The Eye
The human eye is shown in the diagram opposite. Light enters the eye through the transparent curved surface of the conjunctiva and cornea. The surface here is kept moist and clean by secretion of lachrymal fluid (tears) from the lachrymal glands in the upper eyelid. As light rays hit the curved surface of the cornea they are refracted (bent) so that they converge towards each other. The light rays pass through the gap at the centre of the iris called the pupil and then pass through the lens where they are again refracted. The degree of refraction caused by the cornea and the lens can be altered by changing the curvature of these structures. This is achieved by contraction or relaxation of the ciliary muscle which runs as a ring just behind the iris. As the ciliary muscle contracts the curvature of the cornea and lens increases and light is refracted more strongly. This will enable the image formed on the retina to be in focus when dealing with light rays coming from a nearby object.
When the ciliary muscle relaxes the pressure in the eye causes the cornea to become less curved and the increased tension on the suspensory ligaments causes the lens to become less rounded. Light rays are refracted less, and as a result far away objects will be accurately focused on the retina.
The diagram above shows the effects of contraction of the ciliary muscle causing increased curvature of the cornea and lens.
You should realise that when the ciliary muscle relaxes it cannot pull at all and so the eye relies on its internal pressure to force the edges of the eye outwards when the ciliary muscle relaxes.
The process of refraction relies upon the change in the speed of light as its passes from a less dense to a more dense medium and then back again. To promote accurate focusing with minimal distortion of the image the materials making up the eye form a graded series of transparent structures with different refractive indices. The fluid in the rear section of the eye has a very high refractive index close to that of glass, hence its name vitreous humour, meaning glassy stuff!
The diagram above illustrates the way in which light rays from one point on an object being viewed are all refracted by the curved surfaces of the eye so that they converge to a single point on the retina. The image we see will be formed from thousands of such points of light being formed on the retina. Because the light rays are converging as they are focused they form an inverted image on the retina.
The amount of light entering the eye also needs to be controlled so that the light sensitive cells in the retina can work efficiently and not become over-exposed. This is the job of the iris which consists of two opposing groups of smooth muscle fibres, the circulars and the radials.
When the radial muscles of the iris are made to contract and the circulars relaxed the pupil gets bigger as the iris gets narrower. If the circulars are made to contract as the radials are relaxed then the pupil will get smaller as the iris gets wider. By carefully balancing the degree of contraction of each antagonistic set of iris muscles it is possible to accurately open the pupil so that just enough light enters to form a bright clear image but not enough to bleach the photoreceptors. This control is performed by an unconditioned reflex arc working though the brain at a subconscious level.
The image formed on the retina must now be converted into the form of nerve impulses to be transmitted to the brain’s visual cortex where the image will be analysed and interpreted. There are two types of photoreceptor in the retina, the rods and the cones. Rods are able to detect all wavelengths of visible light and work at low light intensities. They are therefore most useful at night and will only produce images in shades of grey, no colour! The cones are designed to respond to particular wavelengths of light, in humans there are red, green and blue receptive cones. In ducks there are about five different colour receptors! By mixing different proportions of red, blue and green light it is possible to produce any of the colours visible to us. Cones require a high light intensity to be able to function and therefore are not able to function without an artificial light source at night.
The rod cells (shown above) contain a pigment called rhodopsin which consists of a vitamin A derivative called retinene which is combined with a protein called opsin. It has a purplish colour when combined and loses its colour when split into its components. The retinene can exist in two forms which are isomers of each other, cis-retinene and trans-retinene. When light hits a rhodopsin molecule the cis-retinene is converted to trans-retinene and this cannot stick to the opsin protein so they split apart. When this happens protein channels in the membrane of the rod cell are closed and as the influx of sodium and calcium ions stops a change in electrochemical potential occurs. This reduces the output of neurotransmitter from the rod and alters the activity of the attached multipolar neurone. This alteration in the rate of nerve impulse emission from the attached neuron will be detected within the brain's visual cortex as a change in light intensity in that part of the image.
The cones work on a similar principal but contain a visual pigment called IODOPSIN which responds to light at higher intensity and whose isomers can be reconverted in relatively bright light. The structure of the protein component appears to differ slightly in the different forms of iodopsin which respond to different wavelengths of light.
The retina has the structure shown in the diagram above. The choroid layer behind the retina has a dense concentration of the pigment melanin which absorbs light rays and prevents them being reflected back onto the rods and cones once they have passed through them. You should be aware that the light would be travelling from the bottom of the diagram towards the top and therefore must pass through the layer of sensory neurons and bipolar neurons before it hits the rods and cones. This type of retina is called inverted because the photosensitive layer is below the neurons which carry the nerve impulses out of the eye.
The rods and cones in the human retina are not equally distributed in all areas, the cones are concentrated around the central region, the fovea centralis or yellow spot. Here, their individual connections to a single neuron running to the brain enables us to produce accurate colour images representing the centre of our field of view. The rods are increasingly abundant as we move towards the edges of the retina, here, colour vision is poor and the image becomes increasingly vague and ill-defined. The poor detail produced by the rods is a consequence of having a couple of hundred rod cells connected to a single neuron running to the brain. This does not allow the brain to determine which rod in each group has been hit by a light ray and forces the brain to produce an image from large patches of light or dark....hence the terrible vision we have at night! If we liken the retina to a computer screen, each point on the screen is a pixel and if we make a picture out of very small pixels it is clear and accurate, but if we make a similar picture from large pixels it becomes less well-defined. The level of detail of our visual images is referred to as our visual acuity, rods have poor acuity while cones give good acuity. Where the sensory neurons running across the surface of the retina join together to form the optic nerve there are no rods or cones hence its name blind spot.
Another problem with our vision is that when light rays of different intensity or colour are hitting groups of adjacent photoreceptors they overlap forming an intermediate zone and this blurs the edges of distinct objects. In diagram 1, the central group of cones is receiving a bright light, the side groups no light and the cones in between just a little light. When this happens the cones being stimulated most switch the cones next to them off by sending inhibitory impulses through the horizontal or amocrine cells. This effectively sharpens the edges of the image, as in diagram 2.
Colour Vision:
The trichromatic theory of colour vision is really quite simple, it suggests that the cones of the retina function a little like the red, green and blue spots making up the pixels on a TV screen By comparing the activity of the red, blue and green cones in any area of the retina the brain can calculate the colour of that point in the image.
Evidence to support this theory comes from study of wavelength sensitivity of the cone cells and from complementary colour images formed when a person stares at a particular colour pattern for a while and then transfers their view to a plain sheet of white paper. Instead of white they report seeing a complementary colour to the one they had been previously looking at. This is explained by the fact that if you have been looking at a bright red patch for a while the red cones will have used much of their photosensitive pigment, when the view is transferred to plain white, the blue and green cones give a big burst of nerve impulses while the red cones now give far fewer impulses so the brain has to assume that it is seeing something with a lot of blue and green light and therefore it produces a cyan image. Red colour blindness is known to be associated with a reduced frequency of red sensitive cones in the retina which is further evidence for the trichromatic theory.
Other experiments have thrown some doubt on the trichromatic theory, it has been found that people can still tell the colours of objects when they are viewed under a single wavelength of light, effectively their brain is able to decide that an object is red from the amount of say green light it is reflecting! Also, green colour blindness has been found to be caused not by the lack of green sensitive cones, but the inability of the brain to decide how to use the information the cones are sending in a correct way, and this is further evidence that the central nervous system has a lot to do with colour vision.
Processing of a visual image:
The retina acts as a preprocessor, enhancing edge definition etc, but the brain has the job of deciding what we are seeing. The nerve impulses travel along the optic nerves to the visual cortex of the brain. Here the neurons of the cortex form layers which each detect particular features of the image, diagonal lines, circles, vertical lines, horizontal lines, movement, colour. This information is then compared to a databank of information in the visual association area surrounding the visual cortex and a decision is made as to what the image contains.
To give a three dimensional image the information from the left sides of our two retinas is sent to one side of the brain while the information from the right sides of the retinas is sent to the other side of the brain, this allows the position of objects as viewed by the two eyes to be compared very rapidly. the overall left image and the overall right image is then correlated by interconnections between the two sides of the brain.
Every sensory area of the brain is divided into two sections, a processing area which analyses the main features of the sensations, and an association area which has recorded the various features of the sensations we have previously come in contact with. In a very small proportion of individuals the association areas for the different senses are wired incorrectly so that hearing a sound may evoke the smell of an object, or seeing a colour may evoke a particular sound. Usually these miswirings are additions to the normal connections which means the person can also make sense of the sensation, but with added dimensions which the rest of us miss!
The blind spot gives us no information as there are no rods and cones present in the area. The brain fills in the missing area with information gathered from the surrounding regions so that we don’t have a gaping hole in our field of view.
Try this test: Close the left eye and look at the cross with the right eye. Move the page slowly from arms length towards the eye. At one point the black spot will vanish and the gap is filled in with stripes!