“A black cat went past us and then another that looked just like it”. Remember when Keanu Reeves experienced déjà vu and was told it indicated a glitch in the matrix? Not restricted to Hollywood sci-fi, in fact almost all of us have had the disconcerting feeling that what is happening now has happened before (even when it hasn’t). After 150 years of scientific research, there are plenty of theories, but little consensus as to why we have these experiences.

Do you feel like we’ve been here before? Déjà vu still has scientists baffled.

The French term déjà vu translates as ‘already seen’ and was first used in 1876. Psychiatrist Vernon Neppe defined déjà vu in 1983 as “any subjectively inappropriate impression of familiarity of the present experience with an undefined past.” More than just sense of familiarity, it’s the unnerving sensation that you’ve had an exact experience before. It’s weird precisely because to the best of your knowledge, that isn’t the case. But déjà vu is normal, and a common phenomenon. Given the amazing techniques we have these days to understand how our brains work, why don’t we have a good understanding of what causes déjà vu? It’s at least partly because I’m guessing last time you experienced it, you weren’t sitting in a lab with electrodes stuck to your skull. We don’t have any reliable way to trigger déjà vu and have to rely on peoples’ recollections of their own experiences, which will always be subjective. So despite déjà vu being a common topic in popular culture, our understanding of the scientific basis for it is still pretty murky.

What do we know?

Déjà vu is extremely common: studies have found between 60% and 80% of the population has experienced it. There are no gender differences, but more educated people experience déjà vu more often than those with less formal education. Déjà vu occurs more often when you are tired or stressed. Déjà vu appears to first occur in children aged eight or nine, is most common in people aged 15 – 25 years, but then tapers off as we age. As a result of this age-related pattern, researchers have suggested a link to brain development. Scientists have long known some experiences of déjà vu can be linked to temporal lobe epilepsy. Epileptics who have seizures centered on their temporal lobes often experience something similar to déjà vu just before the seizure begins. Because we know epileptic seizures are the result of alterations in electrical activity in the nerve cells, it has been argued déjà vu in non-epileptics is also the result of wayward electrical signals. But others believe the type of déjà vu experienced by epileptic patients is very different to ‘normal’ déjà vu. We also know some people experience constant déjà vu, and these people turn out to have brain damage to the temporal lobes. In one case, a 23-year old man felt as thought he was ‘trapped in a time loop’, an experience he described as very frightening.

Faulty memories, or something else?

Alan Brown, author of “The Déjà Vu Experience,” says there are at least 40 plausible scientific explanations for déjà vu. Many of the theories centre around the notion that our memories are prone to errors. The temporal lobes – the part of your brain behind your ears – are responsible for recognising something as familiar. The hippocampus is involved in recalling something has happened before. One explanation of déjà vu is that these two processes get out of sync. The way we form memories could also play a role. Experiences normally get stored in short-term memory before later making it into long-term storage. But if an experience accidentally bypasses the short-term, and instead goes straight to long-term memory, we can feel like the present has happened to us before. It could also be our brains get distracted by something after we’ve already unconsciously started taking in a scene around us. You know when you suddenly realise you’ve been “on another planet”? And when you snap back to reality, it feels like you’ve already been there. (D’Oh! That’s because you have been, a few seconds earlier.) Of course it may also be simply that something in our present situation reminds us of something we really have experienced before. The two situations aren’t exactly the same, but there are enough similarities that the new experience feels familiar. Maybe a new airport transit lounge has exactly the same layout as one you have spent lots of time waiting in. There is some evidence déjà vu occurs more frequently in people suffering anxiety disorders. Another possible explanation is that a person actually has experienced the situation or place before but has simply forgotten it! Or perhaps we are recalling our dreams. There are other explanations too. Carl Jung attributed déjà vu to the ‘collective unconscious’, others take déjà vu as evidence that we have some recall of our past lives, or alien abductions. Whatever the cause, I’m bummed that at my age I’m likely to experience déjà vu only about once a decade. That’s a long time to wait if I want to carry out some scientific research on my own déjà vu.

Links and stuff

• SciShow Déjà vu video
• VSauce Déjà vu video
• It’s ok to be Smart Déjà vu video
• The experience of constant déjà vu – in the New York Times

Radio on demand

 This post accompanies a radio segment on Triple R’s Breakfasters program on Wednesday 15 April 2015.