Key ideas

• Trichromatic theory says three types of cones (red, green, blue) combine their activation to let us see all the colors, such as red plus green producing yellow.
• Opponent-process theory adds a second stage: ganglion cells process color in opposing pairs, red-green, blue-yellow, and black-white.
• Opponent cells signal one color by firing above baseline and its opponent by firing below baseline, so we never see red and green together.
• Afterimages occur because cells fatigued by staring at one color drop below baseline afterward, signaling the opponent color.
• The two theories describe different stages of color vision, and problems in either cones or opponent ganglion cells can cause color vision deficiencies. Dichromatism and monochromatism are two such color deficiencies.
• Visual signals leave the eye through the optic nerve and travel to the thalamus, then to the occipital lobes.
• Damage to brain areas that handle vision can cause prosopagnosia (face blindness) or blindsight.

Key terms

trichromatic theory

The theory that three types of cones (red, green, blue) are activated in different combinations to produce all the colors we see.

afterimage

A brief image in the opponent color seen after staring at a color, such as green after red.

opponent-process theory

The theory that, after the cones, ganglion cells process color in opposing pairs: red-green, blue-yellow, and black-white.

ganglion cells

Retinal cells that receive cone signals and process color in opposing pairs by raising or lowering their firing rate.

color vision deficiency

Trouble seeing colors caused by irregularities in cones or in the ganglion cells involved in opponent processing.

dichromatism

A color vision deficiency involving difficulty distinguishing either reds and greens or blues and yellows.

monochromatism

A color vision deficiency with little or no ability to distinguish different colors.

prosopagnosia

Face blindness: a person can see a face and its features but has difficulty recognizing whose face it is.

blindsight

After damage to a visual brain area, a person says they cannot see an object yet their behavior shows they process it.

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