Have you ever had the feeling that you have experienced a situation in the present even if it did not happen?

This bizarre effect is called déjà vu and the term comes from French, its translation meaning already seen. People who travel frequently or watch many movies are more likely to experience déjà vu than other people.

Although the sensations of déjà vu appear throughout life, doctors have been able to identify the age ranges in which they occur most often. The critical ages are those between 15-19 years and 35-40 years. If in the case of adolescents a possible explanation for the appearance of déjà vu is given by the brain's reaction to new and intense experiences that the subject has not lived, in the case of middle-aged people, guilty of the mysterious sensations would be the age crisis.

Over 70% of the Earth's population claims to have experienced, at least once, this feeling that they know a situation, a person's face, a place, even if these things should be completely unknown.

For a few seconds, we are firmly convinced that we have experienced the moment when the déjà vu effect happens, this effect being so strong that we can almost predict what will happen next. However, this amazing feeling passes quickly and we return to our reality.

Despite the fact that the real reason for déjà vu has not yet been confirmed by science, over 40 theories have been presented trying to explain the phenomenon.

Carl Gustav Jung, the illustrious Swiss psychologist and founder of analytical psychology, is the one who described a strong feeling of déjà vu when he was in front of a painting illustrating a doctor. The psychologist's feeling of familiarity with the shoes and clothes of the character in the painting eventually led him to conclude: the painted person was himself during a previous life, a rather strange explanation, given that it is one of the best-known scientists of the twentieth century.

The first to analyze the sensations of déjà vu and the one who, moreover, made this term known, was the French doctor Emil Boirac. In his volume «L’Avenir des Sciences Psychiques», published in 1876, the French scientist defined three types of déjà vu: déjà vecu-already lived, déjà senti-already felt and déjà visite-already visited. Subsequently, derived from the first three types, was defined the feeling of déjà entendu-already heard, as well as the antonym jamais vu-unseen, a feeling opposite to déjà vu, in which a person returned to a familiar place, for example, no longer recognizes anything.

The fact that the déjà vu sensations are not announced by symptoms and do not last more than 30 seconds, the phenomenon itself is extremely difficult to study. One of the first scientists to attempt a deepening of the mysterious feeling was Sigmund Freud. In fact, the phenomenon described by Freud, called paramnesia, was generally accepted for a long time in the twentieth century, until the moment when deja vu re-entered in the attention of scientists.

The following are some theories about the déjà vu effect:

• Split perception

The theory of split perception suggests déjà vu happens when you see something two different times. The first time you see something, you might take it in out of the corner of your eye or while distracted. Your brain can begin forming a memory of what you see even with the limited amount of information you get from a brief, incomplete glance. So, you might actually take in more than you realize. But your brain recalls the previous perception, even if you didn’t have total awareness of what you were observing. So, you experience déjà vu. In other words, since you didn’t give the experience your full attention the first time it entered your perception, it feels like two different events. But it’s really just one continued perception of the same event.

• Delayed Processing

You observe something, but the information you take in through your senses is transmitted to your brain along two separate routes.

One of these routes gets the information to your brain a little more rapidly than the other. This delay may be extremely insignificant, as measurable time goes, but it still leads your brain to read this single event as two different experiences.