Spatial anisotropy in crowding and ensemble perception.

Accurately perceiving the emotions of other people is an important task for the human visual system. Falsely judging a group of people to be happy when they are angry can have major negative consequences. While the effects of surrounding faces on the perception of a target face (crowding) and the perception of ensemble features of face groups (ensemble perception) have been extensively investigated separately, here we investigated how observers perceive the emotion of both individual faces and groups of faces with identical stimuli to better understand the relations of crowding and ensemble perception. In two experiments, we asked observers to report either the emotion of a central face surrounded by eight faces (central task) or the average emotion of all nine faces (averaging task). In both tasks, only one to two faces were strongly weighted compared with the other faces in the group, suggesting a similar number of sampled items across both tasks. However, we found a strong task- and location-dependent asymmetry: in the central task, the central face and the innermost face on the horizontal meridian (closest to fixation) were strongly weighted, whereas in the averaging task, mainly the two innermost faces on the horizontal meridian and in the upper visual field were strongly weighted. The results indicate that the perception of emotions in groups is strongly constrained by anisotropic weighting mechanisms due to discriminability; however, target discriminability alone did not predict weighting, which was modulated by task requirements. We suggest that the visual system optimizes perceptual selection and information integration according to task demands and visual uncertainty.