In my free time I was surfing the web and I came across this example of the figure ground principle in an African Children Initiative advertising campaign. I'm not very good at geography and it took me awhile to realize that there was anything in the picture besides the child staring up at the adult. This made me realize that certain uses of figure ground are dependent on the audience of the advertisement having a background knowledge of certain things to be able to see that there is a second way of looking at the picture if they shift their focus. If I'd shown this ad to someone who wasn't familiar with geography, or a third grader who had not yet studied a map of the world in school, their mind would likely not allow them to shift their focus at all and the effect of the technique would be lost. This is true for some ads where one of the two images depends on prior knowledge as opposed to ads like this:

which anyone, regardless of background or education, could perceive the double image in if they shift their focus. The figure ground principal therefore is limited in its power to draw attention and create involvement when it relies on the audience to have prior knowledge of one of the images because if your brain doesn't recognize one of the two images at all from memory or association, it's unlikely that you will be able to perceive a) that a second image is there, and b) what it is.