Eye Care Blog

It would seem appropriate, therefore, to regard the whole visual system at birth as a more or less ready-to-use computer that has not yet been programmed. The actual programming takes place soon after birth; but just when this happens is not the same with all living beings. Once the programme has been supplied it remains built-in for life; but to function well it requires frequent use, especially in the early years.

Society does not permit the sort of experiment carried out on cats, apes and monkeys to be performed with humans. Even so, there is at least one instance of a medical or social custom that has some of the features of an experiment, and which sheds some light on the development of our sight. For some time before, and for a little while after, World War II there was a trend among a minority of the Japanese population to perform surgical operations on the eyes of their baby girls. These operations consisted in making cuts on the eyelids, so that when the child grew up she would be ’round-eyed’. The real reason for doing this was almost certainly cosmetic, even though some doctors tried to justify what they were doing on the medical grounds that the oriental eyelid restricts the natural movements of the eye.

This operation was usually carried out when the child was between one and two years of age. Unfortunately it was very rare that both eyes were treated simultaneously. First one eye would be done, and then it would be kept covered for a week or two; then the other eye. The consequence was that the eye treated first nearly always became visually dominant later on.

The upshot of all this is that, although humans develop their visual mechanisms reasonably quickly, it is clear that the process of secondary ‘programming’ continues well into early childhood.

This corresponds with what we know about the physical growth of the organ itself. At birth the human eye is about two-thirds the size it will eventually attain in adulthood. But by the age of four the eye is almost fully grown. To put this in perspective, compare it to the growth-rate ofyour body. At birth you are one-third or less of your final height, at four perhaps not half. To reach its final limits the body takes fifteen to twenty years, but not so the eyes. The head and brain likewise mature early—it is the skeleton that lags behind. From an early stage the mechanisms are set to make the child a good receiver of sensations, many of which are stored as memories to be used later by the adult when he/she takes on all the functions of a fully mobile animal.

The eye is developed to provide you with a field of vision almost a full 180 degrees to the frontal plane of your body. Unlike many birds and reptiles you cannot see behind you without turning your head. Instead your eyes are set in such a way that they can work together. You are one of the few animals that can fuse the two pictures, or sets of information, coming from your two eyes. This is called stereoscopic, as opposed to simple binocular, vision. But because the two pictures are slightly different, their fusion can only occur using slightly different levels of perception in the brain. The result is interpreted by the brain as depth. The greater the differences between the two pictures the greater will be the sense of depth, or space. Hence the very precise impression of space and distances between objects that are close to, and a correspondingly vaguer sense of distances when you look at a horizon. If for any reason the pictures are too dissimilar to be synthesized, the developing child will not have single fused vision. The six muscles of each eye must work not only as a team to point the eye accurately to wherever the brain wants it to look, but they must also work together with the muscles of the other eye. Co-ordination of this kind has generally occurred by the age of six months.

In the first year of life, changes are happening to the inside of the eye. The optical system of the eye may be compared to the lens system of an expensive camera. Light is refracted not just through one lens, but through many different lenses, each with its own particular function. Biologically a lens may be defined as any part of the optic system through which light passes before reaching the retina. As light passes through the human eye there are four main changes in optical density, hence four main lenses. These are, from front to back:

1. The cornea: the outer surface of the eye where that surface is transparent, and by far the most powerful of the four lenses, accounting for as much as two-thirds of the average eye’s refractive capacity.
2. The area between the cornea and the ‘crystal’ lens, filled with fluid, but as a lens extremely weak.
3. The crystal or crystalline lens, accounting for almost a third of the eye’s refractive power, and capable of increasing its power when the eye is being used, for example, for reading.
4. Thevitreous lens, from the back of the crystal lens to the retina, a jellyish fluid of no significant power.

Between birth and the age of four these lenses are rapidly changing shape so as to provide the child with the required visual functions. The front lens, or cornea, is circular in shape. When you look at an eye, either your own in a mirror or someone else’s, the coloured part is the iris, and the black hole in the middle of the iris is the pupil. These in fact lie just behind the cornea. The coloured iris, when looked at closely, reveals a very delicate and intricate pattern, peculiar to each person and race. In most cases the colour of the baby’s iris will change as it grows older, becoming darker. The reason for this is that, at birth, the darker pigments have not yet been laid down in the iris. In albino children there are no dark pigments, and so light will make their eyes look pink. What in fact happens here is that red light, or the red end of the colour spectrum, is reflected forward from the back of the eye.

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