Better living by not completing: a wonderful peculiarity of pigeon vision?

Abstract

Whereas many non-human species have been demonstrated to visually complete partly occluded figures, pigeons have been repeatedly failed to do so. We asked whether this failure reflected the pigeons' lack of perceptual process for completion or their decision among completed and non-completed figures. Four pigeons searched for a red lozenge target having one of its four contours punched in a rectangular edge out among three intact lozenges. All of these four stimuli had a white square next to them. After obtaining consistent search performances, the pigeons were tested with the punched target in a variety of locations relative to the white square, including right at the edges. Humans tested in the same task needed longer times before detecting the target when the square was placed right at the punched edge, suggesting automatic completion in humans. In contrast, the pigeons showed no similar difficulty. This result has two important suggestions: first, pigeons fail to complete partially occluded objects at the perceptual level, and second, this lack of completion is sometimes advantageous for them.