White matter correlates of hemi-face dominance in emotional expression

The neural underpinnings of human emotional expression are thought to be unevenly distributed among brain hemispheres. However, little is known on the anatomy supporting this claim, particularly in the cerebral white matter. Here we explored the relationship between hemi-face dominance in emotional expression and cerebral white matter asymmetries in 33 participants. Measures of emotional expression were derived from picture of their faces in a happy smiling and a sad frowning conditions. Chimeric faces were constructed by assembling right and left hemi-faces, respectively, with their mirrored version. These faces were presented in competition at the extremities of a 155mm segment to be marked according to the emotional intensity. A jury made of 10 additional participants evaluated the set of chimeric facial stimuli. Hemispheric asymmetry of the uncinate, the cingulum and the three branches of superior longitudinal fasciculi were derived from diffusion-weighted imaging tractography dissections. Group effect analyses indicated that the degree of asymmetry in emotional expression was not as prominent as reported in the literature with a large inter-individual variability. Secondly, anatomical dissections replicated previous finding in white matter anatomical lateralisation. Lastly, a strong relationship was revealed between SLF I and happy hemi-face dominance so that the greater the hemispheric lateralisation of SLF I, the more the expression of happiness in the contralateral half of the face. This result brings up interesting hypotheses on the relationship of the SLF I and emotional expression, especially its role in mood disorders.