Rods and Cones

Modified: 2024-06-17 9:04 PM CDST

• The human eye is a specialized organ whose job is to convert or transduce light energy into neural impulses.
• Before we look at how that transduction takes place, let us look at the anatomy of the eye.
  • Refer to the eye diagram.
    • Starting from the outside, note the following parts.
      • First, start with the cornea, or the clear outer covering of the eye.
        • Contact lenses sit on the cornea.
      • Behind the cornea is the iris.
        • The iris controls the amount of light that enters the eye by forming the pupil.
        • The pupil is the hole formed by the action of the iris .
      • Behind the pupil is the lens, the lens focuses the light so that it lands on the retina.
        • The retina is where the photosensitive cells are located.
      • In particular, the lens focuses light so that when you stare directly at an object, the light hits the fovea.
  • The retina is composed of a number of layers.
    • Only the top three layers are involved in vision.
    • The third layer down is where the photosensitive cells, the rods and cones, are located.
    • So, light must travel through the two other layers of cells before it can be detected.
  • In the human eye, the cone cells are located primarily in and around the fovea.
    • The cones are responsible for color vision and daylight vision.
    • The cones also provide us with our sharpest vision, or highest acuity of vision.
    • Finally, the cones are much less numerous than the rod cells.
  • The rod cells are responsible for night vision and for seeing in black and white. (e.g. , "gray scale").
    • So, people who are totally colorblind must not have functioning cone cells.
    • Furthermore, at night, you cannot see colors (you just think that you do). But, that is a topic for the later in this chapter.
    • In the human eye, rods are found everywhere in the retina, except in and near the fovea.
    • Rods do not detect light as sharply as the cones do, but rods are much more sensitive to low light levels than the cones are.
    • Finally, there are many more rods than there are cones.