In a previous study with dextral males, Richardson and Bowers (1999) digitized real time video signals and found movement asymmetries over the left lower face for emotional, but not non-emotional expressions. These findings correspond to observations, based on subjective ratings of static pictures, that the left side of the face is more intensely expressive than the right (Sackeim, 1978). From a neuropsychological perspective, one possible interpretation of these findings is that emotional priming of the right hemisphere of the brain results in more muscular activity over the contralateral left than ipsilateral right side of the lower face. The purpose of the present study was to use computer-imaging methodology to determine whether there were gender differences in movement asymmetries across the face. We hypothesized that females would show less evidence of facial movement asymmetries during the expression of emotion. This hypothesis was based on findings of gender differences in the degree to which specific cognitive functions may be lateralized in the brain (i.e., females less lateralized than males). Forty-eight normal dextral college students (25 females, 23 males) were videotaped while they displayed voluntary emotional expressions. A quantitative measure of movement change (called entropy) was computed by subtracting the values of corresponding pixel intensities between adjacent frames and summing their differences. The upper and lower hemiface regions were examined separately due to differences in the cortical enervation of facial muscles in the upper (bilateral) versus lower face (contralateral). Repeated measures ANOVA’s were used to analyze for the amount of overall facial movement and for facial asymmetries. Certain emotions were associated with significantly greater overall facial movement than others (p < .0001), beginning with surprise and followed by happy > fear > (angry = sad) > neutral. Both males and females showed this same pattern, with no gender differences in the total amount of facial movement under voluntary conditions. In males, movement asymmetries favoring the lower left side of the face occurred for most emotional expressions. For females, all emotions were symmetric over the lower face. Our findings with computer digitizing techniques support the hypothesis that there are gender differences in facial movement asymmetries during the expression of emotion. They further underscore the view that emotional processing may represent a more widely distributed system throughout the brain in women than in men, corresponding to previous reports that language processes are also less lateralized in women.
Psychogenic facial movement disorder: a case report