DÉJÀ VU - The sight, sound, taste or even smell of something makes us think that we've experienced it before, although we know that we couldn't have. 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. Precognitive experiences -- if they are real -- show things that will happen in the future, not things that you've already experienced. An important distinction is that déjà vu is experienced during an event, not before.

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Déjà Vu

How Déjà Vu Works

BY LEE ANN OBRINGER

Have you ever visited ­a store for the first time and had it feel eerily familiar?

Or maybe you're deep in conversation with a friend and you suddenly get the feeling that you've had the exact conversation before, even though you know that you haven't.

If you've ever found yourself in either of these situations, you've experienced déjà vu.

Sixty to 70 percent of us admit to getting this feeling at least once in our lives.

The sight, sound, taste or even smell of something makes us think that we've experienced it before, although we know that we couldn't have.

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.

Déjà vu is a French term that literally means "already seen" and has several variations, including déjà vécu, already experienced; déjà senti, already thought; and déjà visité, 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 Déjà Vu and Precognitive Dreams 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.

Types of Déjà Vu

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.

Studying Déjà Vu

Déjà vu is extremely difficult to study because it occurs briefly, unannounced, only in certain people, and has no witnesses or physical manifestations other than the person saying, "hey, déjà vu!"

Because of this, there is little firm research and no definitive explanations.

Déjà vu studies must depend on personal descriptions and recollection for data.

For two centuries people have tried to come up with reasons we experience déjà vu.

From philosophers, to psychologists, to paranormal experts, they've all had their theories.

Emile Boirac was a French psychic researcher who was the first to use the term déjà vu in his book, "L'Avenir des Sciences Psychiques."

He did not research the phenomenon in depth, however. 

Sigmund Freud theorized that these experiences resulted from repressed desires or memories related to a stressful event that people could no longer access as regular memory.

Scientists used this theory, called paramnesia, to explain déjà vu for a large part of the 20th century.

Over the years, many scientists ignored déjà vu completely due to its frequent association with past life experiences, ESP and alien abductions.

These associations gave the study of déjà vu a bit of a stigma.

Recently, researchers have set aside some of those associations and have begun putting brain imaging technology to work.

Firmly placing déjà vu within the study of memory, they hope to discover more about how memories are formed, stored and retrieved.

They have since determined that the medial temporal lobe is involved in our conscious memory.

Within the medial temporal lobe are the parahippocampal gyrus, the rhinal cortex and the amygdala.

John D.E. Gabrieli at Stanford University found in 1997 that the hippocampus enables us to consciously recall events.

He also found that the parahippocampal gyrus enables us to determine what's familiar and what isn't (and without actually retrieving a specific memory to do it).

While about 60 percent of people say they have experienced déjà vu, the rates are highest among people between the ages of 15 and 25.

The upper age varies among researchers, but most agree that déjà vu experiences decrease with age.

There have also been higher reported occurrences among those with higher incomes, those who tend to travel more and those with higher education levels.

Active imaginations and the ability to recall dreams has also been a commonality among people who report déjà vu experiences.

Some researchers also report that the more tired or stressed you are, the more likely you are to experience déjà vu.

Other researchers, however, have seen just the opposite.

They report that the more refreshed and relaxed you are, the more likely you are to experience déjà vu.

Obviously, the jury is still out about a lot of things related to déjà vu.

One reported finding is that the more open-minded or politically liberal a person is, the more likely they are to experience déjà vu.

However, this may also mean that the more open-minded you are, the more likely you are to talk about something potentially seen as "weird," like déjà vu.

Divided Attention (the Cell Phone Theory)

Dr. Alan Brown has attempted to recreate a process that he thinks is similar to déjà vu.

In studies at Duke University and SMU, he and colleague Elizabeth Marsh put the idea of subliminal suggestion to the test.

They showed photographs of various locations to a group of students, with the plan to ask them which locations were familiar.

Prior to showing them some of the photographs, however, they flashed the photos onto the screen at subliminal speeds -- around 10 to 20 milliseconds -- which is long enough for the brain to register the photo but not long enough for the student to be consciously aware of it.

In these experiments, the images that had been shown subliminally were familiar at a much higher rate than those that were not -- even though those students who had actually been to those locations had been pulled from the study.

Larry Jacoby and Kevin Whitehouse of Washington University did similar studies using lists of words with similar results using lists of words.

Based on this idea, Alan Brown proposed what he calls the cell phone theory (or divided attention).

This means that when we are distracted with something else, we subliminally take in what's around us but may not truly register it consciously.

Then, when we are able to focus on what we are doing, those surroundings appear to already be familiar to us even when they shouldn't be.

With this in mind, it is reasonable to see how we might walk into a house for the first time, perhaps while talking to our host, and experience déjà vu.

It would work like this: before we've actually looked at the room, our brains have processed it visually and/or by smell or sound, so that when we actually look at it we get a feeling that we've been there before.

More Déjà Vu Theories

The Hologram Theory

Dutch psychiatrist Hermon Sno proposed the idea that memories are like holograms, meaning that you can recreate the entire three-dimensional image from any fragment of the whole.

The smaller the fragment, however, the fuzzier the ultimate picture. Déjà vu, he says, happens when some detail in the environment we are currently in (a sight, sound, smell, et cetera) is similar to some remnant of a memory of our past and our brain recreates an entire scene from that fragment.

Other researchers also agree that some small piece of familiarity may be the seed that creates the déjà vu feeling.

For example, you might go for a ride with a friend in an old 1964 Plymouth and have a strong déjà vu experience without actually remembering (or even being aware of the fact) that your grandfather had the same type of car and you're actually remembering riding in that car as a small child.

Things like the smell and the look and feel of the seat or dashboard can bring back memories you didn't even know you had.

Dual Processing (or Delayed Vision)

Another theory is based on the way our brain processes new information and how it stores long- and short-term memories.

Robert Efron tested an idea at the Veterans Hospital in Boston in 1963 that stands as a valid theory today.

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.

Efron found that the temporal lobe of the brain's left hemisphere is responsible for sorting incoming information.

He also found that the temporal lobe receives this incoming information twice with a slight (milliseconds-long) delay between transmissions -- once directly and once again after its detour through the right hemisphere of the brain.

If that second transmission is delayed slightly longer, then the brain might put the wrong timestamp on that bit of information and register it as a previous memory because it had already been processed.

That could explain the sudden sense of familiarity.

"Memories" From Other Sources

This theory proposes that we have many stored memories that come from many aspects of our lives, including not only our own experiences but also movies, pictures we've seen and books we've read.

We can have very strong memories of things we've read about or seen without actually experiencing, and over time, these memories may be pushed back in our minds.

When in we see or experience something that is very similar to one of those memories, we might experience a feeling of déjà vu.

For example, as a child we may have seen a movie that had a scene in a famous restaurant or at a famous landmark.

Then, as an adult, we visit the same location without remembering the movie, and the location appears to be very familiar to us.

THE FULL DEJA VU EXPERIENCE

Below are names for some of the many ways in which the déjà experience may manifest:

·            déjà entendu - already heard

·            déjà éprouvé - already experienced

·            déjà fait - already done

·            déjà pensé - already thought

·            déjà raconté - already recounted

·            déjà senti - already felt, smelt

·            déjà su - already known (intellectually)

·            déjà trouvé - already found (met)

·            déjà vécu - already lived

·            déjà voulu - already desired

Neppe (in conjunction with Prof. B. G. Rogers, Professor of French, University of the Witwatersrand) in 1981 suggested the following additional terms:

·            déjà arrivé - already happened

·            déjà connu - already known (personal knowing)

·            déjà dit - already said/spoken (content of speech)

·            déjà gouté - already tasted

·            déjà lu - already read

·            déjà parlé - already spoken (act of speech)

·            déjà pressenti - already sensed

·            déjà rencontré - already met

·            déjà rêvé - already dreamt

·            déjà visité - already visited

Déjà rencontré appears preferable to déjà trouvé for the already met experience because it specifically relates to interpersonal situations [Source: The Various Manifestations of Déjà Vu Experience].

Déjà Vu and Precognitive Dreams

Some researchers, including Swiss scientist Arthur Funkhouser, firmly believe that precognitive dreams are the source of many déjà vu experiences.

J.W. Dunne, an aeronautical engineer who designed planes in World War II, conducted studies in 1939 using students of Oxford University.

His studies found 12.7 percent of his subjects' dreams to have similarities with future events.

Recent studies, including one by Nancy Sondow in 1988, have had similar results of 10 percent.

These researchers also tied evidence of precognitive dreams to déjà vu experiences that occurred anywhere from one day to eight years later.

The question has been raised about why the experiences themselves are typically mundane everyday things.

One explanation from Funkhouser is that something more exciting is more likely to be remembered, making a déjà vu experience less likely.

Although déjà vu has been studied as a phenomenon for over a hundred years and researchers have advanced tens of theories about its cause, there is no simple explanation for what it means or why it happens.

Perhaps as technology advances and we learn more about how the brain works, we will also learn more about why we experience this strange phenomenon.

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 vuin 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.

Lee Ann Obringer, Contributing Writer 
Lee Ann Obringer holds a bachelor's degree in journalism and advertising with an outside concentration in marketing from the University of North Carolina at Chapel Hill. In­ addition to writing for HowStuffWorks, she works as a freelance marketing communications consultant and designer.

https://science.howstuffworks.com/science-vs-myth/deja-vu.htm