Intro | Amacrine Cells | Bipolar Cells | Cones | Optic Disk and Blind Spot | Ganglion Cells | Horizontal Cells | Optic Nerve Fibres | Receptor Cells | Rods
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Cones are named for their cone-like shape. These receptor cells are sensitive to bright light and are specialized for color vision. Cones are found in greatest density at the fovea. This region consists exclusively of cones and is located at the center of the retina. Because cones are particularly sensitive in bright light and illumination enhances visual acuity, they provide the sharpest and most detailed images of the outside world.
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The transduction process described for rods occurs in the cone cells as well. There 3 types of cone cells that differ based on the wavelength of light absorbed by the photopigments (the opsins) therein; each type responds selectively to short, medium, and long wavelengths. Two models have been offered in explanation of how the cone cells create color vision, the trichromatic (Young-Helmholtz) theory and the opponent-process theory. The trichromatic theory speculates that we perceive color based on the ratio of activity across all three types of cone cells. The opponent-process theory may occur at the level of bipolar cells and beyond, where the cellular characteristics support the encoding of paired opposites such as red versus green, blue versus yellow, and white versus black. For example, many visual cells are excited by green light at the retina and inhibited by red light. Color-blindness may be caused by absent or deficient cones containing any one or all of the photopigments. This defect is passed along genetically as a recessive gene on the X (female) chromosome. For this reason, color-blindness occurs far more frequently in male (8% incidence) than females (1% incidence).