Déjà vu is an uncanny feeling or illusion of having already seen or 
   experienced something that is being experienced for the first time. 
   If we assume that the experience is actually of a remembered event, 
   then déjà vu probably occurs because an original experience was not 
   fully attended to and elaborately encoded.  If so, then it would seem 
   most likely that the present situation triggers the recollection of a 
   fragment from one's past.  The experience may seem uncanny if the 
   memory is so fragmented that no strong connections can be made between 
   the fragment and other memories.  Thus, the feeling that one has been 
   there before is often because one has actually been there before.  One 
   has simply forgotten most of the original experience because one was 
   not paying close attention the first time.  

   The original experience may even have occurred only seconds or minutes 
   earlier.  On the other hand, the déjà vu experience may be due to 
   having seen pictures or heard vivid stories many years earlier, as in 
   the case of Virginia Tighe aka Bridey Murphy.  Those experiences may 
   be part of the dim recollections of childhood, mistakenly believed to 
   have occurred in past lifetimes because one "just knows" they did not 
   occur in this lifetime.  However, it is possible that the déjà vu 
   feeling is triggered by a neurochemical action in the brain that is not 
   connected to any actual experience in the past.  One feels strange and 
   identifies the feeling with a memory, even though the experience is 
   completely new.  That is, déjà vu (French for already seen) may not
   involve the faulty recognition of something one has seen before.  

   The term was first applied by Emile Boirac (1851-1917) who had strong 
   interests in psychic phenomena.  Boirac's term directs our attention to 
   the past.  However, a little reflection reveals that what is unique 
   about déjà vu is not something from the past but something in the 
   present, namely, the strange feeling one has in experiencing déjà vu.
   We often have experiences  whose novelty is unclear and have been led 
   to ask such questions as, Have I read this book before?  Is this an 
   episode of Inspector Morse that I've seen before?  This place looks 
   familiar; have I been here before?  Yet, these experiences are not 
   accompanied by an uncanny feeling.  We may feel a bit confused, but 
   the feeling associated with the déjà vu experience is not one of 
   confusion but of strangeness.  

   There is nothing strange about not remembering whether you've read a 
   book before, especially if you are fifty years old and have read 
   thousands of books over your lifetime.  In the déjà vu experience,
   however,  we feel strange because we don't think we should feel 
   familiar with the present perception.  That sense of inappropriateness 
   is not present when one is simply unclear whether one has read a book 
   or seen a film before.  Thus, it is possible that the attempt to 
   explain the déjà vu experience in terms of lost memory, past lives,
   clairvoyance, etc., may be completely misguided.  
   We should be talking about the déjà vu feeling.  That feeling may be
   caused by a brain state, by neurochemical factors during perception, 
   which have nothing to do with memory.  It is worth noting that the 
   déjà vu feeling is common among psychiatric patients.  The déjà vu
   feeling also frequently precedes temporal lobe  epilepsy attacks.  
   And, when Wilder Penfield did his famous experiment in 1955 in which 
   he electrically stimulated the temporal lobes, he found that about 
   8 per cent of his subjects experienced "memories."  He did not provide 
   support for the claim that what was elicited were actually memories.  
   They could well have been hallucinations and the first examples of 
   artificially stimulated déjà vu.

   Glossary                   Episode 445, scene 2