You've probably experienced the sensation of déjà vu — that eerie sense that you get when you know you have seen or done something before, even if you couldn't have. It's the closest we can come to experiencing a real-life time loop.
How does your brain make things seem familiar when they aren't? Let's take a look! Again! Possibly!
In fantasy series, if a hero suddenly is overwhelmed with a sense of familiarity at a certain place or event, it's usually because they've had a prophetic dream or vision. They were meant to see exactly this, so they could be prepared to fight the monster that's about to jump out at them . . . now! Outside of film and television, déjà vu generally gets us in more prosaic ways. We come to a place that's new, but are sure we've seen it before. We feel a striking sense of, say, telling a stranger that they've dropped their keys before. It may just come to us as a weird sense of over-familiarity in an already familiar setting. Why do we feel this way?
And how could you go about creating this sensation on purpose?
One way to create the sensation is simply to implant false memories in someone — a task that is surprisingly easy. There have been multiple experiments where, with just a little coaching, people will remember a specific memory of being lost in a mall as a child and found by a kindly old lady, despite their relatives confirming that that never happened. Other times, people have been convinced that they saw a person in a Bugs Bunny costume entertaining them during a childhood trip to Disneyland, despite Bugs not being a Disney character. People will remember all kinds of things, with a little prompting.
You could also actually briefly expose someone to an object — a section of town, a painting, a snatch of a filmstrip — and then allow them to wander back and take it in slowly, creating an actual memory that they may not be able to pin down. Researchers have had great success with showing subjects words, names, or faces for a fraction of a second on a computer screen, and inducing a feeling of déjà vu by showing people the same thing for a longer amount of time, just moments later.
To induce a more general and spontaneous sensation of déjà vu, and one that pops up out of the blue, you just have to get the conditions right. The sensation happens to young people more than old people, but mostly because younger people will have more erratic routines. This feeling happens most often when people are tired and stressed, or have a lot of things on their minds. It can happen when people are put under hypnosis to the point where they're out of it. But déjà vu isn't a mental blip brought on by stress. It's not a blip at all. It's a lack.
The feeling of familiarity isn't a by-product of seeing familiar objects. It's a feeling all on its own. Many experts believe it resides within the parahippocampal gyrus, which can spring into action when it's needed during normal activities. We both recognize the object itself, and get the feeling that we've seen it somewhere before. Sometimes that feeling, the recognition memory, leads straight and fast to an actual memory of when we've seen the thing before. Sometimes it's dismissed, since it doesn't matter when we've seen something before. (We can all recognize a Coke bottle but it doesn't matter to us when we last saw it, when we first saw one, or how it became familiar to us.)
Sometimes, however, you'll look at something that pings that vague recognition memory, that sense of being somehow familiar, and you'll want to place it, but we can't. When your brain is firing on all cylinders, in a familiar environment, and in a routine that it often repeats, it's easy to use that sense of familiarity as a guide, and look back until you recall the origin of the memory.
When your brain is tired, stressed, and surrounded by the unfamiliar day after day, it can't comb through the input that it's been exposed to, well enough for that feeling of familiarity to become solidified. Instead, you start having intense feelings of familiarity brought on by things that are evocative of what you'veexperienced before — a particular configuration of coffee cups on a table, the face of someone who is tangentially related to someone you know, a particularly angled street — without being able to pin down why it feels so familiar. The brain decides that it's clearly been here before, and satisfies itself with that explanation, instead of trying to find a better one.
We came up with a word for this phenomenon, and déjà vu became an explanation unto itself.