A ground-like surface facilitates visual search in chimpanzees (Pan troglodytes)

Imura, Tomoko; Tomonaga, Masaki · 2013 · Crossref

DOI: 10.1038/srep02343

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Summary

This study investigates the "ground-dominance effect" in visual search, a phenomenon where humans detect targets more rapidly on ground-like surfaces than on ceiling-like surfaces. While previous research established this advantage in humans due to the ecological relevance of ground surfaces for terrestrial locomotion and object manipulation, it remained unclear whether this bias extends to nonhuman primates. The authors specifically examined chimpanzees (*Pan troglodytes*), a species that utilizes both arboreal and terrestrial environments, to determine if they share this attentional bias with humans. The experimental design involved both chimpanzees and humans performing a visual search task on a computer screen. Participants were asked to locate a target cube with a unique luminance (darker) among distractor cubes. The search items were arranged on imaginary three-dimensional surfaces defined by texture gradients and shading cues, creating either a ground-like or a ceiling-like perspective. The study manipulated two conditions: "in-surface," where the target’s unique face was aligned with the surface plane, and "not-in-surface," where the unique face was on a side perpendicular to the surface. Display sizes varied with four, six, 12, or 18 items. Six chimpanzees and ten humans participated, with response times (RTs) serving as the primary measure of search efficiency. The results demonstrated that both chimpanzees and humans exhibited the ground-dominance effect. For both species, search times were significantly faster under the "in-surface" condition on ground-like surfaces compared to the "not-in-surface" condition. In contrast, no significant difference in search times was observed between the two conditions on ceiling-like surfaces. Statistical analysis confirmed significant interactions between surface type and condition for both groups, indicating that the facilitation of visual search was specific to ground-like displays. Additionally, search times decreased as the number of items increased, consistent with efficient parallel processing. These findings suggest that the ground-dominance effect is not unique to humans but is shared with chimpanzees, implying a common evolutionary heritage in visual attention mechanisms. The results indicate that optical contact between an object and a ground surface facilitates perceptual organization and texture segregation in both species. This shared bias persists despite differences in locomotor habits, suggesting that the visual system prioritizes ground surfaces due to their fundamental ecological importance for object interaction. The study highlights the need for further comparative and developmental research to understand how this effect relates to locomotion and depth perception across different primate species.

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• visual search