In this series bestselling neuroscience author Dr Dean Burnett answers Shambles Patreon subscribers questions about their brains. A sort of neuroscientific agony aunt if you will.

Phoebe asks, ‘Explain Déjà vu!’

Hi Phoebe

Haven’t I already answered this?

HA HA HA HA I AM FUNNY NO YOU SHUT UP!

Anyway, cringeworthy jokes aside, this is something that does come up quite often. It’s a brain phenomenon that everybody recognises, and most would claim direct experience of it. But even so, you don’t get people trying to explain very often. Or ever, in my experience.

So, just so we’re all on the same page, what do we mean when we say ‘Deja-vu’? I’m taking it to mean the phenomenon where we experience recognition, or a sense of familiarity, in response to something that is objectively new and unfamiliar. Put more simply, it’s when we feel we’ve encountered something before, when we know we haven’t.

If there is some completely different phenomenon that happens in the brain that is also called Déjà vu, then I’ve not heard of it. If that was the one you were asking about, then that should have been clearer in the question. Sorry.

Anyway, assuming we all mean the same thing, how and why does Déjà vu happen? What’s going on in the brain to produce such unwarranted recognition or familiarity?

In truth, it’s hard to be sure about this, because Déjà vu, by it’s very nature, is difficult to study. Because it’s very hard to make it happen on command. It’s not really something we can do on purpose. Once something has triggered a Déjà vu reaction, it is recognisable from then on. And trying to recall memories of when you’ve experienced Déjà vu will just complicate matters, because you’re no trying to bring up familiar feelings of unfamiliarity.

It’s a bit like hearing a joke for the second time; it may have been hilarious first time around, but most of the humour will have been in the reveal of the unknown punchline. Once the punchline is known, the brain doesn’t react the same way to it.

But in the more practical sense, it means that Déjà vu is hard to study, because you it’s tricky to put someone in a scanner or attach probes to their head and say “OK, experience Déjà vu… now”.

Well, actually, it’s easy to say that, but it’s a lot harder for anyone to actually do it. And therefore, it’s hard to study it.

Not that scientists haven’t got ways and means to get around this issue, in certain ways.

This has led to the formation of a number of possible mechanisms that could underlie Déjà vu. It could be due one of these. Or more than one. Or all of them. Or none, and something else entirely. Brains are fun like that.

 One possible explanation for Déjà vu is the dual processing model. Whenever we experience anything, it’s the result of multiple streams of information being fed into our brain, and being consolidated into what we know of as our perception. It’s an incredibly complex process, but it usually runs smoothly, with everything being combined harmoniously enough.

But having said that, it wouldn’t be surprising if, for any number of reasons, a glitch or hiccup occurs, leading to some of the stream of information that make up our perception to briefly go out of synch. This could well result in our brain experiencing something via one cognitive stream, and then again a moment later via a different, momentarily delayed, stream.

It’s a bit like when the audio and visual aspects of a streamed video don’t line up. Only instead of an oddly jarring viewing experience, your brain gets to experience the same new thing ‘twice’, leading to an inexplicable feeling of familiarity the ‘second’ time around. And thus, Déjà vu.

A different, albeit related, theory is the neurological model of Déjà vu. This too proposes a ‘glitch’ in the inner workings of the brain, but specifically focuses on neurological mechanisms, like disruption in the signals conveyed by neurons in key brain regions. And these regions are usually within the temporal lobe, which makes sense as that’s where memory, and aspects like familiarity and recollection, take place in the brain. And those things are undeniably a key aspect of Déjà vu.

There’s also the fact that Déjà vu is often something that occurs right before an epileptic fit in temporal lobe epilepsy. Which lends support to the idea that Déjà vu is due to errant neurological activity in the temporal lobe.

Others attribute Déjà vu to specific issues with the memory system. It could be that a new experience is, erroneously, shunted from short term memory to long term memory much more quickly than usual, meaning very new memories (as in, a few seconds old) have an unwarranted sense of familiarity attached to them.

Although exactly how new memories can take a short cut to long term storage is unclear.

A more straightforward memory-based explanation for Déjà vu involves implicit memories. These are memories stored in your brain that you can’t consciously access. This isn’t uncommon; recall is an important part of the memory process, and sometimes memories, due to lack of use or general entropy, become ‘detached’ from the recall mechanism. So, they’re in there somewhere, but we can’t get at them. They’re forgotten, but not gone.

But if we then experience something which has elements in common with this inaccessible memory, it triggers this forgotten memory. We can’t recall it, but it does lead to a sense of familiarity. Recall and familiarity are different processes; recall is when we can retrieve and ‘relive’ a memory and all the information it contains, while familiarity is more the recognition that something has been experienced before.

So if an experience triggers an implicit, forgotten memory, it could activate a familiarity response, without recall. Which, again, is basically what Déjà vu is.

And finally, there’s the attentional model of Déjà vu. As useful as it is, attention is a finite resource for our brain, and has to be used wisely. And sometimes, this leads to problems.

It’s posited by some that, via a few feasible mechanisms, we sometimes don’t pay full attention to something, so our brains take it in on some level, but not in a way that means we’re consciously aware of it. But then, perhaps seconds later, we perceive it again when our full attention is directed to it, to an extent we’re consciously aware of it now.

So, thanks to our limited attentional system, our brains have encountered something twice, but we’ve only become consciously aware of it the second time. And this leads to Déjà vu, where our brains are saying something has been encountered already, but our conscious mind disagrees.

These are, at present, the working theories around why we experience Déjà vu. It would be an amusing irony if the real mechanism underlying it ended up being something completely unfamiliar. But I presume those researching the phenomenon are motivated by aims more noble than ‘it’d be a vaguely amusing meta-joke’.

If you’d like to ask Dean a question, subscribe here and ask away. Dean’s new book Emotional Ignorance is out now.

Dr Dean Burnett is a neuroscientist and best selling author of such books as The Idiot Brain and The Happy Brain. His former column Brain Flapping for The Guardian (now Brain Yapping here on the CSN) was the most popular blog on their platform with millions of readers worldwide. He is a former tutor and lecturer for the Cardiff University Centre for Medical Education and is currently an honorary research associate at Cardiff Psychology School and Visiting Industry Fellow at Birmingham City University.  He is @garwboy on Twitter.

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