The other day I found my self getting in to a debate with my brother about some one on tv looking like a family friend. I had seen the commercial several times before and never saw the similarities until it was pointed out to me by my brother, now every time I see the commercial I see the family friend. I wondered why this was, so I did a little research and found this article about ambiguous images and how they are stored and processed. Essentially it is simple the easiest way for the brain to identify a person or object is to utilize an archetype from our memory. A good example of this is the black and white Dalmatian illusion.
The first time we look at the first image it takes a good bit of processing to deduct what is the image is showing however after we figure out that the image is of a dog we can code it as such. This means that we remember the image not as a random grouping of dots but under the archetype of dog, and now that it is coded as such we can un-see what we have seen.