Constraints on perception : comparative and developmental aproaches

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Human infants recognize unity of two objects moving together behind an occluder (perception of object unity). Such perception shows that humans recognize external objects using some perceptual hypotheses. The purpose of this project was to study the evolution of such perceptual constraints. We experimentally analyzed the process of perceptual completion in nonhuman primates and birds. Major findings are as follows. Tufted capuchin monkeys were trained to match an intact diagonal rod, dissected rods, and unitary rods having irregular contour at their centers. Then the subjects were tested which they would match to the stimuli having their central portions occluded by a horizontal belt. Regardless of whether the sample stimuli moved horizontally behind the occluder or stayed stationary, the monkeys overwhelmingly matched these test stimuli to the intact, straight rod. Thus the monkeys completed the occluded portion in a straight contour. When two rods were placed to form a right-angle bracket (>), the subjects also matched them to the connected rod. When the two rods were placed parallel to each other, however, the subjects matched them to the disconnected rods. Thus the monkeys recognized the unity of the occluded two fragments based on whether the two portions were connective or not. Ongoing study shows that when the contour of the rods is zigzag, the monkeys tend to complete the invisible portions in zigzag lines. The process of perceptual completion seems to follow the Gestalt principles of good shape and/or regularity, like human adults. On the other hand, pigeons never showed evidence for perceptual completion in similar situations. Pigeons may have evolved with a visual information processing algorythm that is completely different from humans'. We are now testing pigeons using pictures of food. Other species having different food habits are also being tested. Developmental study of macaque monkeys reached no good conclusion.