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