The structure of face--space is tolerant to lighting and viewpoint transformations

According to the face-space framework, faces are represented as locations in a multidimensional space, where the distance separating representations is proportional to the degree of dissimilarity between faces. The present study tested whether similarities between faces, and thus the structure of face-space, were tolerant to ("invariant" under) identity-preserving transformations such as changes in lighting or view. To examine the correspondence between the configurations of face-space under different transformations, perceived similarity was rated for two variants of a set of faces, differing either in illumination (Experiment 1) or viewpoint (Experiment 2). We found that similarity ratings within the first variant were highly correlated with ratings within the second variant. In addition, based on these ratings, a separate face-space was constructed for each variant using multidimensional scaling. Procrustean analysis revealed that the different spaces shared comparable structures. This correspondence serves as a face-space manifestation of the tolerance of identity representations. Accordingly, we suggest that tolerance may rely on the fact that similarities between faces under one transformation are isomorphic to similarity patterns under a different transformation. Thus, recognizing faces under varying viewing conditions may only require similarity evaluations within--rather than across--different transformations.