Familiar face + novel face = familiar face? Representational bias in the perception of morphed faces in chimpanzees

Abstract

Highly social animals possess a well-developed ability to distinguish the faces of familiar from novel conspecifics to induce distinct behaviors for maintaining society. However, the behaviors of animals when they encounter ambiguous faces of familiar yet novel conspecifics, e.g., strangers with faces resembling known individuals, have not been well characterised. Using a morphing technique and preferential-looking paradigm, we address this question via the chimpanzee's facial-recognition abilities. We presented eight subjects with three types of stimuli: (1) familiar faces, (2) novel faces and (3) intermediate morphed faces that were 50% familiar and 50% novel faces of conspecifics. We found that chimpanzees spent more time looking at novel faces and scanned novel faces more extensively than familiar or intermediate faces. Interestingly, chimpanzees looked at intermediate faces in a manner similar to familiar faces with regards to the fixation duration, fixation count, and saccade length for facial scanning, even though the participant was encountering the intermediate faces for the first time. We excluded the possibility that subjects merely detected and avoided traces of morphing in the intermediate faces. These findings suggest a bias for a feeling-of-familiarity that chimpanzees perceive familiarity with an intermediate face by detecting traces of a known individual, as 50% alternation is sufficient to perceive familiarity.

Keywords: Categorical perception; Chimpanzee; Face perception; Familiarity; Novelty; Preferential looking.

Figure 1. Visual preferences of chimpanzees for different types of faces.

(A) An example of three different types of stimuli: familiar face (i), intermediate face (ii) and novel face (iii). (B) The mean-proportional fixation duration for each of the face types. (C) The mean-proportional fixation count for each of the face types. (D) The mean-proportional saccade length for each of the face types. Boxplots describe the responses to each type of facial stimuli. *p < 0.05, **p <0.01 (Friedman test).

Figure 2. Visual preferences of chimpanzees for different face types.

(A) The mean proportional fixation duration for each of the face types: a 100% novel face and a 50%–50% morphed face with different novel faces. (B) The mean proportional fixation count for each of the face types. (C) The mean proportional saccade length for each of the face types. Boxplots describe the responses to each type of face stimuli. n.s.: no significant difference (Wilcoxon signed-rank test).