Chapter 5

• What was Ibn al-Haytham’s evidence that we see only because light enters the eyes?


  We see distant objects faster than sight rays could reach them and only see light that reflects off objects into our eyes.
• How does the brain code information about visual perception?


  The brain codes information based on which neurons are active and their level of activity at any moment.
• What does the Law of Specific Nerve Energies state?


  It states that whatever excites a particular nerve establishes a unique kind of energy for that nerve.
• What would you perceive if the auditory receptors in your ear were electrically stimulated?


  You would perceive it as sound due to the law of specific nerve energies.
• What would happen to your perceptions if your brain were flipped upside down?


  Your perceptions would not change because they depend on which neurons are active, not their physical location in the brain.
• What is the function of the pupil and retina in vision?


  • Pupil: opening in the center of the iris
  • Retina: rear surface of the eye lined with visual receptors
• Describe the pathway of light from the receptors to the brain.


  1. Receptors to bipolar cells
  2. Bipolar cells to ganglion cells
  3. Ganglion cells' axons form the optic nerve
• What role do amacrine cells play in visual processing?


  Amacrine cells refine input to ganglion cells, enabling responses to specific shapes, movements, and colors.
• What is the blind spot in the eye?


  The blind spot is the area at the back of the eye where there are no receptors because ganglion cell axons exit through it.
• Why do we not notice our blind spot?


  Your brain fills in the gap, and anything in the blind spot of one eye is visible to the other eye.
• What is the fovea and its significance in vision?


  • Tiny area specialized for acute, detailed vision
  • Nearly unimpeded vision due to absence of blood vessels and ganglion cell axons
• How do midget ganglion cells contribute to vision?


  Midget ganglion cells respond to a single cone and provide 70% of the brain's input.
• How do birds' foveas differ from those of humans?

  Many birds have two foveas per eye, one pointing ahead and one to the side for better peripheral detail.
• Compare foveal vision and peripheral vision.


  • Foveal vision: sensitivity to detail
  • Peripheral vision: sensitivity to dim light
• What are rods and cones in the human retina?


  Rods are abundant in the periphery and sensitive to dim light, while cones are abundant near the fovea and responsible for color vision.
• What is the ratio of rods to cones in the human retina?


  The ratio is approximately 20:1, with rods being more abundant than cones.
• How does the structure of the fovea differ from the periphery in terms of receptor connections?


  In the fovea, each cone has its own line to the brain, while in the periphery, each receptor shares a line with many others.
• What are photopigments and their role in vision?


  Photopigments are chemicals that release energy when struck by light, consisting of 11-cis-retinal and opsins.
• What happens to 11-cis-retinal when light is absorbed?


  It is converted to all-trans-retinal, releasing energy that activates second messengers within the cell.
• Why might you see a faint star better by looking slightly to the side?

  Looking slightly to the side allows light to fall on an area of the retina with more rods and convergence of input.
• What can be inferred about a species with a high ratio of cones to rods?


  It is likely to be highly active during the day and seldom at night.
• What are the shortest and longest wavelengths of light visible to humans?

  • Shortest wavelength: violet
  • Longest wavelengths: red
• What is the Trichromatic Theory of color vision?


  It states that we perceive color through the relative rates of response by three kinds of cones, each sensitive to different wavelengths.
• What does the Opponent-Process Theory explain about color perception?


  It explains that we perceive color in terms of opposites, such as red to green and yellow to blue.
• What is the Retinex Theory of color vision?


  It explains color constancy, the ability to recognize colors despite changes in lighting.
• According to the trichromatic theory, what causes the perception of red?


  A high ratio of response from the long-wavelength cone relative to the other two types of cones.
• Under what circumstance would you perceive a white object as blue according to the opponent-process theory?


  If you stared at a bright yellow object for a minute and then looked at a white object, it would appear blue.
• What accounts for the black perception on a television screen when it is on?


  The black experience arises by contrast with the brighter areas around it.
• Why should we not call light at about 510 nm “green light”?


  Color perception depends on the light from surrounding areas, not just the wavelength of light from a given spot.
• What causes color vision deficiency?

  It is caused by certain genes failing to develop one type of cone or developing an abnormal type of cone.
• What is the prevalence of red-green color deficiency?


  It affects about 8% of northern European men.
• Why is "color vision deficiency" a better term than "color blindness"?


  Very few people see the world entirely in black and white; most have difficulty discriminating certain colors.
• Describe the overview of the mammalian visual system.

  • Rods and cones synapse with horizontal and bipolar cells
  • Ganglion cell axons form the optic nerve
  • Optic chiasm: half of the axons cross to the opposite side
  • Lateral geniculate nucleus processes most ganglion cell axons
• What is the role of lateral inhibition in the retina?


  Lateral inhibition sharpens contrasts to emphasize the borders of objects.
• What is a receptive field in the visual system?


  • Area in visual space that excites or inhibits a neuron
  • Each cell has a receptive field
  • Receptive fields can be excitatory or inhibitory
• How do receptive fields change from bipolar cells to ganglion cells?


  Receptive fields become larger as inputs from earlier levels converge.
• What are the differences between parvocellular and magnocellular systems?


  Parvocellular neurons have small cell bodies and are specialized for detail and color, while magnocellular neurons have larger cell bodies and are specialized for movement and patterns.
• What is the primary visual cortex and its significance?


  • Located in the occipital cortex (area VI)
  • Damage results in loss of conscious vision and visual imagery
  • Blindsight can occur with damage
• What is blindsight?

  Blindsight is the ability to respond to visual information without conscious perception.
• What are the two explanations for blindsight?


  1. Small islands of healthy tissue remain in a damaged visual cortex. 2. The thalamus sends visual input to other brain areas.