1. Color Perception Requires receptor cells that differ in their sensitivity to different wavelengths
The trichromatic hypothesis of color perception:
Three different types of cones
Each responds to a different part of the spectrum
Each has a separate pathway to the brain
The opponent-process hypothesis of color perception:
Four unique hues and three opposed pairs of colors—blue versus yellow, green versus red, and black versus white.
Three systems that produce opposite responses to different wavelengths
Each human cone has one type of pigment—with a different peak of sensitivity:
Short (S)–peak sensitivity at 420 nm
Medium (M)–530 nm
Long (L)–560 nm

2. Spectral Sensitivities of Human Photopigments
The trichromatic hypothesis of color perception:
Three different types of cones
Each responds to a different part of the spectrum
Each has a separate pathway to the brain
Each human cone has one type of pigment—with a different peak of sensitivity:
Short (S)–peak sensitivity at 420 nm
Medium (M)–530 nm
Long (L)–560 nm

3. A Model of the Connections of Wavelength Discrimination Systems in the Primate Retina
The opponent-process hypothesis of color perception:
Four unique hues and three opposed pairs of colors—blue versus yellow, green versus red, and black versus white.
Three systems that produce opposite responses to different wavelengths
Most retinal ganglion cells and cells in the parvocellular layer of the LGN:
Fire in response to some wavelengths but are inhibited by others
A parvocellular LGN cell fires at wavelengths above 600 nm, where L cones are most sensitive.
The cell is inhibited at shorter wavelengths where L cones are less sensitive than M cones.
The cell is a +L / –M cell.

4. Responses of the Four Main Types of Spectrally Opponent Cells in Monkey LGN
A parvocellular LGN cell fires at wavelengths above 600 nm, where L cones are most sensitive.
The cell is inhibited at shorter wavelengths where L cones are less sensitive than M cones.
The cell is a +L / –M cell.
A plus L/minus M (+L/–M) cell is spectrally opponent; it has opposite firing responses to different regions of the spectrum.
Spectrally opponent ganglion cells receive input from different types of cones and record the difference.
Spectrally opponent neurons are the second stage in color processing.
They are not color cells because:
Outputs go to other circuits for form, depth, and movement detection.
Their peak sensitivities do not correspond to the wavelengths for the main colors.

5. Responses of the Four Main Types of Spectrally Opponent Cells in Monkey LGN (Part  1)
A plus L/minus M (+L/–M) cell is spectrally opponent; it has opposite firing responses to different regions of the spectrum.
Spectrally opponent ganglion cells receive input from different types of cones and record the difference.

6. Responses of the Four Main Types of Spectrally Opponent Cells in Monkey LGN (Part  2)

7. Responses of the Four Main Types of Spectrally Opponent Cells in Monkey LGN (Part  3)

8. Responses of the Four Main Types of Spectrally Opponent Cells in Monkey LGN (Part  4)

9. A Model of the Connections of Wavelength Discrimination Systems in the Primate Retina
The opponent-process hypothesis of color perception:
Four unique hues and three opposed pairs of colors—blue versus yellow, green versus red, and black versus white.
Three systems that produce opposite responses to different wavelengths