Notice how the image fluctuates between the two possibilities even though the image on your retina remains constant. It is difficult to perceive both meaningful images simultaneously. 

So What's Going On?

     The Rubin vase/profile illusion is an ambiguous figure/ground illusion. This is because it can be perceived either as two black faces looking at each other, in front of a white background, or as a white vase on a black background. 

     In the case of a figure/ground reversal one line can have two shapes. The shape of the contour formed depends on which side of the line is regarded as part of the figure. This is important, because the visual system represents or encodes objects primarily in terms of their contours.  Also, elements that are close to one another or alike or homogeneous in certain respects tend to be grouped together. This is called grouping. The sudden reversal that you perceive may be due to your shift of attention on the shape of the contour. The observer's "perceptual set" and individual interests can also bias the situation. Biasing the shapes or contours can make one interpretation stronger than the other one. As one can see in the three-dimensional model of the vase, which biases the vase.

     There is no doubt that this particular illusion occurs involves higher cortical processing. This is because you have stored information in your brain that contains knowledge about vases and profiles. 

     Your brain needs to be able to interpret the patterns in your eye in terms of external objects. To do this your visual system needs to be able to distinguish objects (figure) from their background (ground). Most of the time this is relatively easy, but sometimes, as in the case of camouflage, it can be made much more difficult.

     The vase/profile illusion is important because it shows that perception is not solely determined by an image formed on the retina. The spontaneous reversal that you observe illustrates the dynamic nature of subtle perceptual processes. These processes underscore how your brain organizes its visual environment.

Origins of the Rubin Face/Vase Illusion

     The vase/profile illusion was made famous by Danish psychologist Edgar Rubin in 1915. Its pedigree, however, is much older. Examples can be found in 18th century French prints, in which the portraits not only define a vase, usually in a naturalistic setting, but the profiles themselves differ, each representing a particular person.
 
 
 
 

Stanford psychologist Roger Shepard made the drawing seen above. In this case, you can see the profiles of two women, or you can see an obscured face in back of the candlestick. Here, normality depends on seeing one face rather than two.
 
 
 
 
 
 

Dutch graphic artist M. C. Escher's ability to design ambiguous figure/ground prints made him famous. More examples of his work can be seen in the Illusion Art gallery contained on the CD.