thus, the common practice of assuming an increase in callosal connectivity based on mixed or left hand preference is likely invalid.

Keywords: Brain asymmetry; Corpus callosum; Hand preference; Handedness.

Fig. 1

Overview of study identification, screening, and eligibility assessment. All included studies and datasets are presented in Table 1. All studies that were initially considered eligible but did not provide sufficient information for a statistical inclusion or were redundant (i.e., sample overlap) to other included reports can be found in Supplement Table S5 (with a reason for the exclusion)

Fig. 2

Illustration of corpus callosum subdivision used in the meta-analyses. The approach followed the straight-line method introduced by Jancke et al. (1997). The outline of the corpus callosum (black line) is divided into thirds relative to its anterior–posterior extend. The posterior third is additionally split into a posterior fifth (i.e., the splenium) and the isthmus

Fig. 3

Forest plot of the meta-analysis of studies comparing dominant right hand (dRH) and dominant left hand (dLH) samples (dependent variable: total corpus callosum size). The total sample size across all k = 14 studies was n = 1910 for dRH and n = 646 for dLH sample. Negative values indicate the dLH group to have a larger corpus callosum, positive values indicate the dRH group to have a larger corpus callosum. HCP 2017 = Human Connectome Project, data release 2017 (see also Van Essen et al. 2013)

Fig. 4

Forest plot of the meta-analysis of studies comparing consistent right-handers (cRH) and non-cRH (NcRH) (dependent variable: total corpus callosum size). The analysis included k = 12 studies with a total sample of n = 1149 for dRH and n = 1121 for NcRH. Negative values indicate the NcRH group to have a larger corpus callosum, positive values indicate the cRH group to have a larger corpus callosum. HCP 2017 = Human Connectome Project, data release 2017 (see also Van Essen et al. 2013)

Fig. 5

Overview of subsection meta-analyses comparing cRH and NcRH samples. The graph presents the effect size (Hedges’ g) and standard error of the effect size (se(g)) for each study. Negative values indicate the subsection to be larger in the NcRH group, positive values indicate the subsection to be larger in the cRH group. The provided average effect is estimated using a random-effects model. The values in brackets are the 95% confidence interval. Color coding was based on the Cohen’s effect-size heuristics (Cohen 1992) as indicated in the figure legend. Note, for some studies data was not available (n.a.) for some of the subsections. HCP 2017 = Human Connectome Project, data release 2017 (see also Van Essen et al. 2013)

Fig. 6

Forest plot of the meta-analysis of studies comparing consistent right-handers (cRH) and consistent left-handers (cLH) (dependent variable: total corpus callosum size). It included k = 11 studies with a total sample of n = 1142 cRH and n = 306 cLH participants. Negative values indicate the cLH group to have a larger corpus callosum, positive values indicate the cRH group to have a larger corpus callosum. HCP 2017 = Human Connectome Project, data release 2017 (see also Van Essen et al. 2013)

Fig. 7

Forest plot of the meta-analysis of studies comparing consistent right-handers (cRH) and mixed-handers (MH) (dependent variable: total corpus callosum size). The analysis included k = 11 studies with a total sample of n = 1139 cRH and n = 810 MH participants. Negative values indicate the MH group to have a larger corpus callosum, positive values indicate the cRH group to have a larger corpus callosum. HCP 2017 = Human Connectome Project, data release 2017 (see also Van Essen et al. 2013)