This illusion is similar to the Two Grey Steps illusion and the Horsetail Illusion.
All these other illusions work because of the way your eyes see colours and edges.
The unreal shapes in this illusion are called “subjective contours”. They were first described by Schumann in 1904. The contours, or outlines, are not real but are subjective — part of your interpretation of what you see. They depend on your perception and your earlier experiences.
These subjective contour illusions have helped scientists to work out what parts of a picture are important for vision. Because you see edges and patches of brightness where there are none, we know that the brain must be adding information to what we see to help us understand it.
In our day to day experiences, it is common to see near objects blocking our view of more distant ones. This is why many people “see” shapes which seem to be blocking their view of the circles. It is more common to see a full circle with something in front of it than a circle with a bit cut out of it.
Another way of interpreting the pattern is as a set of familiar shapes with bits missing. This interpretation is just as valid, but most people do not see things this way. By moving the circles around, you will be more likely to see the shapes as circles with bits cut out.
One explanation of why the “triangles” and other shapes look brighter than their surroundings relates to our perception of near and far objects. Usually, a near object looks brighter than a more distant one of the same colour. Your brain interprets the illusory triangles as being closer than the circles. This means you see them as being brighter than the things that you think are further away — the background.
The brain often uses the brightness of things to work out which ones are closer. When there are few objects around to compare things to, it can be difficult to tell how close each is. In the picture on the left, it is not entirely clear which object is closer because we don't know how big each of the two objects is.