COLOR VISION

FUNCTIONS OF COLOR VISION
Perceptual Organization: determining the boundaries of overlapping objects
important for survival (energy cost/gain): detecting fruit in forest
distinguishing illumination contours / light-shadow boundaries
Signaling: many animal species use color as part of mating displays
color of finch's beak and legs can indicate health and readiness to mate
leg banding experiments, color and symmetry important

WHAT IS COLOR?
Describing color
achromatic: black, white, and grey
chromatic: red, blue, purple, green, orange, etc. = Hue
Saturation: inversely related to amount of whiteness in a color; the less white (more pure spectral color) the more saturated is the color
- 200 colors X 500 brightness levels X 20 saturation values = 2 Million diff. colors
- Cultural studies show similarity in color name development across societies:
white & black - red - green & yellow - blue
Color perception is also a product of the light illuminating the object

COLOR IS A REFLECTION
e.g., clothes shopping, bathroom mirror, 'warm' and 'cold' (fluorescent) light
- Sunlight is white light: has = intensities across all wavelengths (w) of the spectrum
compare with tungsten bulb, with higher # of long wavelengths
Selective Reflection: colored objects only reflect back some of the spectrum
blue object absorbs med & long w, blue bounces off into your eyes
- objects do not have intrinsic colors, color is an interaction (like all perception)

PSYCHOPHYSICAL COLOR RESEARCH
- Before advanced physio techniques, scientists could make theories of color perception mechanisms through psychophysical experiments
2 major theories proposed in mid-late 1800's

1) Trichromatic or Young (1802)-Helmholtz (1852) Theory
Technique: Color-Matching Experiments
Ss asked to match comparison field to test field's color
need minimum of 3, in diff. proportions, to match any single w light of a particular w stims 3 receptors to diff degrees
pattern of activity = perception
Trichromatic Theory alone doesn't explain some important observations
2) Hering's Opponent-Process Theory (1878)
Technique: Phenomenological Observation
afterimages - red / green and blue / yellow
colorblind people - if blind to red, also blind to green
Hering proposed 3 mechs: Black+/White- Red+/Green- Blue+/Yellow-
build-up or break down of chemicals in retina (Wrong...)

PHYSIOLOGICAL COLOR RESEARCH
Evidence for Trichromacy
Measured absorption spectra of pigments using microspectrophotometry(1983)
Verified above with electrophysiology in human cones (1987)
3 Cone Pigments/Types: short (419nm / blue), medium (531nm / green), long (558 / red)

- Influence of range of w - given cone has response curve
- Influence of intensity - in a single receptor, any w could generate any response depending on its intensity
Metamers: 2 lights w/physically diff w dist can appear identical if they stimulate the receptors in the same way

Evidence for Opponent-Process
"S-potential" found in fish retina - neuron responds positively to one end of spectrum, negatively to the other end

Opponent cells in rhesus monkey LGN and Cortex
Two types of color opponent cells: Type 1 and Double Color-Opponent

Cytochrome oxidase staining reveals color-sensitive Blobs

COLOR DEFICIENCY
Monochromat - no functioning cones, "color-blind", poor visual acuity, sensitive to glare
Dichromat
Protanopia & Deuteranopia are X-chromosome linked
1.0% males, .01-.02% females
Blue up to neutral point - 492nm for Protonopes, 498nm for Deuteranopes, then Yellow
Protonopes missing long wavelength pigment, Deuteranopes missing medium wavelength pigment
Tritanopia - .002% males, .001% females
Blue up to neutral point (570nm) then Red
Tritanopes probably missing short wavelength pigment