Why do people get deja vu?

13 August 2019

BRAIN

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Question

Why do people get deja vu?

Answer

Neuroscientist Duncan Astle delved into Paul's question...

Duncan - Who knows what déjà vu means? Who’s good at French? ‘Already seen’. It’s that sense of familiarity but in the absence of an explicit memory of something. So as you can imagine there are lots of different theories as to why we experience déjà vu. I'm going to give you my favourite one, because I think it's probably the most plausible one: and that is that déjà vu is the moment when you become consciously aware of a discrepancy between two different memory signals that come from two different memory systems. And it's the signal about what you think you've experienced before, and the signal about what you've actually experienced before.

So there are actually multiple different memory systems that we use all the time. They have slightly different underlying neurobiological implementations; some of them are really to do with learning about and remembering episodes, so facts that have happened. Others are more to do with learning about the general principles of the world around us. For instance I knew the way here but I couldn't remember the first time that I had been here.

Now you can imagine the situation where these two systems give contrary indications, so that you get a very strong feeling of familiarity but in the absence of an explicit memory. And one theory of déjà vu is it’s the moment when you consciously realise that you're getting these two conflicting signals about one particular experience.

Chris - What tends to make it happen?

Duncan - No one knows for sure. For instance when people are more sleepy they tend to be more...

Chris - I was going to say, ‘cause I know I've had this and it's happened when I've been jet lagged. It tends to happen to me when I'm really tired or when I've done a night shift at the hospital or something, I get it then. Is that common?

Duncan - It is. When studies try and look at it experimentally, one of the ways that they try and induce it is by sleep depriving people. 

Chris - And is that just because one of the memory circuits you allude to is failing to imprint a memory properly and another one thinks it has, or one of them thinks it's put something into long term memory and it actually hasn't, and so there is this disparity arising. Is it a memory failure or is it just spurious signals being generated by a tired brain?

Duncan - I suspect it's that when you're really tired you're less able to disentangle these two sources of information and that's why you get this moment of conflict.

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