Laterality of asymmetry in movements of the corners of the mouth during voluntary smile.

OBJECTIVES

To examine (1) the laterality of asymmetry in movements of the right and left corners of the mouth in space during voluntary smile and (2) the laterality of asymmetry in relation to the difference between the right and left hemiface size and the handedness.

MATERIALS AND METHODS

Participants were 155 volunteer Japanese female adults. They were categorized into the symmetric group (n = 120) and the right-side hemiface dominant group (n = 26) according to the hemiface size. In addition, the symmetric group was categorized into the right-handed group (n = 98) and the left-handed group (n = 22) according to the Edinburgh Handedness Inventory. Position vectors of the right and left corners of the mouth were obtained from the three-dimensional facial images for the rest, the maximal lip corner retraction, and the portrait smile. The displacements of the right and left corners of the mouth for each expression and the proportions of the subjects with the right- and left-sided laterality were compared.

RESULTS

The left corner of the mouth showed significantly greater displacement (P < .01) than the right in the symmetric group for the portrait smile. The left-sided laterality was found regardless of the handedness.

CONCLUSIONS

Displacements of the right and left corners of the mouth during voluntary smile were asymmetric, and the left-sided laterality was found. Also, the laterality of the facedness differed in relation to the hemiface size, but was not related to the handedness.