There are more than 40 theories as to what déjà vu is and what causes it, and they range from reincarnation to glitches in our memory processes. In this article, we'll explore a few of those theories to shed some light on this little understood phenomenon.

What Déjà Vu Is...And What It Isn't

Chronic Déjà Vu Recently, there have been studies of people who have what researchers are terming "chronic déjà vu." Four senior citizens in the United Kingdom have experienced déjà vu in a constant state. They refused to watch the news because they felt like they already knew what was going to be said (even though they really didn't). Or, they wouldn't go to the doctor because they felt like they had already been and didn't see the point. Researchers have suggested that these individuals have experienced a failure in the temporal lobe. The circuits that are activated when you remember something have gotten stuck in the "on" position, so to speak. This has essentially created memories that don't actually exist [ref].

Déjà vu is a French term that literally means "already seen" and has several variations, including déjà vecu, already experienced; déjà senti, already thought; and déjà visite, already visited. French scientist Emile Boirac, one of the first to study this strange phenomenon, gave the subject its name in 1876.

There are often references to déjà vu that aren't true déjà vu. Researchers have their own definitions, but generally déjà vu is described as the feeling that you've seen or experienced something before when you know you haven't. The most common misuse of the term déjà vu seems to be with precognitive experiences -- experiences where someone gets a feeling that they know exactly what's going to happen next, and it does. An important distinction is that déjà vu is experienced during an event, not before. Precognitive experiences -- if they are real -- show things that will happen in the future, not things that you've already experienced. (However, one theory about déjà vu deals with precognitive dreams that give us a déjà vu feeling afterwards. See the Leading Theories section.)

Hallucinations that are brought on by illness or drugs sometimes bring a heightened awareness and are confused with déjà vu. False memories that are brought on by schizophrenia can be confused with déjà vu as well. Unlike true déjà vu, which typically lasts from 10 to 30 seconds, these false memories or hallucinations can last much longer.

Defining types of déjà vu is a very slippery area. Those who have studied it have applied their own categories and differentiations -- each usually tied to a specific theory about what causes déjà vu. Alan Brown, a professor of psychology at South Methodist University and author of "The Déjà vu Experience: Essays in Cognitive Psychology," has three categories for déjà vu. He believes there is déjà vu caused by biological dysfunction (e.g., epilepsy), implicit familiarity and divided perception. In 1983, Dr. Vernon Neppe, Director of the Pacific Neuropsychiatric Institute in Seattle, proposed four subcategories of déjà vu, including epileptic, subjective paranormal, schizophrenic and associative.

Taking a very broad look at the research and resources available, we can put déjà vu experiences into two categories and then see the more subtle distinctions that researchers have placed on it:

• Associative déjà vu
  The most common type of déjà vu experienced by normal, healthy people is associative in nature. You see, hear, smell or otherwise experience something that stirs a feeling that you associate with something you've seen, heard, smelled or experienced before. Many researchers think that this type of déjà vu is a memory-based experience and assume that the memory centers of the brain are responsible for it.

• Biological déjà vu
  There are also high occurrences of déjà vu among people with temporal lobe epilepsy. Just before having a seizure they often experience a strong feeling of déjà vu. This has given researchers a slightly more reliable way of studying déjà vu, and they've been able to identify the areas of the brain where these types of déjà vu signals originate. However, some researchers say that this type of déjà vu is distinctly different from typical déjà vu. The person experiencing it may truly believe they've been through the exact situation before, rather than getting a feeling that quickly passes.

Déjà vu also occurs with some predictability in major psychiatric disorders, including anxiety, depression, dissociative disorders and schizophrenia.

Next, we'll look at how researchers have studied this phenomenon.