1. The Visual System: Color Vision
Lecture 16

2. The Trichromatic Theory
Young-Helmholtz (1802)
3 types of color receptors
Cones
Differential sensitivity to light wavelengths
red (long)
green (medium)
blue (short) ~

3. The Trichromatic Theory
Perceived color
overall pattern of stimulation
Like mixing paint
Negative After-image?

4. 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? ~

5. Hering Opponent Process Theory
Competing theory
Center-Surround organization
BP, RGC, & LGN
Antagonistic for color
Red-Green
Blue-Yellow
Black-White

6. Hering Opponent Process Theory
Center- surround antagonistic
Each color can be excitatory (+) or inhibitory (-)
12 combinations total ~

7. Which Theory? Both are correct in retina Photoreceptors: trichromatic
Higher levels: Opponent Process
BP, RGC, & LGN
More complex at cortical level
Retinex Theory ~

8. 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 ~

9. Lateral Geniculate Nucleus
6 layers
dorsal  ventral
6  1
RFs center-surround
Input from each eye
monocular
2, 3, 5 from the ipsilateral eye
parallel processing ~

10. 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 ~

11. Color Processing: Primary Visual Cortex

12. 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 ~

13. Modular V1 Organization
L R L B L O
2 & 3 4
LGN
5 & 6

14. 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 ~

15. Double-Opponent Process Blob Cells
R-G+
R-G+
R+G-
R+G-

16. Higher Level Color Processing
V1  V2  V4
V4 in medial occipital cortex
V4 damage
disrupts color constancy
achromatopsia
unilateral – 1 visual field
bilateral – no color perception ~