firing
frequency.
VISION
Vision is our primary distance sense. Its stimulus is light, which can vary in intensity and
wavelength. Some structures of the eye, such as the iris and the lens, control the amount of light
entering the eye and form a proper proximal stimulus—the retinal image. Once on the retina, the
light stimulus is transduced by the rods and cones. Acuity is greatest in the fovea, where the
density of cones is greatest. Rods and cones differ markedly in function. The rods operate at low
light intensities and are insensitive to differences in hue. The cones function at much higher
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