In full daylight the cones are the principal photoreceptors. Most people experience color vision - individual objects have their own intrinsic color. An apple appears to us as green and a tomato as red. This has obvious advantages in distinguishing the different objects in the environment but how is this remarkable feat achieved? People with normal color vision can match the color of an object by mixing varying amounts of just three colored lights-blue; green, and red. This suggests that there should be three different cone pigments, one sensitive to blue, one to green, and one to red. This supposition has now been validated by direct measurements of the absorption of pigments found in In full daylight the cones are the principal photoreceptors. Most people experience color vision-individual objects have their own intrinsic color. An apple appears to us as green and a tomato as red.

This has obvious advantages in distinguishing the different objects in the environment but how is this remarkable feat achieved? People with normal color vision can match the color of an object by mixing varying amounts of just three colored lights-blue, green, and red. This suggests that there should be three different cone pigments, one sensitive to blue, one to green, and one to red. This supposition has now been validated by direct measurements of the absorption of pigments found in individual human cones. The blue-sensitive cones show an absorption maximum at 419nm, the green-sensitive cones have a maximum absorption at about 530 nm, and thered-sensitive cones absorb maximally at 560 nm. The rod pigments have an absorption maximum at 496 nm.

The absorption spectra measure the likelihood that a pigment will absorb a photon of light at a given wavelength. Thus at least two different pigments are required for any color vision and the brain must be able to compare the intensity of the signals emanating from different cones. For normal human color vision, a green light is seen when the green-sensitive cones are more strongly stimulated than the red- and blue-sensitive cones, and so on. White light reflects equal intensity of stimulation of all three types of cone. This is the basis of the ttichromatic theory of color vision proposed by Thomas Young at the beginning of the nineteenth century.

Useful as this theory is, it fails to explain some well-known observations. First, certain color combinations do not occur, such as a reddish green or a bluish yellow. Yet it is possible to see a reddish yellow (orange) or a bluish green (cyan). Second, if one stares at a blue spot for a petiod of time and then looks at a white page, a yellow after-image is seen. Similarly, a green afterimage will be seen after looking at a red spot. To answer these and other difficulties, E. Heting proposed the existence of neural processes in which blue and yellow were considered to be opponent colors, as were green and red. This color opponent theory (with some later modifications), together with the trichromatic ptinciple enunciated by Young, provide a basis for understanding color vision. Experimental evidence in favor of the coloropponent theory is found in the retina. For example, some retinal ganglion cells are excited by a red light in the center of their receptive field but inhibited by a green light in the surround.

Source(s): All above information & images are based on information collected from chapter on eyes from the book Human Physiology by Gillian Pocock and Christophor D. Richards. All rights reserved by respective owners.

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About 200 children are diagnosed with retinoblastoma (eye cancer) each year in the United States. This cancer affects about one out of every 20,000 children, accounting for 3.1% of all childhood cancers. Most children with retinoblastoma are under four years of age. About 75% of children with retinoblastoma have a tumor in one eye. In about 25% of cases, both eyes are affected.

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