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. 2021 Jul 5;31(8):3780-3787.
doi: 10.1093/cercor/bhab048.

Left-Handers Are Less Lateralized Than Right-Handers for Both Left and Right Hemispheric Functions

Leah T Johnstone  1   2 , Emma M Karlsson  1 , David P Carey  1
Affiliations

Left-Handers Are Less Lateralized Than Right-Handers for Both Left and Right Hemispheric Functions

Leah T Johnstone et al. Cereb Cortex. .

Abstract

Many neuroscientific techniques have revealed that more left- than right-handers will have unusual cerebral asymmetries for language. After the original emphasis on frequency in the aphasia and epilepsy literatures, most neuropsychology, and neuroimaging efforts rely on estimates of central tendency to compare these two handedness groups on any given measure of asymmetry. The inevitable reduction in mean lateralization in the left-handed group is often postulated as being due to reversed asymmetry in a small subset of them, but it could also be due to a reduced asymmetry in many of the left-handers. These two possibilities have hugely different theoretical interpretations. Using functional magnetic resonance imaging localizer paradigms, we matched left- and right-handers for hemispheric dominance across four functions (verbal fluency, face perception, body perception, and scene perception). We then compared the degree of dominance between the two handedness groups for each of these four measures, conducting t-tests on the mean laterality indices. The results demonstrate that left-handers with typical cerebral asymmetries are less lateralized for language, faces, and bodies than their right-handed counterparts. These results are difficult to reconcile with current theories of language asymmetry or of handedness.

Keywords: brain asymmetry; fMRI; handedness; left hemisphere; right hemisphere.

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Figures

Figure 1
Figure 1
Threshold-dependent group activation maps for individuals left lateralized for verbal fluency, as a function of handedness. (LH = 43; RH = 31). The data are visualized at a threshold of P < 0.001 with family-wise error (FWE) correction at the cluster level.
Figure 2
Figure 2
Threshold-dependent group activation maps for individuals right lateralized for bodies (top row), faces (middle row), and scenes (25; bottom row) as a function of handedness. The data are visualized at a threshold of P < 0.001 with FWE-correction at the cluster level.
Figure 3
Figure 3
Mean laterality index (LI) scores for the four functions, only in individuals who show typical dominance for each. Error bars indicate standard error. LI values > 0 represent threshold-independent left-hemisphere dominance. The mean LI asymmetry is reduced in all four left-handed samples. Note that the bars for the four different asymmetries were derived from slightly different individuals, as the only inclusion criteria for each was typical dominance for that function. All P values are one-tailed.
Figure 4
Figure 4
Mean laterality index (LI) scores for the three non-language functions, only in individuals who show typical dominance for each, without the left-handed language atypicals (who happened to be typical on these functions). Error bars indicate standard error. The mean LIs remain significantly reduced in the left-handers for face and body processing.
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