The Visual System:
Color Vision
Lesson 18
The Trichromatic Theory
Young-Helmholtz (1802)
 3 types of color receptors
 Cones
 Differential sensitivity to light
wavelengths
 red (long)
 green (medium)
 blue (short) ~

The Trichromatic Theory
Perceived color
 overall pattern of stimulation
 Like mixing paint
 Negative After-image?
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The Trichromatic Theory?
What colors do you see?
 How can you see colors that weren’t
there?
 Negative After-image
 Does it fit Trichromatic Theory? ~
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Hering Opponent Process Theory
Competing theory
 Center-Surround organization
 BP, RGC, & LGN
 Antagonistic for color
 Red-Green
 Blue-Yellow
 Black-White

Hering Opponent Process Theory

Center- surround antagonistic
 Each color can be excitatory (+) or
inhibitory (-)
 12 combinations total ~
Which Theory?
Both are correct in retina
 Photoreceptors: trichromatic
 Higher levels: Opponent Process
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BP, RGC, & LGN
More complex at cortical level
 Retinex Theory ~
Retinex Theory
Perception of visual stimulus
 subjective
 includes context & past experiences
 color & brightness constancy
 Color constancy
 e.g., grass in sun vs. grass in shade
 perceived as same color green
 e.g., Rubic’s cube ~
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Lateral Geniculate Nucleus

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6 layers
 dorsal  ventral

6  1
RFs center-surround
Input from each eye
 monocular
 2, 3, 5 from the ipsilateral eye
 parallel processing ~
LGN: Parallel Processing


Parvocellular system
 layers 3-6
 small RFs
 info from cones
 color & form
Magnocellular system
 Large RFs
 layers 1 & 2
 info from rods
 form only, no color ~
Color Processing:
Primary Visual Cortex
V1 Organization
6 layers
 Most input  Layer 4
 1st binocular receptive fields
 cells get input from both eyes
 ocular dominance
 Modular organization
 Blob cells: color processing
 Wavelength specific ~

Modular V1 Organization
L
2&3
4
LGN
5&6
B
L
O
B
R
L
Blob cells & Color Perception
Same color perceived differently
 Double Opponent Process Cells
 Different from RGC & LGN
 center-surround
 Center:
 red  excitation
 green  inhibition
 Surround:
 red  inhibition
 green  excitation ~

Double-Opponent Process
Blob Cells
R-G+
R-G+
R+G-
R+G-
Higher Level Color Processing

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V1  V2  V4
V4 in medial occipital cortex
V4 damage
 disrupts color constancy
 achromatopsia
 unilateral – 1 visual field
 bilateral – no color perception ~