Body perception in chimpanzees and humans: The expert effect

Abstract

Both humans and chimpanzees have better performances when recognizing faces or bodies when the stimuli are upright compared to inverted. This is called the inversion effect. It suggests that these two species use a specific way to process faces and bodies. Previous research has suggested that humans also show the inversion effect to objects that they have expertise about, and this is called the expert effect. We investigated whether chimpanzees show the expert effect and how humans and chimpanzees differ by testing chimpanzees (human experts) with human body stimuli and testing humans (chimpanzee experts) with chimpanzee and human body stimuli in body recognition tasks. The main finding was that humans (chimpanzee experts) showed the expert effect to chimpanzee bodies, while chimpanzees partially showed it to human bodies. This suggests that compared with chimpanzees, the special processing in humans can be more flexibly tuned for other objects. We also tested humans that were not chimpanzee experts using chimpanzee body stimuli. Although they showed similar performances as the chimpanzee experts, the two groups had differences in some situations, indicating the effect of expertise. This study revealed the important role of experience in object processing in humans, and our evolutionary relatives, chimpanzees.Faces and bodies provide very important social cues for animals. It has been widely reported that humans have a special way to process faces and bodies, as opposed to other objects, through configural processing (e.g., faces: 1-4 ; bodies: 5-7 ). Compared with other objects such as houses or pictures of scenery, the recognition of faces or bodies significantly deteriorates with inversion of the stimuli, which is called the inversion effect. The inversion effect of faces and bodies suggests that humans perceive them as a pre-set template and a holistic configuration rather than as combinations of local features, as is the case with other objects.Humans' perception narrows specifically to conspecifics 8 . Infants of 6 months old could discriminate faces of other species, while older infants and adults could not. However, despite the perception narrowing, many studies have found that the configural processing can be tuned for objects of expertise. In Diamond and Carey's classic study 2 , it was found that dog experts showed the inversion effect when recognizing dogs, while dog novices did not. The researchers claimed that dogs, and all other biological classes whose members are individualized (in other words, having distinct bodies), can invoke configural processing, because they all share the same configuration. Dog experts are familiar with dogs, and are able to exploit the distinguishing internal relational features of dogs better than novices can, so they showed the expert effect and perceived dogs in the same way that people do with human faces (and bodies). This experiment has not always been successfully replicated 9,10 . However, there were several studies that showed the effect of expertise. Gauthier e...

How chimpanzees and children perceive other species’ bodies: Comparing the expert effect

“…Chimpanzee participants showed the inversion effect to chimpanzee bodies, regardless of body posture. As human experts, our chimpanzee participants did not show the inversion effect to bipedal humans with unfamiliar postures in a previous study (Gao et al, 2020), but they showed the inversion effect to bipedal humans with familiar postures, as well as crawling humans, in this study. The inversion effect was also found in the horse and monkey conditions although the chimpanzees had never seen horses.…”

“…The chimpanzees showed the inversion effect to both chimpanzee body conditions (quadrupedal and bipedal), regardless of their postures. They also showed the inversion effect to crawling humans and bipedal humans with familiar postures in this study, while they did not show this response to bipedal humans with unfamiliar postures in the previous study (Gao et al, 2020). These results suggest that, for conspecifics, the postures do not matter, whether they are Chimpanzees used both visual and embodied cues when processing human bodies.…”

“…The number of pictures was the same as chimpanzee experiment. The bipedal human stimuli were the same as that used in the previous study (Gao et al, 2020). The images included human adults standing and showing biologically possible postures.…”

“…The six kinds were: quadrupedal chimpanzees, bipedal chimpanzees, crawling humans, bipedal humans with familiar postures, horses, and monkeys ( Figure 2). In the previous study in chimpanzees (Gao & Tomonaga, 2018), the stimuli were a mix of chimpanzee In the previous study (Gao et al, 2020) TA B L E 1 General characteristics of the seven chimpanzees chimpanzees saw in their daily interactions with humans, and were rated as being more familiar than those used in the previous study before being chosen to use. The horse and monkey pictures were all in natural, quadrupedal postures.…”

“…Captive chimpanzees are supposed to be experts at humans. However, they did not show any inversion effect when recognizing human body stimuli (Gao, Kawakami, & Tomonaga, 2020). Another study also failed to observe the expert effect in dog experts (Robbins & McKone, 2007); the authors argued that this may be due to a lack of embodied experience with dog postures.…”