and understating others.
Visual sensations vary in color; and color sensations can be ordered on the basis of their
hue, brightness, and saturation. Normal human color vision is trichromatic, depending on three
cone types. However, some facts do not fit with this trichromatic conception, because colors
come in pairs—as shown by the phenomena of complementary colors, color contrast, and
negative afterimages. Shape perception depends on specialized detector cells that respond to
certain characteristics of the stimulus, such as curves and straight edges. The optimal input for
each cell—that is, a stimulus of a certain shape and size at a certain position—defines the cell’s
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