What is normal vision?

Normal vision occurs when there is no abnormality of structure or function of:

• the eye, the eye muscles or orbit,
• the nerve pathways and blood supply to the eyes,
• the structures in the orbit,
• the optic nerves; and
• the subsequent pathways in the brain to the visual cortex in the back part of the brain and associated visual areas in the brain.

How do we see?

Light enters the eye through the cornea, at the front of the eye. This is a transparent dome, which has both protective and focussing properties. Light is refracted at the junction of air and the tear film on the surface of the cornea. The cornea is kept clear by the properties of the tear fluid and replenishing the tear film by the blink action of the eyelids.

It then travels through the pupil (the aperture in the centre of the iris, the coloured part of the eye), which varies in diameter, depending on the amount of light entering the eye. If the environment is bright, the pupil constricts and if it is dark, the pupil dilates. The lens sits behind the iris and it again, refracts light, to focus it on the retina. The lens, controlled by a muscle, can change its shape to ensure the image of both near and distant objects are focused on the retina. The lens will become thicker to focus on near objects and thinner for looking into the distance.

The retina is composed of millions of light-sensitive cells (photo receptors) called rods and cones. Cones are vital for the clarity and sharpness of vision, as well as being sensitive to colour and are concentrated around the macular area where vision is at its sharpest. The rods are responsible for peripheral vision and night vision, but are not colour sensitive. They are more numerous than cones and much more sensitive to light and form the large majority of the photoreceptors in the remaining retina, being grouped mainly in the periphery. They do not contribute to visual clarity as cones do.

Each eye records an image on the retina, converting the light energy from the image to nerve impulses through a series of chemical reactions in the cones and rods. As the two eyes are spaced apart on the face, the images are slightly dissimilar with the right eye recording more of the right and the left eye more of the left side of the image. The nerve impulses of each image travel through the optic nerve from the retina of each eye. Approximately half of the optic nerve fibres (the nasal or inner half) from each optic nerve cross over at the optic chiasm. The subsequent visual pathway travelling through the length of the brain has fibres from both eyes (outer half from the same eye and inner half from the other eye). The nerve impulses are relayed to the visual cortex also called the primary visual cortex in the occipital area (the back) of the brain. Here the two visual pathways relay the image from each eye and compare the slightly dissimilar images from each eye. After an initial analysis of the image, nerve impulses are sent to the secondary visual areas (associated visual areas) for a detailed analysis of the image for all visual functions such as contrast, colour, motion, depth and shape, resulting in a final percept and recognition of the image.

Incidence and Prevalence

In 2008, the number of registrations in the UK for visual impairment was:

• Development of the eye
• Structure of the eye