It identifies patterns.

It groups shapes together.

It separates objects from backgrounds.

It predicts what certain arrangements of lines and shadows are likely to represent.

This usually helps us navigate the world efficiently.

But optical illusions take advantage of the same system.

The artist has arranged the image so that your brain initially settles on one interpretation:

landscape.

The puzzle becomes difficult because you need to convince your brain to consider another interpretation:

human face.

The Role of Figure-Ground Perception

One important concept behind illusions like this is known as figure-ground perception.

It refers to the way our visual system distinguishes an object—the “figure”—from everything surrounding it—the “ground.”

Normally, this process works automatically.

You see a person standing in front of a wall.

Your brain separates the person from the wall.

You see a tree against the sky.

Your brain identifies the tree as the object and the sky as the background.