“Perception depends heavily on conceptual schemata. "There is no innocent eye." The raw material of vision cannot be extracted from the finished product. Our schemata (schema: a pattern imposed on complex reality or experience to mediate perception) may change and evolve, be revised or replaced, be suggested or informed, by factors of all kinds, but without some schema there is no perception.”
In Languages of Art, Nelson Goodman, 1972.
What do you see in this red dot?
And in this green dot?
For some people it is immediate. And once you have seen it you will see it again immediately.
Your brain affects how you see the world and can be trained to see.
You have to learn how to decode pictures like photographs, paintings and drawings.
Of course there are tons of these but I had never encountered this one.
I cheated in a sense by putting the concepts of "what do you see", "dot", "red" and "green" a priori in your brain.
Update: New cool illusion involving dots.
Stare at the white dot on the image on the left for 15 seconds then look at the blank square on the right.
When staring at the image on the left the receptors on your retina are stimulated and send a signal to your brain. After looking away they deactivate themselves with the opposite stimulus. This takes a while and the brain now gets the opposite signal. To me the image tends to float around in circles before disappearing.
Stare at the red dot and the blue circle will disappear.
More dots here. Can you see the 12 dots all at once?
Now for a test involving seemingly random dots. What number do you see?