The Vertical Horopter
An important, but little known characteristic of the visual system is the vertical horopter. The word horopter, loosely translated, means the horizon of vision. The horopter is the locus, or grouping, of points in our field of view that appear to us as single images. Points that are anywhere else in space appear as double images.
Horizontally, the horopter is curved, with the sides coming closer to the observer. The vertical horopter, however, starts somewhere between the viewer’s waist and feet. It projects outward, intersecting the point of fixation and continuing in a straight line. If an observer looks at the center of a vertical wire at a close distance in front of them, both ends of the wire will be seen as double in peripheral vision until the wire is tilted backward, with its top farther away from the observer.
The development of a backward-tilted vertical horopter corresponds to another characteristic of our visual environment: the orientation of surfaces. Look at something in the distance, or focus at a spot on the ground. What’s below the spot you’re looking at will usually be closer to you, and what’s above the spot will usually be farther away.