Nearly two thirds of us have experienced déjà vu at least once in our lifetime. And the encounter is so uncanny, you won’t forget it.

You visit your friend’s house for the first time. As you walk into the dining room, everything seems strangely familiar. The peculiar arrangement of the tables and chairs, the scent of the newly baked apple pie, even the sight of the hanging ornament on the wall seems to derive a strange sense of familiarity, as if you have seen it in the past. Perceiving this feels strange because it seems choreographed. But in reality, you know that everything is happening for the first time.

Déjà vu is a French term, which still lacks an English counterpart. It literally means, “already seen”. It’s colloquially defined as a subjective feeling that you have already experienced something that is actually happening for the first time. It is a strange experience of a situation, which vaguely feels more familiar than it should be. Familiarity is, indeed, the cornerstone of this phenomenon.

A brief history of déjà vu

Historically, the earliest account of déjà vu-like experience is referred to Saint Augustine in 400 AD, who named it as “falsae memoriae”. The term déjà vu was first used by Emile Boirac in 1876 in his Revue Philosophique, wherein he mentioned “le sensation du déjà vu” to recall this experience in his letter to the editor.

However, the term, déjà vu wasn’t recognized in the scientific community until 20 years later, in 1896, when F.L. Arnaud, a French neurologist proposed to use it. He presented a case in a scientific meeting of the Societe Medico-Psychologique. He described the case of Louis, a 34 year old who suffered from amnesia after becoming infected with cerebral malaria. After recovering from his illness, Louis was reported to have a vague sense of familiarity with everything that he encountered, including the events that are clearly unique and didn’t happen in the past (such as Pasteur’s state funeral).

The numerous theories to explain this phenomenon ranged from a clairvoyant “message from God” to scientific “delayed neural transmission”. The question remains, is this experience a normal occurrence or is it pathologic? In reality, to this date, nothing is scientifically demonstrated to explain its occurrence. Since it is described as a feeling or a sensation that occurs briefly, scientists find it difficult to study it. It has been linked to memory, dementia and even epilepsy. The recent advances in brain imaging and cognitive psychology have narrowed down the theories that attempt to explain this phenomenon.

Dual processing theory

Dual processing takes place when two pathways for cognition which are supposed to occur simultaneously go out of synchrony. Two common cognitive pathways, which should be well coordinated, are familiarity and retrieval. This basically means that something has to be familiar in order for you to remember it.

For example, while you are visiting your friend’s house, his dog suddenly barks as you walk into the dining hall. As you hear the dog bark, your brain also processes other information that you perceive, like the scent of the apple pie, the arrangement of the tables and chairs and the ornament hanging on the wall. All this information is processed by the brain into a single moment of occurrence. However, in the dual processing theory, there is a slight delay in processing one of the pieces of information that is being fed to your brain. The dog bark becomes discordant with the rest of what you have perceived. Due to this delay, the information is processed as if the events are not happening at the same time. The end result is a false sense of familiarity to one of the events, as if it has already happened.

Hologram theory

Memory is hypothesized to be stored in our brain in the form of holograms, wherein one fragment is only needed in order to remember the entire picture. For example, your brain has identified the ornament in the wall of your friend’s house as a thing of the past. Maybe you recall it from your cousin’s house. However, your brain has already processed the entire memory of the ornament, without identifying it, leaving you with a vague sense of familiarity without remembering it really. Despite the fact that you have never been in this house, you have seen that ornament, but you fail to clearly identify where you know it from. This theory is a confusion of past memories rather than a mistake of the present.

Divided attention theory

This theory is based on the assumption that simultaneous perceptions of a single moment could elicit the déjà vu phenomenon. This reportedly occurs when our brain subliminally takes in a certain aspect of the environment, a particular observation that distracts us, taking all the attention away from the other aspects of the moment we are engaged in. When our full attention returns, we get that sensation of vague familiarity that is observed in the déjà vu phenomenon.

As in our previous example, your attention shifts to the apple pie, but you failed to take note of the barking dog or the hanging ornament on the wall. Although your brain readily perceives what is seen in your peripheral vision, it has been doing so below the normal conscious awareness levels. When you finally shift your attention away from the apple pie, you feel as if you have been there before because you just weren’t paying full attention.

Delayed neural transmission

This theory basically states that there is a glitch in the transmission of information from one nerve to another. There are two pathways described in this theory. In the single pathway, there is a slight delay in the arrival of a single piece of information to the brain – this results in a false sense of familiarity when the transmission is actually delivered. In the dual pathway, one of the pathways is slowed down resulting in a vague feeling of prediction. A rapid change in focus between these two pathways could give a sense of the déjà vu experience. This theory is possible to test scientifically by measuring neural transmission speeds, but because déjà vu is a subjectively brief experience, finding suitable test subjects have been very difficult.

Despite all these theories, the modern day concept of déjà vu remains elusive. Although the advancements in modern medicine has helped narrow down the theories which attempt to explain it, further clinical research is still needed to explain the cause of this widely prevalent and very curious phenomenon.

by Viatcheslav Wlassoff For Brain Blogger