Facial Recognition
Faces are an important component of social communication as they provide identifying information and also signal emotional content. The information we extract from individual faces guides many of our interactions and provides cues to adapt our behavior. Most nonhuman primates live in complex groups similar to humans with rules of behavior. It is essential that individuals are able to identify others and communicate aspects of social relationships, such as bonds and dominance status, in order to properly interact.
Humans are thought to have a specialized neural mechanism dedicated to face processing and the processing of highly familiar stimuli. This is part of a broader network of neural areas supporting face processing. Studies in nonhuman primates, specifically rhesus macaques, have also found a network of neural areas involved in face processing. These include the inferior temporal gyrus, the superior temporal sulcus (STS), the frontal cortex and the amygdala. Certain populations of neurons in the STS code for particular facial expressions, are sensitive to gaze direction or views of the faces and are responsive to particular individuals irrespective of any facial expression or change in viewpoint. This provides a neurological basis for the ability to discriminate between faces of different individuals while also generalizing across different viewpoints of the same individual.
Facial recognition has been studied in several primate species primarily in macaques and chimpanzee and less so in New World monkeys. Several studies that have been conducted with monkeys have used implicit tasks, such as the visual paired comparison task. Typically these tests present the identical image as image the subject habituated to which makes it difficult to know whether subjects would generalize across viewpoints of those individuals. Explicit tasks, such as match-to-sample, have also been used. The findings suggest that monkeys are able to discriminate the faces of conspecifics but often this requires hundreds and sometimes thousands of trials in which to do so. This leaves open the possibility that subjects learned a stimulus set not that they are able to recognize the faces of conspecifics from different viewpoints.
We investigate the face recognition abilities of capuchin monkeys, a New World species. It is useful to study these processes in distantly related species to determine whether face recognition is a general primate ability or if it has evolved more recently and is therefore limited to Old World primates, or even just to apes and humans. In general, incorporating information from a greater variety of primate species helps to identify which processes are similar as well as aid in determining where along the evolutionary split these abilities originated.
To examine the face recognition skills of the capuchins, we employed an oddity paradigm, presenting three different images of one individual and one image of a different individual. Subjects successfully performed this task and positively transferred to a new set of images within in the first 50 trials (Pokorny, J. J, & de Waal, F. B. M., in preparation). The findings demonstrate conclusive evidence from an explicit task that a New World monkey species can recognize the faces of conspecifics. This also allows us to ask questions about their social environment using facial images of their conspecifics.
