Abstract

Researchers have proposed several different models that invoke specialization of function of the right, the left, or both hemispheres to explain asymmetries in facial actions. Some models implicate specialization for emotion; others, for non-emotional processes such as perception of expressions. This study employed three major strategies for assessing the viability of these various models. First, asymmetry of individual muscular actions was measured to determine whether all show the same asymmetry, as some models imply. Second, several conditions elicited two major types of facial movement, deliberate and spontaneous, each subserved by different neural pathways. Differences between these two types of movement indicated whether models implicating emotional processes are tenable. Third, asymmetry of deliberate actions was measured in four ways: 1) which side of bilateral actions had stronger contractions, 2) which side could, on request, show a more unilateral action, 3) which side subjects preferred for unilateral actions, and 4) which side subjects rated easier for unilateral movement.

Laterality varied for different actions for measures 1 and 2. These results indicated that all asymmetry in facial actions is unlikely to be caused by specialization of a single hemisphere for one kind of process. Measures 3 and 4 showed only weak or no laterality.

Spontaneous actions involved in positive emotion and startle reflexes were generally more symmetrical than deliberate actions and rarely showed laterality. Lateralization of deliberate actions, when observed, was mostly opposite to that predicted by theories of right hemispheric specialization for negative or avoidance emotions and left specialization for positive or approach emotions. These results indicated that asymmetry of deliberate actions is not caused by hypothesized laterality of emotion. Instead, evidence suggested that asymmetry was related to differentiated motor control on each side of the face.

Only half of the correlations between the four asymmetry measures for individual, deliberate actions were significantly positive and their magnitude was modest. These and other results suggested that although the measures tapped common variance, they did not always measure the same thing.

No single model fits all the results of this study, indicating the inadequacy of existing and the need for new conceptualizations.