These authors suggested that, although information gained from the mouth region is highly socially relevant, revealing the size of the canines, viewing such features in agonistic displays, like threats and bared-teeth displays, might be aversive. Using a technique that scans the gaze patterns made by rhesus monkeys when processing images, Nahm and colleagues ( Nahm, Perret, Amaral, & Albright, 1997) found viewing preferences for the eyes over the mouth region when monkeys were shown naturalistic photographs of conspecifics' open mouth threats, fear grins (i.e., bared-teeth display), and neutral faces, but greater visual exploration was directed to the mouth region for yawns and lip-smack displays. Strong traditions in ethology and animal behavior research have provided detailed descriptions of primate facial expressions, but only a handful of studies to date have examined how nonhuman primates discriminate their own facial expressions and which features are most salient or conspicuous. To understand the evolutionary significance of primate facial expressions, comparative studies must examine in detail how similar these expression are to human emotional expressions and, moreover, whether nonhuman primates process these expressions using similar cognitive and perceptual strategies.
This universality, as it is described, provides support for the idea that emotional facial expressions are strongly biological in nature and, therefore, any similarity between facial expressions in human and nonhuman primates might reflect a similar underlying emotion and/or motivation ( Chevalier-Skolnikoff, 1973 Darwin, 1872/1955). Among humans, it has long been claimed that specific facial expressions correspond to basic emotions, such as anger, fear, happiness, sadness, surprise, and disgust, and that these are universally recognized across different cultures ( Ekman, 1973 Ekman & Friesen, 1971). The ability to discriminate facial expressions and interpret their meaning during ongoing social interactions are critical skills for many animal species, but particularly for primates that rely on visual cues as one of their primary modes of social communication. Therefore, similar to humans, both configuration and component movements are important during chimpanzee facial expression processing. However, these were not necessarily the only movements implicated in subject's overall pattern of errors. For each expression category, at least 1 component movement was more salient or representative of that expression than the others. Finally, subjects were asked to match each facial expression category using only individual component features. Third, a multidimensional scaling analysis examined the perceived dissimilarity among these facial expressions revealing 2 main dimensions, the degree of mouth closure and extent of lip-puckering and retraction.
First, chimpanzees were found to accurately discriminate between these expressions using a computerized matching-to-sample task, and recognition was impaired for all but one expression category when they were inverted. Four experiments examined chimpanzee facial expression processing using a set of standardized, prototypical stimuli created using the new ChimpFACS coding system. The ability to recognize and accurately interpret facial expressions are critical social cognition skills in primates, yet very few studies have examined how primates discriminate these social signals and which features are the most salient.
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