Colour Perception

Why has colour vision developed? After all, if you see an old black-and white film on television, it is perfectly easy to make sense of the moving images presented to your eyes. There are two main reasons why colour vision is of value to us (Sekuler & Blake, 1994):

• Detection: colour vision helps us to distinguish between an object and its background.

• Discrimination: colour vision makes it easier for us to make fine discriminations among objects (e.g., between ripe and unripe fruit).

In order to understand how we can discriminate about five million different colours, we need to start with the retina. There are two types of visual receptor cells in the retina: cones and rods. There are about six million cones, and they are mostly found in the fovea or central part of the retina. The cones are specialised for colour vision and for sharpness of vision. There are about 125 million rods, and they are concentrated in the outer regions of the retina. Rods are specialised for vision in dim light and for the detection of movement. Many of these differences stem from the fact that a retinal ganglion cell receives input from only a few cones but from hundreds of rods. As a result, only rods produce much activity in retinal ganglion cells in poor lighting conditions.

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