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Visual Impairment :
Special Educational Needs
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Photomicrograph showing rod and cone cells

The retina of each eye contains two types of light-detecting cells called rods and cones. Each retina contains about 120 million rods and 5 million cones.

The electron microscope photo shows their structure. The long cells are rods and the shorter pointed cells are cones.

Cones are responsible for photopic vision, i.e. colour and high resolution, detailed vision, and need bright light conditions to work. They are concentrated in the centre of the retina (the macula) and are densest in the fovea, which is 0.3 mm in diameter.

The light sensitive pigments in the different cones can detect red, green and blue light. The level of the pigments in cones is much less than the pigments found in rods.

64% of cones (Long wavelength; L-cones) detect the red region of the spectrum, 32% (Medium wavelength; M-cones) detect the green region and the rest (Short wavelength; S-cones) detect light in the blue region.

Graph showing wavelegths detected by the cones

Rods are responsible for dark adapted (scotopic) vision, and are 1,000 times more sensitive than cones. Rods contain a pigment called rhodopsin which is sensitive to shorter wavelengths of light. They are extremely sensitive and better at detecting rapid movement. They do not detect colour and are predominantly found in the peripheral areas of the retina. There are fewer in the macula area and they are totally absent from the fovea.

The graph shows the distribution of rods and cones from the centre of the retina to the peripheral regions. The blue line shows the distribution of the cones and the red line shows the rods.

Graph showing distribution of rods and cones in the retina

The diagram shows the rods and cones in the retina

Diagram showing the structure of the retina

When light hits the retina it stimulates the light sensitive pigment in the outer segment of the rod and cone cells, depending on the wavelength of the light. This causes the cell to initiate a nerve impulse which travels through the network of neurones (horizontal cells and bipolar neurones) to the optic nerve. The optic nerve transmits the nerve impulses to the visual cortex of the brain.