We have said that looking at an object with our two eyes is like taking a picture of the same object at the same time with two cameras placed side by side in such a manner that the two pictures taken are almost identical. No matter how closely together we may place the two cameras, however, the pictures that they take will never be exactly identical because each camera would be taking its picture from a slightly different angle or view than the other camera. In the same way, the images of the objects in the world around us that are formed in our two eyes and sent to the brain to be fused are not exactly alike, because each eye sees things from a slightly different position in the head. The brain, however, handles this slight difference in the two images that it receives and fuses very nicely and even gets information about how far away the object is located from the eyes. In other words, we have depth perception partly because our two eyes never see the same object from exactly the same position and thus each eye sends a slightly different picture of the object to the brain to be fused.
The outfielder in the cricket game needs good depth perception in order that he may correctly judge the ball and position himself at the point where the ball will fall to the ground in order to catch it. If you do not think this requires good depth perception and the use of the two eyes, have a friend throw a ball to you and try to catch it with one eye closed. You will find it very difficult, if you can do it at all. Or try playing a game of tennis or badminton with one eye closed and see how well you do it. You will be lucky to hit the ball or the shuttle cock at all.
You use depth perception when driving your car along a highway, particularly in passing another car on a busy road. You must judge the speed of the car ahead of you, but this is usually easy because your speedometer tells you how fast you are driving. The hard part is to judge the speed of cars traveling towards you in order to decide whether or not you have time to pass the car ahead of you before the other cars coming toward you get there.
You even use depth perception in eating a meal when you reach for objects on the table which are located at varying distances from you.
The preceding discussion does not mean that a person with only one eye has absolutely no depth perception at all because this is not true. There are other clues to depth perception beside the slight difference in the images of objects seen by the two eyes and sent to the brain. Shadows, color haze, and the known comparative size of other objects all help the one eyed person in attaining some degree of depth perception. However a one eyed person can never have as keen depth perception as a person with two eyes functioning normally together as a team.
Fusion with good depth perception is, therefore, the ideal goal in the treatment of crossed eyes, although it is not always possible to attain it for one reason or another.
There are numerous questions that occur in the parents when it is first recognized that the child’s eyes are not straight. A sincere attempt is made through this book to answer the majority of these questions.
To the question, why are my child’s eyes turned? We know several immediate causes but only a few of the underlying ones. Honest parents will often ask themselves; is it our fault that our child’s eyes are turned, and is there anything we could have done about it. The correct answer to this question is usually “Yes” to the first part and “No” to the second. Barring accident, injury, or disease, turned eyes are known to run in families. Thus there is fairly good evidence to show that children inherit certain factors from their parents which may result in crossed eyes. There is, however, nothing the parents could have done about such a familial trend. It is a well known fact that all the members of a family need not necessarily show same family characteristic in order to pass it on to other members of the same family in the next generation. Thus a mother and father with perfectly normal and straight eyes may have relatives with crossed eyes, and these normal appearing parents may have one or more crossed-eyed children. The pattern of heredity seem strange indeed to the average person who has not, like the geneticist ,spent a lifetime studying such problems.
Squint is a term that covers all the conditions in which the two eyes are not properly aligned. The types of squint vary in degree. Some children have eyes that are straight most of the time-rarely turning in or out. In such a situation, the condition is called intermittent squint. The degree of turning or angle of squint is a matter of importance both from cosmetic and medical point of view. Oftentimes a squint of a very small angle is harder to treat successfully than a large-angle squint.