Infant Macaque Monkeys Respond to Pictorial Depth

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Abstract
The studies described here are the first to demonstrate that a nonhuman primate species is capable of responding to pictorial depth information during infancy. In two experiments, pigtailed macaque ( Macaca nemestrina) infants were tested for responsivity to the pictorial depth cues of texture gradient/linear perspective and relative size. The procedures were adapted from human studies and are based on the proclivity of infants to reach more frequently to closer objects than to objects that are farther away. The stimulus displays included two equidistant objects that, when viewed monocularly, appear separated in space because of an illusion created by pictorial depth cues. When presented with these displays, animals reached significantly more often to the apparently closer objects under monocular conditions than under binocular conditions. These findings suggest that infant macaques are sensitive to pictorial depth information, the implication being that this ability has ancient phylogenetic origins and is not learned from exposure to the conventions of Western art.