Asymmetric dependencies in perceiving identity and emotion: experiments with morphed faces

We investigated whether an asymmetric relationship between the perception of identity and emotional expressions in faces (Schweinberger & Soukup, 1998) may be related to differences in the relative processing speed of identity and expression information. Stimulus faces were morphed across identity within a given emotional expression, or were morphed across emotion within a given identity. In Experiment 1, consistent classifications of these images were demonstrated across a wide range of morphing, with only a relatively narrow category boundary. At the same time, classification reaction times (RTs) reflected the increased perceptual difficulty of the morphed images. In Experiment 2, we investigated the effects of variations in the irrelevant dimension on judgments of faces with respect to a relevant dimension, using a Garner-type speeded classification task. RTs for expression classifications were strongly influenced by irrelevant identity information. In contrast, RTs for identity classifications were unaffected by irrelevant expression information, and this held even for stimuli in which identity was more difficult and slower to discriminate than expression. This suggests that differences in processing speed cannot account for the asymmetric relationship between identity and emotion perception. Theoretical accounts proposing independence of identity and emotion perception are discussed in the light of these findings.