The mind shouldn't be allowed to play tricks on us.
It is, after all, our only interface with reality, processing every sensation in every moment to define our very existence.
So when the brain repeats itself, we've got all kinds of questions for it. Was that real? Didn't I already have that same experience? Have I lived this life more than once?
Most of us, at some point, in our lives have experienced déjà vu. It's essentially our brain messing with us.
But there's another less common but much creepier version of the "been-here-before" phenomenon. It's called déjà rêvé.
What's the difference?
Literally, déjà vu translates from French as "already seen." As in, this isn't the first time I've stood over a pile of dirty dishes with the tap running, while a blackbird in a tree outside stared back at me.
Déjà rêvé, on the other hand, means "already dreamed."
It's a scene or a memory or even just a feeling that you've experienced in a dream. And when that same experience pops up again in the waking world, it's déjà rêvé.
While it's certainly unsettling to experience things that you've previously dreamt, the concept isn't entirely new. It's essentially good old-fashioned prophesying — a gift for which people people were once esteemed (See: Moses) or condemned (See: Witch trials).
But science doesn't have time for the supernatural. Research on déjà vu has already pricked the paranormal bubble that once surrounded it — you don't actually see the same thing twice, but rather your brain is having trouble accessing the memory of a previous experience.
In other words, a new experience may remind you of a past experience, but your brain can't quite recall it specifically.
Those kinds of gaps leave the door wide open for us to indulge in supernatural explanations: I've been here before… in a past life.
Research on déjà rêvé, on the other hand, is a little sparser. Part of the reason for it may be that it's long been confused with déjà vu. Did you dream the dream or experience the experience first? Or did you live it first? It's the old chicken-or-the-egg routine, only our nebulous, often imperfect, memories muddied the picture even further.
A French study from last year sought to isolate déjà rêvé in people who suffered from partial epilepsy. The participants had reported experiencing déjà rêvé in the past, particularly when they were having seizures. For the study, they agreed to have their brains electrically stimulated, allowing scientists to not only induce déjà rêvé, but also zero in the specific areas of the brain involved in it.
The scientists established three broad types of the phenomenon.
The first, called episodic-like déjà rêve, was the most literal: The experience could be immediately traced to a specific dream. Things get murkier with the next category. Dubbed familiarity-like déjà rêvé, that's where participants could only remember the vague, fuzzy outlines of the dream that came before the real-world experience. Subjects may have remembered a dream, but not which dream or even when.
The plot gets thicker — and more bizarre — with the third type, called dreamy state déjà rêvé. That's when participants had trouble differentiating reality from dream. Am I still dreaming? Or did I really experience that? It's almost like having a dream about déjà vu.
The researchers aimed not only to help find an effective treatment for epilepsy, but also put déjà rêvé on the map as a legitimate experiential phenomena — one that's distinct from its cousin.
"This study demonstrates that déjà-rêvé is a heterogeneous entity that is different from déjà-vu, the historical 'dreamy state' definition and other experiential phenomena," the study authors note.
And, perhaps, along the way, they might be able to keep our brains a little more honest.