Impossible colours refers to those colours that do not appear in the ordinary visual functioning, it is believed that these colours cannot arise in normal vision.

Impossible colours refers to those colours that do not appear in the ordinary visual functioning, it is believed that these colours cannot arise in normal vision.

For example if one try to imagine reddish green (a colour that is somewhat like red and somewhat like green), or yellowish blue — not green, but a hue similar to both yellow and blue, the mind will draw a blank. The reason is that even though those colours exist, you’ve probably never seen them. Red-green and yellow-blue are the so-called “forbidden colours.” Composed of pairs of hues whose light frequencies automatically cancel each other out in the human eye, they’re supposed to be impossible to see simultaneously.

Impossible colours are of two types:

1. Forbidden colours: That would be seen if the signals processed at the brain’s visual cortex, from the retina’s three types of cone cells, could be set to combinations which the eye does not produce, whatever colour or colours of light it is exposed to.

2. Chimerical colours: That are not seen as a direct response to any output from a single retinal location but can be generated, in the visual cortex, by mixing successive colour signals from one location, or simultaneous signals from two locations or from the two eyes. Examples are yellowish-blue (seen as similar both to yellow and to blue) and reddish-green (similar both to red and to green).

In-spite of this a paper was published in 1983 by Hewitt Crane, a leading visual scientist, and his colleague Thomas Piantanida that appeared in the journal Science. Titled “On Seeing Reddish Green and Yellowish Blue,” it argued that forbidden colours can be perceived. The researchers had created images in which red and green stripes (and, in separate images, blue and yellow stripes) ran adjacent to each other. They showed the images to dozens of volunteers, using an eye tracker to hold the images fixed relative to the viewers’ eyes. This ensured that light from each colour stripe always entered the same retinal cells; for example, some cells always received yellow light, while other cells simultaneously received only blue light.

The observers of this unusual visual stimulus reported seeing the borders between the stripes gradually disappear, and the colours seem to flood into each other. Amazingly, the image seemed to override their eyes’ opponency mechanism, and they said they perceived colours they’d never seen before. Wherever in the image of red and green stripes the observers looked, the colour they saw was “simultaneously red and green,” Crane and Piantanida wrote in their paper. Furthermore, “some observers indicated that although they were aware that what they were viewing was a colour (that is, the field was not achromatic), they were unable to name or describe the colour. One of these observers was an artist with a large colour vocabulary.”

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