The apparent size of objects situated in front of or behind the horopter.

Objects in the visual world that are located outside the horopter project upon non-corresponding parts of both retinae. Hence, most environments will generate--within a portion of the binocular field of view--retinal images that contain clearly dissimilar pattern elements at corresponding retinal points. Fusion is impossible there and the brain has to decide locally from which eye conflicting information is to be chosen. The binocular field of view thus consists of parts where information from both eyes contributes to perception and others where only one eye is 'switched on' at a given time. Two easily performed perception experiments demonstrate the advantage of having two eyes, even if fusion of parts of the images they receive is impossible: the apparent size of large objects outside the fixation plane (i.e., visual obstacles) is reduced. Quantitative measurements agree well with the predictions derived from a simple geometrical model. It is also shown that only a very limited number of objects can elicit 'true' (physiological) double images.