Introduction

There are a number of ophthalmic instruments that are designed to assess various aspects of binocular vision and functions, such as

1. (a) simultaneous perception, fusion and stereopsis

2. (b) accommodation/convergence relationships

3. (c) presence and magnitude of phorias.

These will now be discussed in greater detail.

Simultaneous perception, fusion and stereopsis

The simplest level of binocular vision is simultaneous perception, the ability to see two images, one formed on each retina simultaneously. For normal binocular vision, it must be possible to see two images, one formed at the fovea of each retina, and to superimpose them. This is called simultaneous foveal perception.

Fusion is a more refined level of binocular vision, in which two similar images, one formed on each retina, are seen at the same time and are blended as one. That there is a blending, and not just a superposition, can be tested by placing a small prism in front of one eye. This will displace the retinal image from the fovea. If fusion is present, a corrective eye movement will be made to place the image back at the fovea.

When a solid object is viewed, the two retinal images are slightly different because the two eyes view the object from different perspectives. Stereopsis is the ability to see slightly different images and to blend them as one with a perception or appreciation of depth. This is the most refined level of binocular vision.

A more detailed account of these binocular vision levels can be found in texts such as Duke-Elder (1973).