red green and blue yellow, two colors we almost can never see
http://www.livescience.com/17948-red-green-blue-yellow-stunning-colors.html
"Cells in the retina called "opponent neurons" fire when stimulated by
incoming red light, and this flurry of activity tells the brain we're
looking at something red. Those same opponent neurons are inhibited by green
light, and the absence of activity tells the brain we're seeing green.
Similarly, yellow light excites another set of opponent neurons, but blue
light damps them. While most colors induce a mixture of effects in both sets
of neurons, which our brains can decode to identify the component parts, red
light exactly cancels the effect of green light (and yellow exactly cancels
blue), so we can never perceive those colors coming from the same place.
Almost never, that is. Scientists are finding out that these colors can be
seen — you just need to know how to look for them."
.."The observers of this unusual visual stimulus reported seeing the borders
between the stripes gradually disappear, and the colors seem to flood into
each other. Amazingly, the image seemed to override their eyes' opponency
mechanism, and they said they perceived colors they'd never seen before."
"Crane's and Piantanida's paper raised eyebrows in the visual science world,
but few people addressed its findings. "It was treated like the crazy old
aunt in the attic of vision, the one no one talks about," said Vince
Billock, a vision scientist. Gradually though, variations of the experiment
conducted by Billock and others confirmed the initial findings, suggesting
that, if you look for them in just the right way, forbidden colors can be
seen."
"When the experiment is done correctly, he said, the perceived color was not
muddy at all, but surprisingly vivid: "It was like seeing purple for the
first time and calling it bluish red.""
