Charles B. Brenner & Jeffrey M. Zacks in Scientific American, December 13, 2011

So there's the thing we know best:  The common and annoying experience of arriving somewhere only to realize you've forgotten what you went there to do.  We all know why such forgetting happens: we didn't pay enough attention, or too much time passed, or it just wasn't important enough.  But a "completely different" idea comes from a team of researchers at the University of Notre Dame.  The first part of their paper's title sums it up:  "Walking through doorways causes forgetting."

…

The doorway effect suggests that there's more to the remembering than just what you paid attention to, when it happened, and how hard you tried.  Instead, some forms of memory seem to be optimized to keep information ready-to-hand until its shelf life expires, and then purge that information in favor of new stuff.  Radvansky and colleagues call this sort of memory representation an "event model," and propose that walking through a doorway is a good time to purge your event models because whatever happened in the old room is likely to become less relevant now that you have changed venues.  That thing in the box?  Oh, that's from what I was doing before I got here; we can forget all about that.