CHAPTER 3 - The Eye and Retina

Question 1

Describe light, the structure of the eye, and the rod and cone receptors. How are the rods and cones distributed across the retina?

Answer 1

• Light: The Stimulus for Vision
  
  • Visible light: a band of energy within the electromagnetic spectrum
    
    • Electromagnetic spectrum is a continuum of electromagnetic energy that is produced by electric charges and is radiated as waves
    • the energy in this spectrum can be described by its wavelength –> the distance between the peaks of the electromagnetic waves
      
      • range from extremely short-wavelength gamma rays to long-wavelength radio waves
  • visible light: the energy within the electromagnetic spectrum that humans can perceive
    
    • has wavelengths ranging from about 400 to 700 nanometers
    • for humans and some other animals, the wavelength of visible light is associated with the different colours of the spectrum, with short wavelengths appearing blue, middle wavelengths green, and long wavelengths yellow, orange, and red
• The Eye
  
  • Light reflected from objects in the environment enter the eyes through the pupil and is focused by the cornea and lens to form sharp images of the objects on the retina, the network of neurons that covers the back of the eye and that contains the receptors for vision (aka photoreceptors)
  • Two types of photoreceptors
    
    • Rods and cones so called because of the rod- and cone-shaped outer segments
  • The outer segments are the part of the receptor that contains light-sensitive chemicals called visual pigments that react to light and trigger electrical signals
  • Optic nerve: contains a million optic nerve fibers that conduct signals toward the brain
  • 1. One small area, the fovea, contain only cones. When we look directly at an object, the object’s image falls on the fovea
  • 2. The peripheral retina, which includes all of the retina outside of the fovea, contains both rods and cones. It is important to note that although the fovea has only cones, there are also many cones in the peripheral retina
       - Fovea is so small that it contains only about 1%, or 50 000, of the 6 million cones in the retina
  • 3. The peripheral retina contains many more rods than cones because ether are about 120 million rods and only 6 million cones in the retina
  • Macular degeneration destroys the cone-rich fovea and a small area that surrounds it
    
    • This creates a blind region in central vision
  • Retinitis pigmentosa is a degeneration of the retina that is passed from one generation to the next
    
    • First attacks the peripheral rod receptors and results in poor vision in the peripheral visual field
    • In more severe cases, the foveal cone receptors are also attacked, resulting in complete blindness
  • Blind spot: area in the retina where the nerve fibers that make up the optic nerve leave the eye, and where there are no photoreceptors
    
    • Why aren’t we always aware of our blind spot?
      
      • The blind spot is located off to the side of our visual field, where objects are not in sharp focus  so, we don’t know where to look now what to look at, our blind spot is difficult to detect
      • A mechanism in the brain “fills in” the place where the image disappears
        
        • It creates a perception that matches the surrounding pattern –> how the brain creates a coherent perception of our world

Question 2

How does moving an object closer to the eye affect how light reflected from the object is focused on the retina?

Answer 2

• Cornea: the transparent covering of the front of the eye, accounts for about 80% of the eye’s focusing power, but it is fixed in place  it can’t adjust its focus
• Lens: supplies the remaining 20% of the eye’s focusing power, can change its shape to adjust the eye’s focus for objects located at different distances
  
  • Change of shape  achieved by the action of ciliary muscles, which increase the focusing power of the lens by increasing its curvature
• Why does the eye need to adjust its focus:
  
  • Person with normal (20/20) vision
    
    • If the object is located more than about 20 feet away, the light rays that reach the eye are essentially parallel, and the cornea-lens combination brings these parallel rays to a focus on the retina
    • But if the object moves closer to the eye, the light rays reflected from this object enter the eye at more of an angle, and this pushes the focus back

Question 3

How does the eye adjusting the focusing of light by accommodation? Describe the following refractive errors that can cause problems in focusing: presbyopia, myopia, hyperopia. How are these problems solved through either accommodation or corrective lenses?

Answer 3

• Accommodation
  
  • The change in the lens’ shape that occurs when the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker
    
    • Increases the bending of the light rays passing through the lens
  • Everything is not in focus all at once
  • Unconscious phenomenon
  • Makes it possible to adjust vision for different distances
• Refractive Errors
  
  • There are a number of error that can affect the ability of the cornea and/or lens to focus the visual input onto the retina (refractive errors)
  • Example of refractive errors
    
    • As people get older, their ability to accommodate decreases due to hardening of the lens and weakening of the ciliary muscles –> become unable to accommodate enough to see objects, or read, at close range
      
      • Presbyopia –> can be dealt with by wearing reading glasses – bringing near objects into focus by replacing the focusing power that can no longer be provided by the lens
    • Myopia (nearsightedness) –> an inability to see distant objects clearly
      
      • Occurs when the optical system brings parallel rays of light into focus at a point in front of the retina –> the image is blurred
      • 2 possible causes
        
        • 1. Refractive myopia –> the cornea and/or the lens bends the light too much
        • 2. Axial myopia –> the eyeball is too long
      • Corrective lenses can solve this problem
    • Hyperopia (farsightedness) –> can see distant objects clearly but have trouble seeing nearby objects because the focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short
      
      • Young people can accommodate
      • Older people, who have difficulty accommodating, often use corrective lenses that bring the focus point forward onto the retina
  • Vision doesn’t occur on the retina but in the brain
    
    • Before the brain can create vision, the light on the retina must activate the photoreceptors in the retina

Question 4

Where on the retina does a researcher need to present a stimulus to test dark adaptation of the cones? How is this related to the distribution of the rods and cones on the retina? How can the adaptation of cones be measured without any interference from the rods? How can adaptation of the rods be measured without any interference from the cones?

Answer 4

• The fovea –>

Question 5

Describe how rod and cone sensitivity changes starting when the lights are turned off and how this change in sensitivity continues for 20 to 30 minutes in the dark. When do the rods begin adapting? When do the rods become more sensitive than the cones?

Question 6

What happens to visual pigment molecules when they (a) absorb light and (b) regenerate? What is the connection between visual pigment regeneration and dark adaptation?

Question 7

What is spectral sensitivity? How is a cone spectral sensitivity curve determined? A rod spectral sensitivity curve?

Question 8

What is a pigment absorption spectrum? How do rod and cone pigment absorption spectra compare, and what is their relationship to rod and cone spectral sensitivity?

Question 9

What is convergence, and how can the differences in the convergency of rods and cones explain (a) the rods’ greater sensitivity and (b) the cones’ better detail vision?

Question 10

What is a receptive field? What did Hartline’s research indicate about receptive fields?

Question 11

Describe the experiment that demonstrated the effect of lateral inhibition in the “Limulus”

Question 12

What is center-surround antagonism? Describe how lateral inhibition and convergence underlie center-surround antagonism

Question 13

Discuss how lateral inhibition and center-surround receptive fields can lead to edge enhancement

Question 14

What is the Chevreul illusion? What does it illustrate about the difference between physical and perceptual?

Question 15

What does it mean to say that early events are powerful shapers of perception? Give examples

Question 16

What is the young infant’s visual acuity, and how does it change over the first year of life? What is the reason for (a) low acuity at birth and (b) the increase in acuity over the first 6 to 9 months?