Stereoblindness (also stereo blindness) is the inability to see in 3D using stereopsis, or stereo vision, resulting in an inability to perceive stereoscopic depth by combining and comparing images from the two eyes.
Individuals with only one functioning eye have this condition by definition since the visual input of the second eye does not exist. The condition also results when two eyes do not function together properly as described here.
Most stereoblind persons with two healthy eyes do employ binocular vision to some extent, albeit less than persons with normally developed eyesight. This was shown in a study in which stereoblind subjects were posed with the task of judging the direction of rotation of a simulated transparent cylinder: the subjects performed better when using two eyes than when using their preferred eye. They appeared to judge the direction of rotation from the images in each eye separately and then to combine these judgments, rather than relying on differences between the images in the two eyes.[1] Also, purely binocular motion stimuli appear to influence stereoblind persons' sensation of self-motion.[2] Furthermore, in some cases each eye can contribute to peripheral vision for one side of the field of view (see also monofixation syndrome).
However, there is an exception to this: Those with true congenital alternating squints have two healthy eyes, and the ability to switch (by choice) between seeing with either eye. However, stereoscopic and three dimensional vision can never be achieved in this condition (attempts to train those with true congenital alternating squints into binocular vision results in double vision, which can be irreversible).[citation needed]