15 April 2012

Déjà vu

Déjà vu is one of the coolest phenomena ever.  I know I already said that about change blindness.  I also said I’d change my mind.

I love déjà vu in the same way that I love sneezes and yawning and blind spots and dreams and migraines.  These things all make me very happy.  They’re small reminders that my brain is still there, it’s organic, it does things I can’t predict.  They point out that we are laughably unaware of the mucky mushy underpinnings of our lofty cognitive musings.  Déjà vu makes us remember we’re only human.

Well, maybe that’s not true for everyone.  Déjà vu means different things to different people.  What it certainly is not is a literal re-experiencing of a moment that happened in that exact same way at some previously unspecified time.  This is an incorrect interpretation of the phrase, because even though the direct translation of “déjà vu” is “already seen,” the definition of the word includes the notion that one is re-seeing something one knows one couldn’t possibly have seen before. 

Now, some say that déjà vu is some special form of extra-sensory perception, or it’s a signal the Matrix has been altered, or it tells us things about our past lives, or it’s some sort of breakdown between all the versions of our lives we’re simultaneously living.  Here’s the thing.  Déjà vu is already a beautiful miracle without making it anything paranormal or supersensory.  It’s a truly incredible process and a delightful experience.  This may sound weird coming from an urban fantasy writer, but I just don’t like the taste of forcing supernatural elements where they don’t belong.

Scientific theories posit numerous explanations for déjà vu, most having to do with the medial temporal lobe (remember the hippocampus…?).  Keep in mind that researchers very rarely have the opportunity to study déjà vu, given how transient and unpredictable it is – in fact, apparently only about 60-70% of people report having ever experienced the phenomenon at all (yet another variable thing I thought was common to everyone!).  So even some of the so-called “scientific research” discussed here tends to wax philosophical.

According to these researchers, déjà vu may occur because:
(1) Some aspect of the current experience excites the brain pathways that produce a sense of familiarity with the event, but not those that support proper recollection of a previous event, creating a disconnect that makes us feel like we know it without being able to pull out exactly when or where we experienced it before.  
(2) Our brains probably store memory in such a way that a small stimulus (a smell, a color) can trigger the incomplete recall of a real but different memory… and in some cases this might give us a sense that the current experience has already been experienced.  (The first part of this is certain – it’s the second that’s up in the air.)
(3) Our two brain hemispheres might sometimes get slightly out of sync when processing an input, such that one side gets that direct input fractions of a second earlier than usual and therefore misinterprets the added information from the other half of the brain as a repeat of an already-experienced memory. 
(4) We “experience” many types of things in media like books and movies, which allow us to feel strong familiarity for things we’ve never actually experienced in real life – and when we see it in real life for the first time we might accidentally think we’ve already seen it. 
(5) Some researchers believe that precognitive dreams (i.e. dreams which predict future events) may create a sense of déjà vu later on when they are properly experienced.  I’ll tackle this one shortly.  
(6) And lastly – and this is the least controversial of the theories because it’s the most testable – déjà vu can occur as a result of an epileptic event, like a seizure, in the medial temporal lobe. 

(There are plenty of other theories I’ve decided to let you discover on your own, seeing how long that paragraph has become already.)

I like aspects of a lot of these, but I want to put my money down on the first and last – the disembodied familiarity thing and the seizure thing.

There’s a lot of evidence that one’s concrete knowledge of a previously-experienced event (call it recollection) and one’s comparatively vague sense of familiarity with an event are different things that are processed differently by different brain regions – recollection by the hippocampus, and familiarity by… well, parahippocampal and/or perirhinal cortex, depending on who you talk to (they’re both structures basically adjacent to the hippocampus).  In the rare déjà vu experience, it’s possible that something about the current environment differentially stimulates the familiarity and recognition brain structures, creating a detached sense of familiarity.

Notice that in the previous sentence, I said it was something about the external environment causing the brain activation.  But it’s also possible that your brain just does this stuff to itself, without any outside help.  For example, people with temporal lobe epilepsy sometimes report feeling déjà vu right before a seizure strikes.  But you don’t have to have epilepsy to have epileptiform brain activity, and in fact every single person on the planet has endured some level of seizure-like activity in his or her brain.  Basically, every once in a while some tiny group of neurons goes a little haywire and activates for no good reason, but it’s natural and nothing to worry about.  Mostly these events don’t impact our conscious lives at all.  But maybe, sometimes these events occur in just the right place at the right time, activating our familiarity structures out of the blue, and suddenly the whole world around us feels like we’ve done it before.

Regardless of whether it’s externally or internally generated, it makes sense that déjà vu is an innocent brain mistake which makes us feel something that’s not really real.  It helps explain why we sometimes feel recursive déjà vu – the sense that we’ve even had this particular sense of déjà vu before, and that we’ve had a déjà vu of that déjà vu of a déjà vu, and so forth.  That’s just our brain accidentally and repeatedly triggering a feeling that this event has occurred before when it hasn’t.  So I’m pretty darn confident that when you experience déjà vu, that exact experience has never happened to you before – no matter how much you want to believe that.  That want, that need – that’s just your brain talking.

Which brings me to precognitive dreams.  I will certainly insult people with my opinion about this, but I’m willing to take that hit and say that the ability to actually foresee future events in a dream is literally impossible.  Let me rephrase that so I can be totally clear – precognitive dreams cannot be the true experience of a real-life event before it happens.

There are just too many problems with the idea that dreams can be pre-plays of real events (not least the violation of causality).  I’ll name a small few.  (1) The vast majority of things that happen to us happen repeatedly, so it’s practically impossible to avoid dreaming up scenarios which will be similar to later life events; also, “similar” is not at all the same as “identical”.  (2) If you compare every dream that you’ve ever dreamed with every event that has ever happened to you, you will absolutely come up with matches, and it has nothing to do with foreseeing anything.  (3) Our brains can make us feel conviction about things we actually can’t remember very well, so when those similar real events happen we can be duped into accidentally overwriting our dreams to match the events (someday I’ll write a post about this point).

Okay, enough of that.  I don’t want to give the impression I don’t believe dreams can be predictive.  Brains are prediction machines.  Especially human ones.  It’s arguably what we do best.  So it’s totally reasonable that your brain makes very, very good predictions about the future while you’re dreaming, using information you might not consciously piece together while going about your daily.  I am a happy believer when someone tells me that they always dream of a white elephant before someone dies – so long as they also tell me the white elephant is their brain’s way of assimilating a host of (subconscious) clues indicating those other people were about to die.  Such a dream would be entirely plausible, and maybe even probable.

What I’m saying is that déjà vu serves as a reminder that our brains are doing a lot of things behind the scenes.  In fact we don’t have conscious access to the majority of the things our brains do.  (Go ahead, try and stop your heart just by thinking it.)  When magical things like déjà vu and prescient dreams happen to us, we can congratulate our brains for being so gosh-darn brilliant without us even knowing it.  They really are capable of miraculous feats.

P.S. I got really sick of seeing the phrase “déjà vu all over again” in article titles as I looked all this up.  I used to love saying that and now it’s tainted for me forever.  So sad…


  1. I think I've read this before....

    Just kidding, obviously. You are brilliant, and I love your take on things.


  2. Let me be more substantive. I have always been quite fascinated by the workings of the brain. There is a statistic quoted repeatedly and so much so that it has become trite to my ears, and I don't even know if it is true. Something to the effect that our brains are only being utilized by about 10%, imagine what we could do if we could access the other 90%. But in my less educated mind, that doesn't make a lot of sense because of course it is in use...

    Please explain...

  3. BAH that drives me CRAZY! This notion came about because when we first started imaging brains, the blobs that light up during any given task are overlaid on only a small portion of the total brain. You know why? Because different parts of our brains do different things. When you're having people tell you whether a shape is a square or a circle, is the part of their brain involved in complex emotional regulation, or in auditory discrimination, really expected to light up? HELL no. You're only using 10% of your brain because you're only asking it to process 10% of the kinds of things it can process. And a lot of these processes are mutually exclusive - you wouldn't WANT more than 10% to light up.

    You know what happens when you access the other 90%? EPILEPSY. Freaking epilepsy. That's the definition of epilepsy. If you want to have a gran mal seizure, go right ahead and "access 100% of your brain" all at once. Yeesh.

    1. I should write a post about this :)

    2. I knew it was something like that. I'm almost as smart as you! Maybe I should be a doctor too. ;)

  4. Seriously... these informative posts are freakin fantastic, just so you know.

  5. Kaitlin you never cease to amaze me. Thank you.

  6. Argg... I didn't mean to be anonymous but I can't seem to post here any other way. It's me, Judi.