Human beings are predominantly visual creatures: ‘seeing is believing’. We receive most of the information about our world through our eyes, augmented chiefly by the sense of hearing and then by the other, lesser, senses. But we do not simply see flat images, as in a photograph or painting. We take in many different aspects of our environment: colours, sizes, shadows, perspective, distance, depth, movement, the position of our head, body and limbs, and so on. All this is accomplished by the eyes receiving images and the brain interpreting them. Many accounts of how the eye works point out that ‘the eye is like a camera’. Of course, it should be the reverse: the camera is the copy of the eye. Nevertheless, the camera serves as an easily-envisaged analogy to explain how the eye itself receives, focuses and records images.

What we actually see are rays of light reflecting off objects and entering our eyes. All our visual information comes from a pair of postage-stamp-sized images thrown on the retinas of our eyes.

Refraction and focusing

Light bends, or refracts, at the point where it passes from one transparent medium to another. A light ray heading for the retina first strikes the cornea, then the aqueous humour, followed by the lens and the vitreous humour; at each interface it is refracted. However, the design of the ‘resting’ eye is such that parallel rays of light from distant objects are automatically focused on the retina. The distance point is the farthest distance away at which an object can be seen clearly.

In the case of diverging rays of light from near objects, the eye must increase its overall refractive power to focus the image correctly. This process, called accommodation, is brought about by three mechanisms. First, the ciliary muscles around the lens contract, which allows the lens to become more globular in shape and hence more powerful at focusing. Second, the muscles in each eye socket turn the eyeballs inwards so they both point directly at the object. This is called convergence. Third, the pupils tend to become smaller. This restricts light rays to the central areas of the lenses which focus more accurately and, as photographers will recall, with greater depth of field. This third process is called the ‘near reflex’. The near point is the shortest distance at which an object is seen distinctly.

Problems in focusing correctly, termed ‘refractive errors’, are extremely common. They include hyper-metropia, or long sight, myopia, or short sight, and astigmatism. In some cases the eyeball is too big or small; in others the ciliary muscles are weak or the lens has become hard and inelastic. These errors are usually corrected easily by wearing spectacles or contact lenses. The cornea provides about four times the converging power of the lens, with the latter being used mainly for fine adjustments to focusing. Even if the lens becomes opaque in old age or because of a cataract and has to be removed, the person can still see quite well with the aid of spectacles.

The pupil and iris

The pupil is the central opening, or aperture, in the iris. The iris controls its size, and so determines the amount of light entering the eye. There are two different sets of muscles in the iris which control the size of the pupil. The radial muscles, called the dilator pupil- 4’ lae, cause the pupil to dilate (become bigger); the circular group of muscles, called the sphincter pupil-lae, make the pupil constrict (become smaller). The size of the pupil is influenced by various factors. It dilates in dim light and also in extreme conditions of fear, pain and excitement. It constricts in bright light and also when viewing a near object (the near reflex). In certain eye examinations drops are introduced into the patient’s eyes to dilate the pupils, so that the doctor can clearly see the inside of the eye. The drops paralyse the mechanism of accommodation so that close work becomes impossible, but the effects wear off after 12 to 24 hours.

From light rays to nerve impulses

The retina, the most complex part of the eye, contains two distinct types of light-receiving cells, called rods and cones because of their shapes. There are about 120 million rods and 6.5 million cones in the retina of the average eye.

The rods’ task is to record images in poor light. They are not sensitive to colour, only shades of grey. The cones detect colour and fine detail, and can work only in bright light. Both types of cell work in the same basic way. They contain chemicals called visual pigments which break down when light falls on them, so triggering a nervous impulse. The visual pigment is resynthesized in a fraction of a second, and breaks down again if light is still falling on it. Thus, continuous exposure to light causes repeated ‘firing’ of the rods and cones, producing a stream of nerve impulses. The impulses from the rods and cones are relayed via the optic nerve to the brain.

Rods and night vision

The visual pigment in the rods is called rhodopsin or visual purple. Vitamin A is necessary for its synthesis, hence a lack of this vitamin causes the condition termed ‘night blindness’ in which vision becomes difficult in dim light.

The rods are situated chiefly at the sides of the retina, because the area at the back (the macula lutea) where most important images fall is mainly cones, in order to detect colour and fine detail. You can demonstrate this for yourself. In low light levels, if you look straight at an object you will not see it very clearly because its image falls on the cone-rich back of the retina. Look slightly to one side of the object so that its image falls on the side of the retina, where there are more rods; you should then be able to make it out clearly, at the sacrifice of colour vision.

Cones and colour vision

The retinal depression of the yellow spot, called the fovea centralis, is exclusively cones. Here images are seen in greatest detail: when you look ‘straight at’ an object you have adjusted your eyeballs so that the image falls on both foveas.

There are different theories to explain colour vision; the most popular one is the trichromatic theory. This assumes three different kinds of cones, each containing a different pigment that is sensitive to a particular region of the visible-light spectrum. These regions correspond to the three primary colours of red, green and blue light.

If red light falls on the retina only the ‘red’ cones will fire impulses, hence we see only the colour red. All other colours are obtained by altering the intensities of the three primary colours. For white light to be seen, all three types of cone must fire impulses equally; yellow is seen when the green cones are firing rapidly and the red cones are firing moderately; orange is seen whf “> green cones are firing moderately and red cones are firing rapidly. About one man in 12 is colour blind – he cannot distinguish colours in the normal way. Colour-blind men outnumber women by 20 to one. A colour-blind person, instead of having the three different types of cone, has one or two. The commonest type of colour blindness is difficulty in distinguishing between red and green. Literal ‘colour blindness’, where everything is seen in shades of grey, is extremely rare.

Dark adaptation

When we go from bright sunlight into a dark room, we feel temporarily blinded. Gradually, as our eyes become accustomed to the darkness, we see more clearly. This improvement continues for about 25 minutes, after which it ceases. How does this improvement in vision, called ‘dark adaptation’, come about? In bright light the cones function well, and because their visual pigment is designed to work in such conditions they are fairly resistant to being broken down completely and so continue functioning. The rhodopsin pigment in the rods is not so resistant, and under continuous exposure to bright light it is all eventually broken down and the rods become temporarily functionless. Now we enter the dark room. At once, the light intensity falls below the critical level required by the cones; they cease to work and send no more impulses to the brain. The rods are also temporarily function-less, because their rhodopsin has been exhausted. For a few seconds we are unable to see. Quickly, however, rhodopsin is resynthesized and as its concentration increases the rods again become sensitive to light. This process goes on for 25 minutes, when maximum rhodopsin levels – and therefore optimum dark-adapted vision – are reached.

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The visual cortex

The cortex is the part of the brain that is specialized to receive and interpret visual information. It is situated at the back of the brain, in the occipital region (which is why a blow to the back of the head may make you ‘see stars’). The brain cells that deal with nerve impulses from each retina are laid out on the cortex like a map, mirroring the retinal structure, but and with the area corresponding to the fovea expanded as the area of greatest acuity.

The visual-cortex cells seem to belong to various types, each detecting only one distinct form of stimulus, such as light coming from certain position in the visual field. These cells are in turn organized into hierarchies. For example, if several light-detecting cells in a row are stimulated they will in turn stimulate a ‘straight-line’ detecting cell at the next level of the hierarchy. Through their thousands of millions of interconnections the cortex cells analyze the dot-like pattern in the visual field and build up a meaningful interpretation of the image. Scientists are only just beginning to understand this complex procedure.

Judging distance

We use many visual cues to estimate distance, and therefore movement. Shadows, perspective, overlap, the expected size of objects and their juxtapositions are all important. But the fact that we have two eyes permits extremely accurate distance judgements, in two ways.

First, when we look at a distant object the lines of sight for each eye are almost parallel; when we look at a nearby object the eyes swivel inwards – the phenomenon of convergence, explained previously. Sensors in the muscles that move the eyeballs inform the brain as to the exact position of each eye, so that by comparing eyeball positions the brain can judge the degree of convergence and so estimate how far away the object is.

Second, each eye sees a slightly different view of the world. This is termed ‘stereoscopic’ vision. The difference is more marked with near objects, because the outline of that object and the parts of the background it obscures may change dramatically from one eye to the other. Again, by comparing the two images the brain can judge distance. This procedure is facilitated by the fact that each optic nerve divides in half on the way to the brain, so that the left-hand part of the visual cortex receives information from the left side of each retina, and vice versa. In pictorial terms, the two images from equivalent sides of each eye are superimposed by the brain to detect perspective differences and so judge distance.