Déjà Vu In All Its Forms.

Déjà vu Déjà vu is the experience of feeling you have experienced or seen a new situation before, that is a past episode is repeating itself in the present, and that you remember it.

Déjà Vécu (already lived) Déjà vécu is what most people think of as Déjà vu. They have the think they have seen or experienced an event in great detail before, they may also have a feeling of knowing what is going to come next.

Déjà Visité (already visited) Déjà visité is the knowledge of buildings and places that you should not know as you have never been there before. You may know your way around a building or town despite never having visited before.
Nathaniel Hawthorne in his book Our Old Home wrote about an experience in which he visited a ruined castle and had a full knowledge of its layout. He later found out that his knowledge of the castle to a poem he had read many years early by Alexander Pope in which the castle was accurately described.

Déjà Senti (Already Felt) Déjà senti is the phenomenon of thinking you have already felt something novel such as a smell or touch.

Jamais Vu (Never Seen) Jamais Vu describes when a familiar situation is not recognized. It is often considered to be the opposite of déjà vu. The observer does not recognize the situation despite knowing that they have been there before. It is commonly explained as when a person momentarily doesn’t recognize a person, word, or place that they know. Studies of Jamais vu show that it may be a symptom of brain fatigue.

Presque Vu (Almost Seen) When one cannot recall a familiar word, name or situation. Presque vu is the sensation of being on the brink of a revelation, that is, having the answer"on the tip of the tongue."

Déjà Entendu (Heard Again) This is the feeling that you have already heard what you are now hearing. Déjà Entendu is the aural counterpart to Deja Vu.

Déjà raconté (Already Told) The feeling that you've already told someone somthing. First mentioned by freud with the possible explanation that you meant to tell someone but didn't, and thus constructed a false memory of having done so.

There are also some less well described forms of the déjà family of experiences.

Déjà Pensé Already thought of
Déjà Voulu Already desired, a memory disturbance were you believe that what you desire now is the same as before, despite now being different.
Déjà Eprouvé Already experienced or tested
Déjà Fait Already done
Déjà Reve Already dreamt
Déjà Pressenti Already sensed
Déjà Voulu Already desired
Déjà Su Already known
Déjà Trouve Already found
Déjà Dit Already said
Déjà Goute Already tasted
Déjà Halluciné Already hallucinated
Déjà Lu The feeling that one may have read the present passage or one very like it before.

Deja vu has commonly been thought to have its genesis in forgotten memories, fantasy or dreams. The feeling of Déjà vu wears off a few seconds or minutes, though its effects may linger in the mind for some time. There are two main classes of Déjà vu, associative and biological.

Associative déjà vu

The most common type of déjà vu experienced. It is associative in nature, that is you may hear, smell or feel somthing that reminds you of a similar experience, thus setting off the experience of Déjà vu.

Biological déjà vu

Biological déjà vu, is as the name suggests, caused by some physical, biological change in the brain. This can be from an injury, or more commonly caused by differing forms of epilepsy.
High occurrences of déjà vu among people with temporal lobe epilepsy have been reported. Just before having a seizure they may experience a strong feeling of déjà vu. There has however been some debate within medical research as to wether or not this is true Déjà vu as the person experiencing it may truly believe they've been through the exact situation before, rather than the traditional feelings associated with getting déjà vu.

Theories on the reasons for Déjà vu

1)The Hologram Theory

Memories are like holograms that you can recreate in your minds eye. The entire three-dimensional image can be recreated from any fragment of the whole memory. The smaller the memory fragment, however, the fuzzier the ultimate picture recreated in the minds eye.
Some detail in the environment such as a sight, sound or smell, similar to some memory fragment of our past, allows the brain to recreate an entire scene from that fragment.
For example, you might go for a walk in the country when you see a red tractor which seems somehow farmiliar. You might have subconsiously remembered a walk taken as a child with your parents were you saw a similar style of tractor in a similar setting.

2)Dual Processing (or Delayed Vision)

Dual Processing is based on the way our brain processes new information and how it stores long and short term memories. First proposed by Robert Efron at the Veterans Hospital in Boston in 1963. He proposed that a delayed neurological response causes déjà vu. Because information enters the processing centers of the brain via more than one path, it is possible that occasionally that blending of information might not synchronize correctly.

3)Other Sources Theory

The Other Sources Theory proposes that we have many stored memories that come not only our own experiences but also outside influences. This could be films, photographs and books. We can have very strong memories of things that we've read about or seen without actually physically experiencing them, over time, these memories may be pushed back in our minds.
When we see or experience something that is very similar to one of those memories, we experience a feeling of déjà vu. An example of this could be recognising a person, but not knowing why. You may have seen their picture in a paper years before but long since consciously forgotton seeing the picture.