Primates-May 2018

**Wilson, D. A., & Tomonaga, M. (2018). Visual discrimination of primate species based on faces in chimpanzees. *Primates,* 1-9.**

Previous literature on facial processing in human and nonhuman primates suggests that experience and familiarity play a large role in face processing and preference. However, there is debate about *how* the processing of facial stimuli actually works (i.e. configural processing vs feature-based processing). Configural processing is defined as “the emergent features of a face that only become apparent when two or more of its basic features (e.g., the eyes, nose, or mouth) are processed at the same time” (Wilson & Tomonaga, 2018). For example, Gothard (2008) found that macaques recognize both same species and human faces but the processing may differ between the two (i.e. processing human faces is feature based while processing monkey faces is both feature- based and configural). In order to determine relevant factors for chimpanzee face discrimination, the authors used a match-to-sample task that presented primate facial stimuli in a variety of conditions. Five adult female chimps participated in the study. Stimuli included photographs of following species’ faces: chimpanzee, gorilla, orangutan, Japanese human, and baboon containing unfamiliar faces with neutral expressions. The task was a standard zero-delay match-to-sample with a sample and three comparison stimuli. Conditions were designed to assess the impact of color, orientation, familiarity and perceptual similarity on primate facial discrimination as well as performance on identical (same image) and categorical (different image, same species) matching. There were five conditions as follows: (1) Identical and categorical matching of chimp, gorilla and orangutan faces in color and greyscale, (2) Identical matching of greyscale stimuli in upright and inverted orientations, (3) Identical and categorical matching of greyscale baboon and capuchin images (unfamiliar/perceptually different); (4) Identical and categorical matching of chimp and human (familiar/ perceptually different); (5) Identical and categorical matching of orangutan and gorilla (unfamiliar/ perceptually similar). It should be noted that perceptual similarity was determined by researchers’ perception of relative similarity between two different species’ faces. Wilson and Tomonaga (2018) found that there were (1) higher accuracy and RTs for color images and identical matching, (2) higher accuracy for upright images, higher accuracy and RTs for (3) capuchin and baboon and (4) human and chimpanzee than for (5) orangutan and gorilla. There was also an interaction with decreased performance for categorical matching of gorilla and orangutan faces. Based on the results, the authors suggest face-specific configural processing due to the inversion effect found in condition two. However, there were no differences found between familiar and unfamiliar faces which is contradictory to previous findings. The offer several explanations for this result: (1) Individual within-species recognition takes evolutionary precedence over discrimination between individuals of other species and between two different species. Between-species recognition may operate at a categorical level rather than individual recognition, and (2) Perceived similarity between faces may have obscured familiarity effects. The authors posit that perceived similarity between faces plays a large role in discrimination with more similar faces being more difficult to distinguish. Future studies could test the generalizability of these claims to see whether perceived similarity is observed between primate species’ faces and whether this may take precedence over previous experience.