Abstract
Recognition and individuation of conspecifics by their face is essential for primate social cognition. This ability is driven by a mechanism that integrates the appearance of facial features with subtle variations in their configuration (i.e., second-order relational properties) into a holistic representation. So far, there is little evidence of whether our evolutionary ancestors show sensitivity to featural spatial relations and hence holistic processing of faces as shown in humans. Here, we directly compared macaques with humans in their sensitivity to configurally altered faces in upright and inverted orientations using a habituation paradigm and eye tracking technologies. In addition, we tested for differences in processing of conspecific faces (human faces for humans, macaque faces for macaques) and non-conspecific faces, addressing aspects of perceptual expertise. In both species, we found sensitivity to second-order relational properties for conspecific (expert) faces, when presented in upright, not in inverted, orientation. This shows that macaques possess the requirements for holistic processing, and thus show similar face processing to that of humans.
Highlights
In primate societies, a crucial socio-cognitive skill is to recognize and individuate faces
We focus on the aspects related to holistic processing of faces, especially the second order relational properties of facial features [7], i.e. the relative spatial arrangement of facial features
Upright faces of conspecifics contained a high degree of eye salience, i.e. viewing times towards the eyes as opposed to nose and mouth
Summary
A crucial socio-cognitive skill is to recognize and individuate faces. First-order relational properties describe the general arrangement of features, i.e. the eyes are above the nose, the nose above the mouth, and allow basic-level categorization of faces, i.e. the detection of a face [3]. Second-order relational properties describe the fine-tuned metrics among the features This information is unique for each individual face and allows classification at the subordinate level, i.e. individuation of faces [1,2]. For non-face objects, the opposite case is true: an image of a dog will be labeled as ‘dog’ first, before being labeled by its breed, or its name [4,5,6] Whether or not these two characteristics, holistic processing and subordinate-level entry point, can be described by the same underlying mechanism remains unclear, . A higher rebound of adaptation for the configurally manipulated stimuli as opposed to the normal control stimuli indicating that monkeys are sensitive to configural changes between the eyes
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