Déjà vu is the experience of finding something familiar when you know that familiarity is inappropriate. The classic example is somebody going on holiday to a place for the very first time, but as they walk around they have the strong impression that they have been there before.
There is a phenomenological component to déjà vu that is captivating and unique. It is associated with sensations that go beyond awe and wonder. Some people even decide that they have “lived” this moment before, and that it must be an evidence of past lives and reincarnation.
Science has always struggled to explain déjà vu. A few explanations have done the rounds. One is that it might be caused by signals from one eye reaching the brain slightly before the other, so the brain encodes something but is then immediately presented with it again – hence the feeling of familiarity. However, the problem with that is that blind people have reported déjà vu.
Psychologists have started looking at déjà vu as potentially a failure to retrieve all of the information you are trying to retrieve. You see a scene and it is similar to something you have seen before but you can’t remember when you encountered it: it just seems familiar in an I-can’t-quite-place-it way.
"When we experience déjà vu, the brain region that signals familiarity is having a wee twitch"
That theory is flawed because when we get those sensations, it’s very apparent to us that elements of the scene are familiar and we engage the brain in a long retrieval process: ‘I know that from somewhere, I just can’t picture where!’ And often, when we come back to it half an hour later, we remember exactly what it reminds us of.
The key is that there are brain regions that separately deal with the act of retrieving a memory and with signaling to the conscious experience that they have recovered it. Usually they are seamless in their operation. But just as you can get eye twitches and muscle twitches, when we experience déjà vu, the brain region that signals familiarity is having a wee twitch.
We recently did some research using a task that comes fairly close to replicating the sensations that people get when they experience déjà vu. We took advantage of a well-known effect within psychology called the DRM effect. Basically, you present people with a list of related words but you leave a critical word off the list.
"The brain is very good at filling in gaps, and that applies even to memory"
For example, I might present you with the words ‘blanket’, ‘duvet’, ‘night’, ‘snooze’ and ‘snore’. A while later I would test your memory of those words. If I re-presented you with ‘blanket’ and ‘night’ you would rightly say that you had encountered those words. If I presented you with ‘cup’ you would correctly said you hadn’t heard it before.
However, if I presented you with the word ‘sleep’, you would more than likely say, yes, you recognized that. The brain is very good at filling in gaps, and that applies even to memory. It is a false memory effect.
For our study, we modified this procedure. When we first gave the words to the participants, we asked them to monitor: Are there any that begin with the letters ‘sl’? At the end of that first phase, they reported back to us that, no, none of the words had begun with those letters.
This meant that when, at test, we presented them with the word ‘sleep’, it felt familiar to them – but they also had this objective experience that it shouldn’t do. It’s precisely the situation that defines déjà vu – a feeling of familiarity with the awareness that the familiarity is inappropriate.
"Déjà vu is a conscious experience of a memory error being spotted, rather than a memory error being made"
A large proportion of the study group reported déjà vu, so we put them into an fMRI scanner and observed their brain activation as they were encountering the words. We found their activity was all in the medial pre-frontal cortex, which is a set of brain regions associated with error detection and with inhibiting responses to erroneous information. They basically stop us making mistakes.
These areas of the brain were very active, while the regions that are associated with retrieving stuff from memory stayed quiet. This suggested to us that the fMRI findings agreed with our theory that déjà vu is a conscious experience of a memory error being spotted, rather than a memory error being made.
Basically, it’s not that we have encountered something before and we are trying to retrieve where and when it was. We know full well that we haven’t encountered it before, and the conscious is signaling that something has gone wrong here. There is a twitch upstream from where we are, and we are stopping it becoming something that we act upon.
"We don’t turn off the TV because we have a déjà vu while watching Sherlock"
That’s actually a really important thing about déjà vu. We don’t tend to act on it. We don’t turn off the TV because we have a déjà vu while watching Sherlock and we think we must have seen it before. We just think, Hmm, that’s weird, and carry on watching. Déjà vu tends to have no behavioral consequences.
Young adults – teenagers through to mid-20s – get far more déjà vu than older people. This makes sense, because young people make fewer memory errors, and also they have more excitatory neurotransmitters in the brain. People also experience déjà vu more when they are tired. This figures. You get more muscle twitches when you are tired, so you would also get more neural twitches – after all, it’s the nerves that control the muscles.
"A lot of people who have epilepsy experience déjà vu as part of their pre-seizure"
There are certain other factors associated with déjà vu. A lot of people who have epilepsy experience it as part of their pre-seizure. Some drugs will also trigger it. Experiencing déjà vu is part of amphetamine psychosis.
Whenever I talk about déjà vu like this, I always get emails from people who experience a lot of déjà vu – so much that it is a problem for them. It’s really difficult to know how to respond to them. There is still so much that we don’t know about it. There are so many unquantifiable things. I certainly don’t know how to stop people having déjà vu.