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After this week's discussion, I found myself wondering about the usefulness of photographic memory.  We discussed whether or not Stephen Wiltshire would be able to navigate the cities that he drew, using the "map" that he created in his head.  We seemed to agree that what he is doing is not exactly photographic memory, but it is certainly a similar concept: being able to look at something and hold it in one's mind.  I wonder if he could reproduce those same cities after extended periods of time.  For example, could he look at Rome, draw Rome, then a few weeks later look at Tokyo, draw Tokyo, and then draw Rome again from memory?  Eidetic images seem fade away after a given amount of time; is this the way that Stephen Wiltshire's brain works too?  If it is, can photographic memory be defined as simply a gift for memorization on a much larger scale than normal?  We have all had the experience of studying for a test, memorizing all of the information necessary, and then being unable to remember that information a few weeks or months later.  Most of the photographic memory tests seem to be testing memorization ability; on the other hand, people with eidetic memory seem to be able to actually see an image after the image is no longer there. 

Like Sasha, I have also been wondering about the evolutionary advantages of eidetic memory or photographic memory.  There are people who can remember every moment of their lives, but these people are miserable.  Most people seem to feel as if we all have a finite amount of brain space.  Once that space is filled, something must be forgotten in order to remember something new.  People who can remember their entire lives would seem to have infinite brain space; does this have something to do with photographic memory?  I can't help but think that if it was evolutionarily advantageous to have a photographic memory and infinite brain space, then it wouldn't be so rare.