If you look at one of the white X-es in the colored circles (just look at the X, nothing else) on left side for 15-20 seconds, and then look at the X on the right side, you probably will see an afterimage, which will be in what Paul Churchland calls chimerical colors. Those are colors which can not appear to you in the normal looking at the things (for explanation of why afterimages happen, you can check this post at Mixing Memory, or check the page with videos on this blog, for a link to a presentation of Paul Churchland). Anyway the short story is that there is neural adaptation, a process where the neuron after given period of being excited with certain input, will “get tired” and go, all by its own, to its “normal” state. If after that the excitatory input goes away, there will be aftereffect such that the neuron will go to inhibited state, even that there is no input which inhibits it. Or the other way around, if inhibitory input is present for a longer time, and then removed, the neuron will go into excited state even there is no excitatory input.
As explanatory model it explains the afterimages (though of course doesn’t explain the phenomenal “look” of the colors, and taken by itself doesn’t explain some other visual illusions). But what is interesting is that this kind of aftereffects can be seen not just in case of color, but also other “more complicated” features. In the following picture there are faces of prototypical female on left side, prototypical male on the right, and a female/male morph image in the center. Try and concentrate on the left picture first (for e.g. 30 seconds), and then look at the center picture. Does it look as male or female? Then concentrate on the right picture for 30 seconds, and then look at the center picture. How about now?
If all is successful, this should show gender adaptation phenomenon, and if we connect this to the theory about how aftereffects in colors happen, should mean that probably there are also neurons in the brain which are used for recognition of gender. (also check Mixing Memory post for more).
BTW, I took the pictures for the second example from the Beauty Check site. It is about how morphing of multiple faces affects attractiveness of the face. Check it out here.