An illusion.
This
is the most common version of an illusion I refered to in a lecture in
2008/9; if you stare at the four dots in the picture
for thirty seconds and then stare at a
white wall or white sheet of paper, you will see a vague negative
image of the picture. This is because image data, in common with other
sensory information, is encoded in the difference in the firing rates
of different neurons, because the cells that were involved in encode
the white areas of the picture have been active, there spike rate has
fallen and so when you are looking at the blank paper those neurons
are relatively less active, giving the illusion of a dark image. This
illusion works with other, similar, images: the obvious example is the
Jim Fitzpatrick screen print of Che Guevara
, the illusion here is less striking than in the previous case, but it is interesting since you can see the spike rate adaptation of the red
photoreceptors leaves a (white-red=)cyan after image.