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I understand that both the Trichromatic Process and the Opponent Process take place in human vision.
Trichromatic Process is widely accepted to happen in the retina,
however I heard differing accounts of where Opponent Process happens - in the retina or in the LGN.

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Opponent process happens BOTH in the retina, and in the brain (Lateral Geniculate Nucleus).

Here is the excerpt from Vision Science: Photons to Phenomenology by Stephen E. Palmer:

In the brain (LGN):

Physiological psychologist Russell De Valois (1965) and his colleagues (De Valois, Abramov, & Jacobs, 1966) measured responses in the LGN of macaque monkeys and discovered color-selective cells whose responses to wavelengths of light were incompatible with trichromatic theory but conformed roughly to the pattern predicted by Hering's opponent process theory. Some were excited by red light and inhibited by green (R+G- cells). Others responded in the opposite way, exciting to green and inhibiting to red (G+R- cells). In addition, they found cells that were excited by yellow and inhibited by blue (Y+B- cells) as well as ones showing the reverse pattern of responses (B+Y- cells). Together, these LGN cells can reasonably be interpreted as a neural implementation of Hering's opponent process theory.

In the retina:

Further research has shown that these patterns of response are also present in the bipolar and ganglion cells of the retina. Output of R+G- opponent cells can be derived by combining excitatory input from the L cones and the inhibitory input from M cones, essentially computing the difference between the output of these two cone systems (L - M).

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