An invisible sign stimulus: completion of occluded visual images in the Bengalese finch in an ecological context.

An object that includes occluded parts is sometimes perceived as a complete image and this phenomenon is known as amodal or visual completion. A sign stimulus is a minimum set of information that elicits a behavior, but this notion raises questions about whether animals ever engage in the behavior when they cannot see the occluded sign stimulus, but they can visually complete it. Male Bengalese finches engage in courtship behavior toward video images of female finches. We conducted three experiments with Bengalese finches to show both sign stimuli and visual completion function in an ecological context. We used three types of visual images recorded from female finches as stimuli: the head, the body, and the whole. Results showed that male Bengalese finches showed courtship behavior toward the head-occluded stimuli whereas they did not toward the headless body image. The results imply that the males completed this occluded sign stimulus through the process of visual completion. Exposure to a sign stimulus combined with the process of visual completion may operate cooperatively to facilitate adaptive responses under conditions of limited information.