Within- and Between-Category Face Identity Discrimination: Association with Physical Facial Features

Facial identity discrimination is crucial for success in the social world. Yet, how this task is accomplished remains incompletely understood. Face perception involves identifying members of same or different of social categories (such as gender and race). We know that differences in facial morphology are critical for discriminating individual faces. What is unclear, however, is whether the same mechanisms are used to discriminate identities when they are members of the same versus different social categories. To address this question, this study examined and compared perceptual performance on withinand between-category facial identity discrimination tasks and evaluated the relationship between the perceptual performances and physical features of face images used for discrimination. Face images from each of five pairs of individuals (two Asian females, two Caucasian females, two African males, one African male and one Caucasian male, or one Caucasian female and one Caucasian male) were morphed to create images along a continuum of facial dissimilarity. Using the original and created images, with psychophysical methods we measured perceptual thresholds for facial identity discrimination in a group of human observers (n=24). For each pair of original face images, the differences in six physical facial features (end-lip raise, mid-top-lip raise, mid-top-lip, mid-low-lip, eye-opening, mid-eyebrow raise and luminance) were also measured. Perceptual thresholds were the lowest (best performance) for the across-race condition, the second lowest for the across-gender condition and the highest for the three same-race and same-gender conditions. Two physical facial features, mid-top-lip raise and luminance, each accounted for a significant portion of perceptual performance (35% and 40%, respectively). These results indicate that between-category facial identity discrimination is more precise than within-category discrimination. Precise discrimination of facial identity is associated with selective physical facial features.