AP PSYCH 3.3 Visual Anatomy

The Process

• In bottom-up processing, the ‘bottom’ of the process is the stimulus
  • In visual processing, that is light waves
• Light waves enter the eye through the thin outer covering of the cornea
  • The first line of defense against debris, also does some rudimentary focusing
• The light then passed through the hole in the iris, the colored muscle that contracts and flexes to let in the appropriate amount of light
• The hole is the pupil
• The light goes through the hole then through the lens, which flips the image onto the retina
  • The lens is always shifting shape to focus on what you’re looking at
  • If the lens flips the image too late or too early, vision problems can occur
• The whole back lining of the eye is the retina
  • The retina is covered in rods and cones
  • Cones are clustered near the fovea (like ‘focus’), where the optic nerve exits, and they detect well-lit colors the best
  • Rods cover most of the outer retina, and detect dark, black-and-white best
  • Once the rods and cones are stimulated, they transmute that light stimulus into neural impulses
  • These impulses travel down the optic nerve and are processed accordingly
• The image is flipped by the lens and isn’t flipped upright until it reaches the brain
  • The occipital lobe, where visual processing occurs, is actually not right behind the eyes, but at the furthest point from the eyes at the back of the brain
  • The tow optic nerves cross each other and the images from each eye are processed in the opposite hemisphere
• Feature detectors detect…
  • Light and color
  • Lines
  • Shapes
  • Angles
  • Motion
  • These help piece together what is being seen and how to react

Color Vision

• In vision, the stimuli for our receptor cells lining the retina are light waves
• Light waves have two properties
• Wavelength
  • Determines hue (color)
  • Wavelength is the distance from one point on a light wave to the same point on the next wave
  • The peaks or troughs are used most often because the points are distinct, but any point can determine wavelength
  • Short wavelengths create cool colors
  • Long wavelengths create warm colors
• Amplitude
  • The height of a wave from its trough to peak
  • Determines intensity/saturation

Color Theories

• Some consider these two theories to be two steps in a single process

Trichromatic Theory

• Developed by Young and Helmholtz
• Photoreceptors work in teams of three (tri → three, chromatic → color)
  • Red, green, and blue
  • Like some TV displays
• Combinations of cones firing make up all colors in the visual spectrum
• Strength of the signal determines how the brain interperets the colors
• As light hits the retina, these cones are stimulates to create a sensation of color

Opponent-Process Theory

• Visual information is transferred from the photoreceptors to the ganglion cells
  • As this occurs, some neurons are excited while others are inhibited
• Hence, they work in an ‘opponent-process’
• Neurons turn ‘on’ and ‘off’ during this process
• Explains the phenomenon of inverted afterimages

Color Blindness

• Color blindness is the result of a lack of functioning photoreceptors for color
• People who are color-blind cannot distinguish excitatory from inhibitory signals or may have unresponsive cones
• Monochromat
  • Can only see black, white, and grey
• Dichromat
  • Red and green or yellow and blue color blindness
  • Most common type of color blindness
• Trichromat
  • Able to see all colors in visual spectrum