Body inversion effect in monkeys

Abstract

Humans visually process human body images depending on the configuration of the parts. However, little is known about whether this function is evolutionarily shared with nonhuman animals. In this study, we examined the body posture discrimination performance of capuchin monkeys, a highly social platyrrhine primate, in comparison to humans. We demonstrate that, like humans, monkeys exhibit a body inversion effect: body posture discrimination is impaired by inversion, which disrupts the configural relationships of body parts. The inversion effect in monkeys was observed when human body images were used, but not when the body parts were replaced with cubic and cylindrical figures, the positions of the parts were scrambled, or only part of a body was presented. Results in human participants showed similar patterns, though they also showed the inversion effect when the cubic/cylindrical body images were used. These results provide the first evidence for configural processing of body forms in monkeys and suggest that the visual attunement to social signals mediated by body postures is conserved through the evolution of primate vision.

Fig 1. Examples of upright stimuli used in the experiments.

(A and B) Whole body postures. (C) Cubic/cylindrical body postures. (D) Part body postures. (E) Scrambled body postures.

Fig 2. Mean percentages of correct responses and mean response times for correct trials in pretest sessions for monkeys.

Error bars indicate bootstrapped 95% confidence intervals. **p < .01.

Fig 3. Mean percentages of correct responses and mean response times for correct trials in Experiments 1 to 4.

Error bars indicate bootstrapped 95% confidence intervals. **p < .01.

Fig 4. Mean percentages of correct responses and mean response times for correct trials in Experiments with humans.

Error bars indicate bootstrapped 95% confidence intervals. *p < .05, **p < .01.