Light. The stimulus for vision is light, which travels in waves. The amplitude (wave height) is associated with the sensory experience of brightness; the wavelength determines the hue (color) of the light; and the wave purity (whether there is more than one type of wave) produces the psychological experience of saturation.

The vision system. Light travels to the eye and passes through the cornea, the pupil (regulated in size by the iris), and the lens and then moves to the retina, where it strikes the photoreceptors for vision, the cones and the rods. The cones, in the center (fovea) of the retina, are responsible for color vision, and operate best in intense illumination. The rods are important for night vision and peripheral vision and have a greater density at the edge of the retina. Visual information proceeds from the eye through optic nerves attached to the retina at the back of each eye; the optic nerves meet and then divide at the optic chiasm in the center of the brain (Figure 1 ). The lateral portion of each optic nerve travels from the optic chiasm to the lateral visual cortex on the same side of the brain (that is, the outside of the right nerve to right visual cortex and the outside of the left nerve to the left visual cortex). However, the medial portion of each nerve crosses over at the optic chiasm and goes to the medial visual cortex on the other side of the brain (medial right nerve to left medial visual cortex and medial left nerve to right medial visual cortex).

Figure 1 The Visual Pathway

• The Young-Helmholtz trichromatic theory of color vision, proposed by Thomas Young and Hermann von Helmholtz, states that the eye has three types of cones with different sensitivities to lights of different wavelengths that produce the primary hues of red, green, and blue.

• Ewald Hering, feeling that the Young-Helmholtz theory did not cover all visual phenomena, offered the opponent process theory to explain visual images that are the complementary color of the image of the stimulus. For example, if you stare at a red dot and then look at a white paper, you will see the afterimage of a blue-green dot. (Blue-green is the complementary color of red.) These theories help to explain some of types of color blindness (some people, dichromats, have a hard time telling green from red or yellow from blue).

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