What to see?
You can notice very easily the double image; even if the image on your retina remains constant you can’t perceive both figures simultaneously. So you have two choices you either can see a vase either two faces. This illusion is well known and it can be used in all kind of exercises to explain the way perception works. Both images can be seen at the same time through a three-dimensional image.
What to do?
This type of illusion is an ambiguous figure/ground illusion and it’s called the Rubin vase/profile illusion. It carries this name because the way we see the image: either as two black faces looking at each other in front of a white background, either as a white vase on a black background.
So depending on what you want to see you have to focus your attention on the black part or the white part of the image and the contour of the figure will reveal the object of your attention.
How it works?
This type of illusion is used by artists to create 2D pieces because they’re using the cognitive ability to separate elements and they base their hypothesis upon contrasts of lights. The shape of the image depends on the side of the line we chose as part of the figure. This is important because the human brain has in general just one object in the field of perception and the other objects go into the background of perception. The reversal you perceive may happen because you shift your attention from one contour to another or from one color to another.
This process happens with a lot of cortical involvement. The human brain tends to store information about everything around us, that means even about contours and vases… so the brain needs to be able to give a certain signification to the external object that you perceive through your eyes. Because the way the visual system works the brain has to distinguish the objects from their background.
This kind of illusion can explain easily the way the perception works. This process shows you how the brain organizes its visual environment through the spontaneous reversal.