Abstract
The neural underpinnings of human emotional expression are thought to be unevenly distributed among the two 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 healthy participants. Measures of emotional expression were derived from pictures of the participant's faces in a 'happy smiling' and a 'sad frowning' conditions. Chimeric faces were constructed by mirroring right and left hemi-faces, as done in previous studies, resulting in a left mirrored and right mirrored chimeric face per picture. To gain measures of hemi-face dominance per participant, a jury of 20 additional participants rated which chimeric face shows the higher intensity of emotional expressivity, by marking a 155mm line between the two versions. Measures of the 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 and showed a large inter-individual variability. The degree of asymmetry in emotional expression was, however, significantly associated with the asymmetries in connective properties of the fronto-temporal and fronto-parietal tracts, specifically the uncinate fasciculus and the first branch of the superior longitudinal fasciculus. Therefore, this result raises novel hypotheses on the relationship of specific white matter tracts and emotional expression, especially their role in mood disorders.
Highlights
The neural underpinnings of human emotional expression are thought to be unevenly distributed among the two brain hemispheres
A second group of 20 participants judged the set of chimeric facial stimuli from the group of posers. They evaluated the Right/Right and Left/Left chimeric faces presented by pairs, at the extremities of a 155mm segment; they had to mark the segment according to the relative emotional intensity of the two stimuli, placing the mark closer to the stimuli that expressed the emotion more intensely, with the centre of the segment representing equal emotional intensity between the 2 stimuli
These subgroups were further subdivided in half, so that some started with the stimuli being presented in a Left/Left – Right/Right hemi-face disposal, and reversed Right/Right – Left/Left disposal, in order to control for pseudoneglect effect (Jewell and McCourt, 2000)
Summary
The neural underpinnings of human emotional expression are thought to be unevenly distributed among the two brain hemispheres. A variety of theoretical approaches have been developed to frame these findings, such as the Right Predominance hypothesis, the Valence & Approach/Withdrawal model (Demaree et al 2005, Silberman and Weingartner 1986) and the Homeostatic Theory of Emotion (Craig, 2005) This has resulted in a further increase in the array of experimental designs and research targets applied by the investigators. A significant preference for the left hemi-spatial field even exists when chimpanzees—our closest primate ancestor—have to discriminate chimeric human smiling faces (Morris and Hopkins 1993) Given these premises, one might expect asymmetrical processing of emotional expression in humans
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