What is déjà vu?

Neuroscientist Philipe Bujold was asked this question from Marika by Chris Smith, and told us all some theories about déjà vu...

Philipe - I’ve had that question a few times. Actually, unfortunately, I can’t give you a hundred percent certain answer because we still don’t know what a deja-vu actually is. We have a few theories though and that involves a lot of the memory circuits. The hippocampus is involved in encoding the context in which memories are formed but right next to that there is a part of the cortex called the rhinal cortex. And that rhinal cortex is divided, but it’s main role would be to recognise what’s familiar. And so one of the big theories at the moment is that this rhinal cortex needs to fire inline with the hippocampus, which encodes context, and then that opens up a memory, so it will fire the network that’s associated with the specific memory. If, unfortunately, you don’t have that context firing, you just have that feeling of familiarity, then that might be why you have a deja-vu. So we’re not sure. The only really studies that have been done into that are starting to show us that that’s the case. The stimulation of these parts of the brain that also seem to create fake deja-vus, if you want, but we still don’t know why it would happen. It seems to happen more in young people, or it might just be that young people are more aware of it because we still have a lot of our brain intact because ageing hasn’t started.

Chris - It happens to me when I’m tired. Does that make a difference?

Philipe - Yes. So stress and fatigue actually are huge links to more deja-vu experiences.

Chris - I suddenly have a sort of flash or recollection and a feeling of oh, I’ve thought of this before and it’s a memory that’s fleeting. It’s there and then it’s gone and I can’t get it back. But it only happens when I’m really tired. After night shifts and things when I was a junior doctor, it would happen quite a bit.

Philipe - Yeah, exactly. So that’s probably when it would happen, especially for undergrads actually. I think it’s something like two thirds of undergrads seem to have experienced a deja-vu feeling.

Chris - Oh, don’t remind me.

Chris - Fran?

Fran - I used to be absolutely convinced when I was seven or eight that a deja-vu was something that I’d dreamt but I’d forgotten the dream, so I was sort of remembering from a dream. Is that at all plausible?

Philipe - I wish I could give you an answer. I don’t actually know. I would not think so because our brain seems to be very adapted to not remembering dreams otherwise we would have a lot of issues functioning in daily life. You already have to remember what’s happening for 12 to 16 hours every day, if you have to remember everything that’s happened overnight also that would be a big issue.

Chris - There are some people that do though aren’t there? These people who have some kind of savantic behaviour and can remember an excruciating episodic detail everything that’s happened to them throughout their lives.

Philipe - Yes. So it does happen for some people. That is fortunately not the norm because it is, like you said, excruciating for these people, but it happens in some cases. I wouldn’t be able to tell you why it happens but it does happen, yes.