The process by which organisms form an internal representation of the external environment on the basis of the pattern of light available to them. The crucial requirement is for a light-sensitive receptor. Higher animals have many such photoreceptors (about 230 million in each eye in humans) making up a photoreceptive surface, the retina. For anything more than crude light detection, an image is essential; this constitutes an orderly representation on the retina of the spatial array of objects in the environment. Information from the retina passes up the 1 million fibres of the optic nerve to the occipital cortex, the first of many brain regions devoted to vision. The way in which the brain encodes basic visual features such as colour, edges, and movement is becoming increasingly understood, but very little is known of how these features are interpreted to enable us to recognize objects.