Visual Search for the Orientations of Faces by a Chimpanzee (Pan troglodytes)
DOI: 10.2354/psj.15.215
archive: archived pipeline: cataloged verified
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Summary
This study investigates whether chimpanzees exhibit the "inversion effect"—the phenomenon where upright faces are processed more efficiently than inverted ones—and whether this effect is specific to faces or generalizes to other stimuli. Motivated by conflicting results in previous primate research, the author employed visual search tasks to determine if upright faces possess a processing advantage over inverted or horizontally rotated faces, and if this advantage persists when distractors are varied. The experiments were conducted with a single female chimpanzee, Chloe, using a touch-screen visual search paradigm. In Experiment 1, the subject searched for target human faces among distractors of the same identity but different orientations (upright, inverted, or horizontal). Experiment 2 expanded the stimulus set to include chimpanzee faces and houses, using a trial-block method to alternate target and distractor roles within sessions. Experiment 3 introduced a critical manipulation: comparing performance when all images on the screen were identical ("Same" condition) versus when all images were different ("Different" condition), to assess whether the chimpanzee relied on image discrimination or orientation detection. The results demonstrated a clear advantage for upright faces. In Experiments 1 and 2, the chimpanzee made fewer errors and responded faster when upright faces were involved compared to conditions involving only inverted or horizontal orientations. This "upright face superiority" was observed for both human and chimpanzee faces but was absent for houses. Crucially, Experiment 3 revealed that while performance dropped significantly for houses and chimpanzee faces when distractors were different, the chimpanzee maintained above-chance accuracy for upright human faces even when all images were distinct. This indicates that the subject could detect the orientation of upright faces independently of specific image features. The findings suggest that chimpanzees possess a face-specific processing mechanism similar to humans, characterized by an inversion effect and search asymmetry. The author concludes that chimpanzees likely categorize stimuli as "faces" through structural coding before analyzing orientation, a process that does not apply to non-face stimuli like houses. This supports the hypothesis that face perception in chimpanzees involves specialized cognitive mechanisms rather than general visual expertise, highlighting evolutionary parallels in primate face recognition.
Provenance
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| Stage | Outcome | Tool | Model | Prompt | Attempts | Completed |
|---|---|---|---|---|---|---|
| discover | success | Crossref | — | — | 1 | 2026-06-11 |
| archive | success | canonical_url | — | — | 1 | 2026-06-25 |
| extract | success | cached | — | — | 2 | 2026-06-25 |
| clean | success | clean | — | — | 1 | 2026-06-20 |
| chunk | success | chunk | — | — | 1 | 2026-06-20 |
| embed | success | embed | Qwen/Qwen3-Embedding-8B | — | 1 | 2026-06-20 |
| enrich | success | openalex | — | — | 1 | 2026-06-20 |
| promote | success | — | — | — | 1 | 2026-06-11 |
| summarize | success | llm | qwen3.6-27b-prismaquant | summ-v5 | 1 | 2026-06-25 |
| tag | success | vector_similarity | — | — | 6 | 2026-06-20 |
| verify | success | — | — | — | 1 | 2026-06-26 |
Summary generated by qwen3.6-27b-prismaquant on 2026-06-25; verification: verified.
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