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Developed in the mid-1800s.
Proposed that human color perception involves three types of cones in the retina, each sensitive to different wavelengths of light.
Key Finding: The cones are responsive to red, green, and blue light.
The theory was derived from experiments on how people perceive different mixtures of light without prior knowledge of the specific types of photopigments in cones.

Developed in the late 1800s.
Contrasts with the trichromatic theory by suggesting that color perception is based on opposing pairs of colors.
The three dimensions of color in this theory include:
- Black to white
- Red to green
- Blue to yellow
Developed from clever experiments that further explored color perception.

A classic demonstration of the opponent process theory.
Setup: View a yellow cross while keeping eyes still; necessary for the demonstration to work.
Concept: Fatiguing the receptors for one color in an opposing pair enhances perception of its opponent color.
- Example: Staring at a yellow cross fatigues the yellow color receptors, leading to a relative increase in sensitivity to blue.

After staring at the yellow cross, the screen is switched to gray.
Gray is expected to stimulate both blue and yellow detectors equally.
However, due to the fatigue of the yellow detectors, blue detectors respond more vigorously, thus producing a bluish afterimage.
Visual Result: The gray screen appears to have a bluish tint due to the fatigue effect.
Feedback: This illustrates how visual perception can be altered based on receptor fatigue.

Both the trichromatic and opponent process theories apply to different stages of visual processing.
Photoreceptors: The trichromatic theory elucidates the color processing in cones (red, green, and blue).
Ganglion Cells: The opponent process theory describes how ganglion cells process the input from cones.
Neural Pathways: Concludes that the output of cones influences bipolar cells, which in turn affect ganglion cells that output into the brain.

Different classes of ganglion cells are activated or inhibited based on the following dimensions:
- Black to White: Some ganglion cells activated by white are inhibited by black, or vice versa.
- Red to Green: Other ganglion cells respond to red and are inhibited by green.
- Blue to Yellow: A third class of ganglion cells is excited by blue and inhibited by yellow, achieving the blue-yellow dimension.

Signal Flow:
- Blue, green, and red cones project to bipolar cells, which then signal to ganglion cells.
- From the blue cone, signaling is excitatory; from the red and green cones, signaling is inhibitory towards the ganglion cell.
- Since yellow is a combination of red and green, the inhibition from both these cones results in the ganglion cell being inhibited by yellow.
Conversely, ganglion cells that react to yellow are excited by input from red and green cones while being inhibited by blue cones.

Both theories—trichromatic theory and opponent process theory—are accurate, each covering essential aspects of color perception.
Together, they provide a comprehensive mechanistic understanding of how color perception functions in humans, involving processing at the cone level and subsequent interpretation by ganglion cells.