Your Visual System

Your Visual System

Your Visual System

Your visual system is the most sophisticated and complicated of your senses. Vision supplies information about the status of your environment, what it contains, and what is moving within it. Vision supplies you with information about shape, size, distance, brightness, and colour.

The seemingly simple task of interpreting shape requires information about vertical lines, horizontal lines, diagonal lines, edges, and shading. Your sense of colour is restricted to light with wavelengths between red (700 nm ) and violet (400 nm). Your eyes are positioned in the front of the skull and slightly separated to provide stereoscopic vision. Each eye provides a slightly different view of an object. These different views provide information about the distance and shape of an object. Both eyes survey the field of view directly ahead of you, while objects to each side can be detected peripherally by just one eye. It takes about 0.1 seconds for your brain to process a field of view and then get ready to for the next field of view. A series of still pictures flashed on a screen at twice that rate is interpreted by your brain as continuous action.

Your eyes convert incoming photons (fundamental bits of light energy) into nerve impulses. Each eye has a number of substructures that contribute to your ability to see:

1. The upper and lower eyelids regularly sweep the surface of your eye and can close rapidly to protect its surface.

2. The cornea is a clear protective layer and acts as a preliminary lens.

3. The aqueous humour is the fluid-filled cavity between the cornea and the lens.

4. The iris is a coloured disk in front of the lens. It has an opening that can vary in diameter from 0.5 to 9.0 mm to control the intensity of light that reaches the retina.

5. The lens is a flexible structure that is normally thick and curved for viewing objects up close. It can be stretched by ocular muscles to a thinner and less curved shape for viewing objects in the distance. With age the lens loses some of its elasticity and can no longer return to its highly curved status. That is when you have to start using reading glasses.

6. The vitreous humour is the liquid-filled region in the centre of the eye. The vitreous humour has to exert enough pressure to maintain the shape of the eye, but too much pressure (glaucoma) can damage the retina.

7. The retina is the lining at the back of your eye that is coated with lightdetecting sensors, called rods and cones. There are about 120 million rods spread over most of the retina to provide a basic black and white view of the universe. In addition there are about 7 million cones concentrated in the region directly behind the lens to provide a high-resolution image, in living colour, of the region directly in front of each eye. The rods are more sensitive to faint light than the cones, so faint scenes are typically perceived in black and white.

The muscles that move your eyes also play a key role in your vision system. Every twentieth of a second your eyes make rapid and abrupt jumps through about 15 degrees, as if they are in a constant search mode. This jumping motion, called a saccade, occurs when the background remains stationary and you are concentrating on an object in the foreground, as when you are reading. The eye muscles can also execute smooth pursuit movements to follow an object as it moves across your field of view. These two types of movements help to keep an object of interest focused on the more sensitive cones at the centre of the retina.

The neural connections associated with the visual system are the most complicated of any of our senses. Behind the eyes, there is a partial switch-over in the optic nerves (called the chiasm) so that all the information from the left half of the field of view, from both eyes, is directed to the right occipital lobe at the back of your brain.

Similarly, information from the right field of view, from both eyes, is channelled to the left occipital lobe. The analysis of visual information is quite complicated. So far neurologists have been able to trace twenty-eight different sub-channels in the brain for analyzing visual information.