The face inversion effect in rhesus macaques

Authors

• O Tomeo
• N Liu
• SW Li
• LG Ungerleider

Abstract

Face perception plays a critical role in social communication and interaction. Given the similarities between monkeys and humans in the neural circuitry underlying social cognition, the rhesus macaque could provide an ideal model to study face processing. Behavioral studies exploring face processing in monkeys, unlike those in humans, have yielded inconsistent results. Here, we sought to clarify face-processing mechanisms by re-examining in macaques the face inversion effect, which refers to greater difficulty in recognizing inverted faces. Rhesus macaques were trained to perform a delayed match-to-sample (DMS) task. Stimuli were from six categories (faces: macaque, chimpanzee, human and sheep; objects: shoes and cars). Faces were forward facing and emotionless. All stimuli were isoluminant and grayscale. Faces were cropped with an oval mask to isolate central face information and exclude peripheral features. Reaction time (RT), accuracy, and pattern of eye movement were recorded; the efficiency score (RT/accuracy) was used to account for the trade-off between RT and accuracy. We found that scan patterns were similar when viewing macaque, chimpanzee and human faces but not sheep faces. However, better efficiency scores for recognizing upright stimuli than the inverted ones were only found for macaque and chimpanzee faces but not for human faces, sheep faces, or non-face objects. These results revealed a face inversion effect only for conspecific (macaque) and heterospecific (chimpanzee) faces, implying that macaques process both macaque and chimpanzee faces holistically. Moreover, our data support the idea that the effect is specific for stimuli for which the individual has developed expertise.

Scientific Focus Area: Neuroscience

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