Perception. It is everything.
Most of these photos when viewed one by one have pleasing and normal results. However, when viewed side by side and focusing in the middle, strange results occur. It is more like something a caricature artist would portray.
By studying illusions such as these, along with research on memory, vision, and brain science, researchers are starting to understand the boundaries of our capacity to interpret the seemingly endless amounts of input that we try to absorb almost all day long. Sometimes we observe things that are not there and do not see things that are.
In the words of Philip K. Dick, “Reality is that which, when you stop believing in it, doesn’t go away.”