6.2

• True or False, when we direct our gaze at something near, the tension on the ligaments holding each lens in place is adjusted by the ciliary muscles, and the lens is flattened.

  False, the lens assumes its natural cylindrical shape.
• True or False, when we focus on a distant object, the lens is cylindrical.

  False, the lens is flattened.
• The process of adjusting the configuration of the lenses to bring images into focus on the retina is called accommodation.
• Why do vertebrates have two eyes?

  Vertebrates have two sides: left and right.
• What are the implications of having two eyes?

  Vertebrates can see in almost every direction without moving their heads.
• Why do some vertebrates, including humans, have their eyes mounted side-by-side on the front of their heads?

  1. To see depth
• True or False, convergence is greatest when you are inspecting things that are close.

  True
• True or False, the positions of the images on your two retinas can never correspond exactly because your two eyes do not view the world from exactly the same position.

  True
• The difference in the position of the same image on the two retinas.

  Binocular Disparity
• True or False, the binocular disparity is greater for close objects than for distant objects.

  True
• Why do a few vertebrate species have their eyes side-by-side on the front of the head while most species have one eye on each side?

  • Predators tend to have the front-facing eyes because this enables them to accurately perceive how far away prey animals are.
  • Prey animals tend to have side-facing eyes because this gives them a larger field of vision and the ability to see predators approaching from most directions.
• The retina is composed of five different types of neurons: RHBAR

  • Receptors
  • Horizontal Cells
  • Bipolar Cells
  • Amacrine Cells
  • Retinal Ganglion Cells
• Order: GABHR


  Ganglion

  Amacrine

  Bipolar

  Horizontal

  Receptors
• Specialized cells for lateral communication:

  Amacrine cells and the horizontal cells
• Communication across the major channels of sensory input.

  Lateral Communication
• Retinal neurons communicate both chemically via synapses and electrically via gap junctions.
• Retinal neurons communicate both chemically via synapses and electrically via gap junctions.
• Retina is in a sense inside-out:

  Light reaches the receptor layer only after passing through the other layers. Then, once the receptors have been activated, the neural message is transmitted back out through the retinal layers to the retinal ganglion cells, whose axons project across the outside of the retina before gathering together in a bundle and exiting the eyeball.
• The inside-out arrangement of the retina creates two visual problems:

  1. Incoming light is distorted by the retinal tissue
  2. For the bundle of retinal ganglion cell axons to leave the eye, there must be a gap in the receptor layer.
• For the bundle of retinal ganglion cell axons to leave the eye, there must be a gap in the receptor layer; this gap is called the blind spot.