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