We perceive the world through our senses, but those perceptions are constructed by the brain. Most of the time, our perceptions consistently reflect what is perceived: an apple looks red whenever and wherever we see it. But sometimes, the brain makes false assumptions when constructing a perception, and what we perceive is an illusion. Following are a few such illusions, showing that perceptions are constructed by the brain.
In the Asahi illusion, the center appears much brighter than the surrounding white color, even though it isn't. This illusion is so strong that it causes the pupils in your eyes to contract.
In the black hole illusion, most people see the central black region expanding, as if one is falling into a hole. This illusion is strong enough to make the pupils in your eyes dilate, as if you are trying to see into the "hole".
Believe it or not, all the balls are the same color!
The two pears are the same color!
There is no light blue disk.
The lines are all straight, and they are all parallel or perpendicular to each other.
The rotation is an illusion: only the colors (black, white, gray) change. Click image for slow motion; double click to stop.
And here's an audio illusion created by visual stimuli.