Have fun with strabismus!

Amblyopia, and more

Chances are you have heard the term "amblyopia" or "lazy eye" at one time or another - and if you were diagnosed with strabismus when you were a child, you may even have grown quite annoyed with these concepts. Oh no, there's that dreaded eye patch again! There are a lot of interesting things to know about amblyopia and stuff like that, though.

The human visual system has been designed with the idea of binocular vision in mind. Roughly, it is intended to function as follows:

Normal Binocular Vision
Whatever is imaged on the fovea ("hotspot") is subjectively perceived as being straight ahead. If two slightly dissmilar objects are imaged on the two foveas (which is most always the case because of the different vantage point in space of each eye), the two objects are superimposed and fused into a single impression. Stereopsis and sensory fusion are the two physiological processes responsible for binocular fusion.

Sensory fusion is the process that takes care of people being blissfully unaware of the discrepancies between the the two retinal images. Corresponding points on the peripheral retinas localize things in the same direction in space, even when the two images cannot be fused, but their directional values can be modified to a certain extent in the presence of fusion, if the stimuli are near enough the corresponding points in the other eye. Binocular depth perception is a high level brain function which integrates the two slightly different images.

So far, so good - this all works very well, provided the two eyes are properly aligned to begin with. All this intriguing functionality develops before you're about 8 years old - what happens when the eyes are not aligned before that time?

In the presence of strabismus, each fovea receives a different image, so different things will be seen in the same place, a phenomenon appropriatetly called visual confusion. For example, the extra flower in the image on the right is in the same place as a dark area in the image on the left.

Also, an object seen straight ahead and imaged on the fovea of one eye, will be imaged on some other peripheral retinal area in the other eye; it will be seen twice, in different locations - which is termed diplopia. In the image on the left, the duplicate letter L is a vague, because the resolution of the peripheral retina is a lot less than in the center.

Both diplopia and confusion are more prevalent when contrast is high and contours are sharp.
Needless to say, neither visual confusion nor diplopia can hardly be useful. A squinter's visual system, provided it's still being developed, will therefore incorporate one or more of the following workarounds:

Suppression
The simplest way to not be bothered by two foveal images that don't match, is to ignore one of them. Suppression occurs in the form of a suppression scotoma, mainly located in the central part of the visual field.

Anomalous Retinal Correspondence
ARC is a way of compensating for the different directional values supplied by the corresponding points of the two retinas. Basically it shifts the image from the peripheral retina of the deviating eye, so the images will overlap, and fusion will be possible.
ARC and suppression are often working together; one picks up what the other can't handle; ARC is employed in the peripheral visual field while a suppression scotoma covers the center.

Amblyopia
Small children who always use the same eye for viewing, while the other is in a deviated position constantly, often develop decreased vision in the eye they don't use. This is different from suppression - suppression occurs only when both eyes are used at the same time, while amblyopia is present in the non-preferred eye even when it is used alone. Eye patches on the preferred eye are used to prevent the development of amblyopia in children.

Later on
If strabismus develops after age 8, due to, for instance a paralysis of an eye muscle, none of the workarounds develop; visual confusion and diplopia will occur.

Only in cases of acquired eye motility problems in adults, diplopia will be as obvious as in the image on the left.

Sometimes the angle of strabismus will change when looking in different directions, and then the compensatory mechanisms may not work every which way one looks. An existing deviation may change over time, sometimes resulting in diplopia that wasn't there before. So not in all cases the existence of well developed suppression and/or ARC will warrant a lifetime without visual confusion or diplopia.

As shown in the image on the left, sometimes an object has just the right contrast and contour sharpness to be hovering around the threshold level for suppression.

Note: the above example is a rather crude approximation of the real thing. (For browsers that don't support moving .GIF's, compare the image on the right to the one on the left.)
See the page about perception for more examples of interesting visual phenomenae related to squint.