HL 2 Biology

E.2b Vision: Retina


<<Rod and Stem Cells>>

  • entered light in the eye – form an image on the retina
  • 2 types of light-sensitive cells: rod and cone cells
    • these detect the image and convert it into nerve impulses
    • rods: very sensitive to light, works well in dim light condition
      • give us black and white vision
      • absorb visible wavelengths of light/don’t distinguish them
      • in bright light – pigments inside bleached, do not work
      • send impulses to brain via single neuron
      • less detail in images to brain by rods
    • cones: 3 types, which absorb different ranges of wavelength of light
      • more detail, each sends impulses to brain via individual neuron
      • 3 types: red, blue, or green 
      • when any color of light reaches retina => red, blue  & green cones each stimulated by a certain amount
      • observing amount of light each were stimulated by, can detect color of light
      • only stimulated in bright light, doest work in dim light

<<Processing of Visual Stimuli>>

  • Retina processes visual stimuli collected by rods & cones and passed via the bipolar cells to ganglion cells
  • Ganglion cell stimulated when: light falls on receptive field  (small circular area of retina)
    • 2 types of ganglion cells:
      • type 1: ganglion stimulated if light falls on center of the receptive field
        • stimulation reduced if light also falls on periphery
      • type 2: ganglion stimulated when light falls on the periphery of the receptive field
        • stimulation reduced if light falls on center
      • both more stimulated if edge of light/dark area is within the receptive field = edge enhancement (explanation of optical illusion/Hermann grid)
    • contra-lateral processing (of visual stimuli): some impulses from the left eye pass to the right side of the brain and vice versa
      • in optic chiasma exchange happens
      • so…right side of brain processes visual stimuli from the left side of the visual field

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