Perception of nonhuman primate facial expressions
Lisa Parr
Yerkes National Primate Research Center, Emory University
Primates send and receive an array of facial signals in order to navigate their social environment, and these communicative displays have undoubtedly been influential in the evolution of both primate mind and society. Yet facial expression is a relatively understudied mode of primate communication. Several recent developments, including FACS-based coding systems for nonhuman primates, have advanced our understanding of these important signals by improving their method of measurement and expanding theoretical questions. In this talk, I will present data on the perception of facial expressions in chimpanzees using standardized stimuli, normalized for individual muscle movements using the ChimpFACS coding system. Chimpanzees were presented with all combinations of these stimuli to discriminate using a matching-to-sample, computerized task. Their pattern of errors was analyzed using multidimensional scaling which identified several key features underlying their perception, including mouth and lip movements. A follow-up experiment presented subjects with the task of matching individual movements to prototypical expression configurations to determine which movements were most salient. For each expression, at least one movement was found to be most prominent. Moreover, subjects’ error patterns were best explained not by the number of key features that two expressions shared but whether or not they shared this key feature. Comparable data on facial expression discriminations will be presented from macaque monkeys, along with a presentation of a comparable FACS system for measuring facial movement in macaques. The overarching goals of this research are to provide more objective and standardized tools for measuring and comparing facial expressions across primate species. This will, in turn, help refine experimental methodologies where expressions can be equated in terms of their component features, and not overall impression. And finally, to help in identifying the specific role facial expressions play in the lives of primates.