Depending on how we are looking at an image, we might see things differently. For example, in one well-known type of optical illusion, we are asked if we see a vase or two faces. In this type of illusion the brain relates the images to its assumptions or knowledge. Typically the brain sees the background as the ground and the object in the center as the "object, but the cases like the "vase" versus the "two faces can be ambiguous. The illusions that depend on brain processing are called cognitive illusions.