Familiarity mediates apes' attentional biases toward human faces

Abstract

In zoos, primates experience markedly different interactions with familiar humans, such as the zookeepers who care for them, compared with those with unfamiliar humans, such as the large volume of zoo visitors to whom they are regularly exposed. While the behaviour of zoo-housed primates in the presence of unfamiliar, and to a lesser extent familiar, humans has received considerable attention, if and how they spontaneously distinguish familiar from unfamiliar people, and the cognitive mechanisms underlying the relationships they form with familiar and unfamiliar humans, remain poorly understood. Using a dot-probe paradigm, we assessed whether primates (chimpanzees and gorillas) show an attentional bias toward the faces of familiar humans, with whom the apes presumably had a positive relationship. Contrary to our predictions, all subjects showed a significant attentional bias toward unfamiliar people's faces compared with familiar people's faces when the faces showed a neutral expression, both with and without a surgical face mask on, but no significant attentional bias when the faces showed a surprised expression. These results demonstrate that apes can spontaneously categorize humans based on familiarity and we argue that the attentional biases the apes showed for unfamiliar human faces reflect a novelty effect.

Attention toward emotions is modulated by familiarity with the expressor: A comparison between bonobos and humans.

“…We believe the study by Lewis et al (2021) and ours complement each other in showing that at least for bonobos, seeing familiar conspecifics brings along a range of potentially relevant social information such as rank and emotional expressions that in turn may modulate attention differently. Furthermore, Leinwand et al (2022) showed that chimpanzees (and gorillas) have an attentional bias toward unfamiliar humans with a neutral expression compared to familiar humans, whereas they do not have a bias toward unfamiliar or familiar humans when they show a surprised expression. Combined, the existing findings show a modulating role for familiarity and emotional expressions, but biases may be informed by species-specific characteristics and by the model species on the stimuli.…”

“…Therefore, we believe an interesting next step would be to study more closely how familiarity with the expressor modulates attention for emotions across different great-ape species. Furthermore, findings by Leinwand et al (2022) show it may be interesting to also include expressions of familiar and unfamiliar individuals of a different great-ape species, for instance to gain insight into (dis)similarities in emotional expressions across the great-ape family.…”

“…However, we could rule this out, because bonobos on average responded as fast to stimuli of unfamiliar (novel) as of familiar individuals. A worthwhile follow-up study is to directly compare familiar and unfamiliar individuals (emotional and neutral) within trials to disentangle the effects of emotion and familiarity (as has recently been done by Leinwand et al, 2022 using human stimuli). In addition, studying an attentional bias toward emotions of familiar and unfamiliar individuals in chimpanzees could be a fruitful next step.…”

“…As attention gates that signal from the environment are preferentially processed, it is therefore plausible that evolution fine-tuned this mechanism to quickly differentiate not only between emotional and neutral cues but also between expressions of familiar, socially close group members and unfamiliar others. Interestingly, a recent study with chimpanzees and gorillas ( Gorilla gorilla ) showed that they have an attentional bias toward unfamiliar human faces showing a neutral expression (Leinwand et al, 2022), but this bias disappeared when the human models displayed an emotional (surprised) face. Currently, we do not yet know if and how familiarity and emotional expressions interact when viewing conspecifics.…”

‘Do I know you?’ Categorizing individuals on the basis of familiarity in kea ( Nestor notabilis )

“…In line with the 'pre-exposure hypothesis' ( postulated by Lee and colleagues [30]), domesticated species and those living in urban environments have frequently been tested [21,25,26,[29][30][31][32][33][34]. However, populations of laboratory animals have generally been ignored with the noteworthy exceptions of the pigeon studies [26,28] and the most recent study by Leinwand et al [35] of a zoo ape population (chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla gorilla)).…”