How we see


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How we see

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How we see

  1. 1. How we see When you take good care of your eyes, you take good care of yourself.       The eye is often compared to a basic camera, and indeed the very first camera was designed with the concept of the eye in mind. We can reduce the complex process that occurs to process light into vision within the eye to a relatively basic sequence of events. First, light passes hrough the cornea, which refracts the light so that it enters the eye in the right direction, and aqueous humour, into the main body of the eye through the pupil. The iris contracts to control pupil size and this limits the amount of light that is let through into the eye so that light-sensitive parts of the eye are not damaged. The pupil can vary in size between 2mm and 8mm, increasing to allow up to 30 times more light in than the minimum. The light is then passed through the lens, which further refracts the light, which then travels through the vitreous humour to the back of the eye and is reflected onto the retina, the centre point of which is the macula. The retina is where the rods and cones are situated, rods being responsible for vision when low levels of light are present and cones being responsible for colour vision and specific detail. All the light information that has been received by the eye is then converted into electrical impulses by a chemical in the retina called rhodopsin, also known as purple visual, and the impulses are then transmitted through the optic nerve to the brain where they are perceived as ‘vision’. The eye moves to allow a range of vision of approximately 180 degrees and to do this it has four primary muscles which control the movement of the eyeball. These allow the eye to move up and down and across, while restricting movement so that the eye does not rotate back into the socket.  
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  3. 3. Rods and Cones Rods are the light-sensitive cells in our eyes that aid our vision in low levels of light. Rods are blind to colour and only transmit information mainly in black and white to the brain. They are far more numerous with around 120 million rods present in every human eye compared to around 7 million cones. Cones are responsible for perceiving colour and specific detail. Cones are primarily focused in the fovea, the central area of the macula whereas rods mainly surround the outside of the retina. Cones work much better in daylight as light is needed to perceive colour and detail. When light enters the eye, it first passes through the cornea, then the aqueous humor, lens and vitreous humor. Ultimately it reaches the retina, which is the light-sensing structure of the eye. The retina contains two types of cells, called rods and cones. Rods handle vision in low light, and cones handle color vision and detail. When light contacts these two types of cells, a series of complex chemical reactions occurs. The chemical that is formed (activated rhodopsin) creates electrical impulses in the optic nerve. Generally, the outer segment of rods are long and thin, whereas the outer segment of cones are more, well, cone shaped. Below is an example of a rod and a cone: The outer segment of a rod or a cone contains the photosensitive chemicals. In rods, this chemical is calledrhodopsin; in cones, these chemicals are called color pigments. The retina contains 100 million rods and 7 million cones. The retina is lined with black pigment called melanin -- just as the inside of a camera is black -- to lessen the amount of reflection. The retina has a central area, called the macula, that contains a high concentration of only cones. This area is responsible for sharp, detailed vision. When light enters the eye, it comes in contact with the photosensitive chemical rhodopsin (also called visual purple). Rhodopsin is a mixture of a protein called scotopsin and 11- cis-retinal -- the latter is derived from vitamin A (which is why a lack of vitamin A causes vision problems). Rhodopsin decomposes when it is exposed to light because light causes a physical change in the 11-cis-retinal portion of the rhodopsin, changing it to all-trans retinal. This first reaction takes only a few trillionths of a second. The 11-cis-retinal is an angulated molecule, while all-trans retinal is a straight molecule. This makes the chemical unstable. Rhodopsin breaks down into several intermediate compounds, but eventually (in less than a second) forms metarhodopsin II (activated rhodopsin). This chemical causes electrical impulses that are transmitted to the brain and interpreted as light. Here is a diagram of the chemical reaction.
  4. 4. Activated rhodopsin causes electrical impulses in the following way: 1. The cell membrane (outer layer) of a rod cell has an electric charge. When light activates rhodopsin, it causes a reduction in cyclic GMP, which causes this electric charge to increase. This produces an electric current along the cell. When more light is detected, more rhodopsin is activated and more electric current is produced. 2. This electric impulse eventually reaches a ganglion cell, and then the optic nerve. 3. The nerves reach the optic chasm, where the nerve fibers from the inside half of each retina cross to the other side of the brain, but the nerve fibers from the outside half of the retina stay on the same side of the brain. 4. These fibers eventually reach the back of the brain (occipital lobe). This is where vision is interpreted and is called the primary visual cortex. Some of the visual fibers go to other parts of the brain to help to control eye movements, response of the pupils and iris, and behavior. Eventually, rhodopsin needs to be re-formed so that the process can recur. The all-trans retinal is converted to 11-cis-retinal, which then recombines with scotopsin to form rhodopsin to begin the process again when exposed to light.
  5. 5. Seeing colour    Colour is not actually inherent in any object. We only see colour because objects absorb some colour from light, and refl ect others. It is the reflected ones that we see and that give an object a set ‘colour’. Therefore, for example, grass is not green, it purely absorbs all other colours in light and refl ects back green. If an object refl ects all colours we will see it as white, if it absorbs all colours we see it as black. We use cones to perceive colour as rods are blind to colour.     Ishihara based his test on pseudo-isochromaticism, but with the intention of delivering results that were more easily interpreted and thus more reliable. Almost nine decades on from its first edition, the Ishihara test remains widely used, able to quickly screen for colour vision defects that other, more exacting tests can then elucidate in detail. The Ishihara test can only detect the more common red-green
  6. 6. colour vision deficiencies (not the rarer blue ones), and then with only limited precision. A mild form of red-green deficiency occurs when either the red or green sensitive photopigment in the retina has an altered response to colour; this results in reduced discrimination between the colours red and green. A more severe deficiency occurs when either the red or green photopigment is missing entirely. Normal colour vision uses all three types of light cones correctly and is known as trichromacy. People with normal colour vision are known as trichromats. The different anomalous conditions are protanomaly, which is a reduced sensitivity to red light, deuteranomalywhich is a reduced sensitivity to green light and is the most common form of colour blindness and tritanomaly which is a reduced sensitivity to blue light and is extremely rare. People with deuteranomaly and protanomaly are collectively known as red-green colour blind and they generally have difficulty distinguishing between reds, greens, browns and oranges. They also commonly confuse different types of blue and purple hues. People with reduced blue sensitivity have difficulty identifying differences between blue and yellow, violet and red and blue and green. To these people the world appears as generally red, pink, black, white, grey and turquoise. People with monochromatic vision can see no colour at all and their world consists of different shades of grey ranging from black to white, rather like only seeing the world on an old black and white television set. Achromatopsia is extremely rare, occuring only in approximately 1 person in 33,000 and its symptoms can make life very difficult. Usually someone with achromatopsia will need to wear dark glasses inside in normal light conditions.
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