Abstract
Previous studies have revealed that non-human primates can differentiate the age category of faces. However, the knowledge about age recognition in non-human primates is very limited and whether non-human primates can process facial age information in a similar way to humans is unknown. As humans have an association between time and space (e.g., a person in an earlier life stage to the left and a person in a later life stage to the right), we investigated whether chimpanzees spatially represent conspecifics’ adult and infant faces. Chimpanzees were tested using an identical matching-to-sample task with conspecific adult and infant face stimuli. Two comparison images were presented vertically (Experiment 1) or horizontally (Experiment 2). We analyzed whether the response time was influenced by the position and age category of the target stimuli, but there was no evidence of correspondence between space and adult/infant faces. Thus, evidence of the spatial representation of the age category was not found. However, we did find that the response time was consistently faster when they discriminated between adult faces than when they discriminated between infant faces in both experiments. This result is in line with a series of human face studies that suggest the existence of an “own-age bias.” As far as we know, this is the first report of asymmetric face processing efficiency between infant and adult faces in non-human primates.
Highlights
Faces convey a lot of information to humans, such as age, identity, gender, and emotional states (Bruce and Young 2012; Rhodes et al 2011)
Dahl and Adachi (2013) conducted a matching-to-sample task in which chimpanzees were required to discriminate between the face identities of familiar group members that were presented in a vertical arrangement and found that chimpanzees have a spatial mapping of the dominance hierarchy similar to humans
Testing spatial mapping of face age was the main purpose of this study, we investigated whether the chimpanzees’ performance in discriminating adult faces and infant faces is asymmetric because face processing is largely modulated by the amount of the experiences
Summary
Faces convey a lot of information to humans, such as age, identity, gender, and emotional states (Bruce and Young 2012; Rhodes et al 2011). Dahl and Adachi (2013) conducted a matching-to-sample task in which chimpanzees were required to discriminate between the face identities of familiar group members that were presented in a vertical arrangement and found that chimpanzees have a spatial mapping of the dominance hierarchy similar to humans They reported that when the rank of the represented individual and the position in the display were congruent (e.g., a high-ranked individual was positioned higher), the response time was faster than when they were incongruent. Older infants (9 months) and adults can discriminate among conspecific faces, but not monkey faces, while younger infants (6 months) can discriminate both of them (Pascalis et al 2002) Such perceptual tuning based on very early experience in life is called perceptual narrowing and is observed in other face categories such as own- versus other-race faces in humans (“ownrace bias,” e.g., Kelly et al 2007). We compared their discrimination performance for adult faces, and that for infant faces to examine if they have age-related asymmetric processing efficiency based on the different amount of the experiences
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