Many of us will have experienced déjà vu, yet none of us seem to know why.
Even someone with their feet planted firmly in the realms of logic and science can't help but feel a little esoteric when déjà vu strikes, like a whimsical jolt of nostalgia from the depths of the ether.
Déjà vu, meaning "already seen," has fascinated researchers and layfolk alike for centuries.
The experience is relatively widespread, too; according to various surveys, almost two thirds of people have experienced this otherworldly flashback feeling.
However, despite déjà vu's prevalence, it has proven a tricky phenomenon to pin down; you can't simply lay someone in an MRI scanner and wait for an episode, as they might be there for weeks.
Although a definitive answer has not been reached, there are a range of theories that drift between the realms of psychology and neuroscience. Here, we will explain some of the most compelling theories.
What we know about déjà vu
Although it is considered bad form to alert readers to the conclusions of an article prematurely, we still do not know why or how déjà vu occurs. So, what do we know about the experience?
Age: Déjà vu appears to occur more commonly in younger people, steadily becoming less common as we age.
Gender: Men and women seem to experience it at roughly the same frequency.
Social: According to some studies, déjà vu is more common among people from higher socioeconomic groups and more highly educated individuals.
Travel: People who travel more frequently are more likely to experience déjà vu. One study carried out in 1967 found that only 11 percent of people who never traveled experienced déjà vu, compared with 41 percent of those who made between one and four trips per year, and 44 percent of those traveling five or more times per year.
Stress: Other studies have demonstrated that déjà vu is more common when we are particularly tired, stressed, or both. For instance, there are multiple reports of troops experiencing déjà vu as they approach battle.
Drugs: Certain drugs may increase the likelihood of a bout of déjà vu. A case study published in 2001 recounts the experience of a mentally healthy 39-year-old male who experienced recurrent déjà vu when taking amantadine and phenylpropanolamine together to treat flu.
Where in the brain does this occur?
Perhaps surprisingly, déjà vu does not appear to correlate with any particular mental disorders. The only condition that is reliably associated with the experience is temporal lobe epilepsy (TLE).
The temporal lobe (shown here in yellow) appears to be important in déjà vu.
In this particular form of epilepsy, as with other types, there is often an "aura" prior to a seizure. For some people with TLE, their auras regularly include déjà vu.
The temporal lobes, involved in visual memories and processing sensory input, seem to be prime suspects for housing déjà vu.
One study carried out in 2012 honed down the search a little further. They found that stimulating the entorhinal cortices (EC) can produce déjà vu-like experiences. The EC, located in the medial temporal lobe, play a role in spatial memory and memory consolidation.
What causes déjà vu?
The region of the brain associated with déjà vu may have been pinned down, but what causes it? In general, theories of déjà vu fit into four categories:
- dual processing
None of the following theories holds all of the answers, but each gives a unique opportunity to revel in the paper-thin yet remarkably robust experience that we call consciousness.
In a nutshell, dual processing theories suggest that two cognitive processes that normally run in parallel are, for a moment, uncoupled. This category of explanations can be further split into four types, depending on which processes are imagined to have uncoupled.
It is worth noting that these are some of the oldest déjà vu theories, and none have any empirical evidence to back them up. They are food for thought, however:
Familiarity and recall: This theory argues that familiarity and retrieval are two cognitive functions that normally work in concert. If, for some reason, familiarity was erroneously triggered, we would feel a baseless sense of having been somewhere before.
Encoding and retrieval: This explanation comes with a handy metaphor: a tape recorder. Normally, the record head (encoding) on a tape player and the play heads (retrieval) operate separately. We are either laying down memory, or we are retrieving it.
The theory goes that sometimes, both heads can accidentally be functioning together. This means that we generate a false sense of familiarity about the sequence of events that is playing at the same time. Although the metaphor is pleasing, scientists are not convinced. Memory formation and retrieval do not work in that way.
Perception and memory: This theory claims that, as we perceive events, memories are formed alongside. Normally, we are focused on perceiving events, but if we are tired or distracted, the formation of a memory can occur at the exact same time as we are perceiving our surroundings. In this way, our perception of "now" would appear as a memory.
Dual consciousness: First considered in the 1880s by Hughlings-Jackson, it posits that we have two parallel streams of consciousness: one monitoring the outside world, and one watching our internal musings. If the primary, more sensible, outward-looking consciousness dwindles due to fatigue, the more primitive consciousness would take over and accidentally mistake new experiences for older, internal experiences.
Although each each of the above are food for thought, none cut the mustard as far as modern scientific theories go.
Could déjà vu and epilepsy be linked?
The neurological explanations of déjà vu are commonly split into "seizure" and "neural transmission delay."
Seizure: As mentioned previously, individuals with TLE commonly experience déjà vu as part of the aura before a seizure. The logic clearly follows that, if this is the case, perhaps déjà vu is a minor type of seizure.
However, data do not back this up. Déjà vu is not more common in people with epilepsy in general, and people who have déjà vu more regularly are not more prone to seizures.
Also, although the connection between déjà vu and TLE is well established, the majority of people with TLE do not experience déjà vu as part of their aura.
Neural transmission delay: There are a couple of versions of this theory. One describes déjà vu as information traveling from the eye via a number of pathways to reach higher centers. If the information from two pathways arrives at different times, for whatever reason, the brain might perceive the second message as old information.
This section of theories concentrates on the way in which memories are stored, held, and retrieved.
One memory-based explanation does have some experimental backing. A study carried out in 2012, using virtual reality, gave an intriguing insight.
Researchers found that if participants were shown a scene that was very similar to a scene that they had been presented with previously but could not recall, a feeling of déjà vu was sometimes sparked.
In other words, if the memory of a scene is not brought to mind as we view a new, similar scene, the previously experienced scene stored in our memory bank still exerts some influence - perhaps a feeling of familiarity.
Another memory theory put forward by researchers Whittlesea and Williams turns our notion of familiarity on its head. Perhaps we have been thinking of "familiarity" in the wrong way. For instance, if we were to see our mailman at the front door of our house - a very familiar scene - it would not produce a sense of familiarity. However, if we were to see our mailman unexpectedly, such as if we were on holiday out of town, it would evoke a sense of familiarity.
Our so-called sense of familiarity does not strike when we see familiar things. If it did, we would be sensing familiarity almost constantly. Rather, this feeling of familiarity occurs when we see something familiar unexpectedly.
When we see something familiar, our brain processes it more quickly and it takes less effort. Whittlesea and Williams' theory has it that if we were to experience something very familiar (but not recognize it at the time) in an unfamiliar setting, the familiar element would be processed quickly (even though we hadn't noticed it), making the whole scene feel familiar.
The fourth strand of déjà vu explanations focus on attention. The basis of these theories is that a scene is briefly witnessed without full attention being given. Then, shortly after, the same scene is perceived again, but this time with full perception. The second perception matches the first and is accidentally assumed to be older than it truly is, thereby triggering déjà vu.
More still to learn
As intriguing as these theories are, none have been proven, and, in reality, all could have some or no truth to them. We assume that déjà vu is a single type of experience, but it could be generated in subtly different ways either between individuals or within the same individual at different times.
Although the temporal lobes appear to be involved, we really are no further forward in understanding why this ubiquitous and unsettling phenomenon occurs.
So, next time you experience déjà vu, make sure to revel in one of human biology's most fascinating unexplained mysteries.