The left side superiority effect for facial expression perception is not a left visual field superiority effect

摘要

The perception of the expression of a centrally placed face is dominated by the information from the right side of face or the left visual field. Such left superiority effect, also called left visual field superiority, has been considered as resulted from the brain lateralization for face processing. We tested this hypothesis with a split face paradigm. The face images were pictures of four individuals selected from a facial expression image database (Chen et al., 2009). We interpolated the pictures of the happy and sad expressions of the same individuals to create a series of morphed images with morphing level from 0% (sad) to 100% (happy) at 20% interval. The test stimuli were split faces with the left and the right sides at different morphing levels. The test images were presented either at the fovea, or 5 degree to the left or the right of the fixation. The task of the observer was to indicate whether the test face was sad or happy. The observers showed a strong left side superiority effect in all viewing conditions. Hence, the left side superiority is not due to a greater sensitivity to the left visual field and thus is not due to cortical lateralization for face processing. Instead, it is likely resulted from an object-based processing. The left side superiority was enhanced in the right visual field condition but reduced in the left condition. Such difference may be due to the cortical magnification that made an observer be more sensitive to the side of the image that was closer to the fixation. The left side superiority was more stable for the positive expression but declined significantly for the negative expression when the stimuli were moved from the fovea to the periphery, indicating separate mechanisms for these two types of expression.