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I just found this remarkable optical illusion:

enter image description here

To do: Fixate your gaze on the center of one of the figures and stare at it for some time (20-30 seconds) while it cycles (without moving your eyes). After several iterations you’ll start noticing that the empty outlines fill in with ghostly redish or bluish colors! These illusory colors are called “afterimages”. Interestingly, the colors of the afterimages vary, which is puzzling because they come from the same original figure. Moreover, the shape of the outlines determines the filled-in color, which is complementary to the color of the same shape in the original figure.

My question:

Is this some sort of soft-coded Grapheme-Color synethesia? Do people with Grapheme-Color synethesia have hard-coded afterimages in their brains?

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up vote 3 down vote accepted

Very cool image! As for your question, your link includes a link to the authors' poster, which allows you to "read more about the illusion and possible explanations."

From the poster:

In conclusion, the observations so far suggest that the afterimage effect is not due to higher level effects of shape-specific coloured afterimages but rather to a rapid spreading of local patch-based afterimages within the outlines presented after the image.

From Wikipedia: "Like all forms of synesthesia, grapheme → color synesthesia is involuntary, consistent..." This illusion operates involuntarily as well, but is manipulable by the image designer, as the poster shows. Hence I think it's safe to say this has nothing to do with synesthesia as an abnormal condition, but it could probably be described in perceptual terms that apply to normative function. You might want to look for more of the authors' work or contact them if you're sufficiently curious! Please share with us here whatever info you receive if you do.

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Nick's answer links to the very interesting geometrical discussion by the authors, but they leave out some background. The color after-image phenomena is best described by opponent-process theory. The basic idea is that the neural systems representing color have a competitive nature. So the system that codes for red and green is the same system and cannot mix red and green. This also explains why, for the most common variant of color-blindness, subjects can not differentiate between red and green.

The wiki gives the primary opponent colors:

opposing colors

The after-image opponent process itself is a static phenomena. You can see it by staring at this flag for 1 minute, then staring at a white surface. The authors extend this in a very interesting way.

flag negative

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My problem with explaining the illusion in terms of complementary colours using the opponent process theory is that the two alternating colours, red and blue, in the illusion aren't actually complementary, at least not according to the colour chart in Keegan's answer where red complements green and blue complements yellow.

The theory might well account for afterimages, but is that what we're seeing here? Could it be rather a pre-image? The brain is filling in the shape with the colour it expects on the basis of past experience. The blind spot in the retina provides another example of visual confabulation/interpolation using surrounding detail. Why one should expect blue for the four-pointed star shape in one orientation and red in the other needs further explanation of course.

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