When the eyes face the front, the central part of the visual field is imaged on both retinas (see Figure 4.4). This bestows certain advantages for depth perception, but also creates a formidable problem for the brain: how to ensure that the two retinal images are transformed to yield a unified perception of the part of the visual field seen by both eyes. Failure to achieve this, which sometimes happens in pathological conditions of the nervous system, results in diplopia, the perception that there are two objects when there really is only one. Diplopia can be demonstrated by pushing gently on the skin at the side of one eye to misalign the two visual axes.

Binocular Single Vision

The encoding of the two retinal images of a single object to yield a unique perception results in perceptual fusion of the two images. In discussing fusion, it is important to distinguish between it and two other phenomena, fixation and focus. If the visual axis of one eye is directed at an object so that the image is positioned on the fovea, the eye is said to fixate the object. It is possible to deliberately place an image outside the fovea, but the term “fixation” is generally used to mean foveal fixation. The fixated object will be in focus only if its distance from the eye and the power of the eye's optics permit the formation of a crisp retinal image.