Introduction

The human visual system has limited access to depth and distance information on its own; binocular disparity (the difference in images from each of our eyes) is our main biologically-based cue for depth. You can see binocular disparity working as a cue for depth yourself by covering one eye and looking straightforward for a few seconds before opening both eyes. The increased depth perception you have after opening both eyes is due to binocular cues. The majority of human judgement regarding depth comes from monocular cues; that is, cues from the two-dimensional image that can be taken in by only one eye. Monocular cues include perspective, nearness to the horizon, texture gradient, shading, and interposition of objects, among others. The Ponzo Illusion shown in this experiment is a powerful demonstration of the influence of these monocular cues on the perception of distance, and, in turn, on our judgement of the size of objects located within an apparently three-dimensional scene.