The image of the external world on the retina is essentially flat or two-dimensional, and yet it is possible to appreciate its three-dimensional character with remarkable precision. To a great extent this is by virtue of the simultaneous presentation of different aspects of the world to the two eyes, but, even when subjects view the world with a single eye, it does not appear flat to them, and they can, in fact, make reasonable estimates of the relative positions of objects in all three dimensions. Examples of monocular cues are the apparent movements of objects in relation to each other ...(100 of 31643 words)