There are four current scientific theories about dÃ©jÃ vu, an uncanny type of memory that two-thirds of the population has experienced at some point. These theories are based on different ideas about how memory functions are organized.
Even if you don’t know any other French, you probably know what déjà vu means. In the middle of a friend’s story about oversleeping on the day of a big test, you suddenly get jolted by the strong sense that you have already experienced this moment. Or during a trip to a new country, you turn a corner and stop in your tracks, flooded with the sensation that you have already walked down this particular street, at the same time of day, with the exact same smell of wood smoke in the air. Other than the oddness of the feeling that we’ve been in this moment before, there is usually nothing special about the moment itself.
Déjà vu is hard to study in a lab, especially since it usually happens in a mundane setting and is so fleeting. However, some neuropsychologists and cognitive scientists think of déjà vu as a common type of cognitive malfunction that can help us better understand how the brain processes memory.
Popular Theories about Déjà Vu
Déjà vu. Two-thirds of the population have probably experienced it. The overwhelming sense that we have already lived the moment we are experiencing before is uncanny, at the least. Some people are convinced that déjà vu experiences are evidence of a past life. Others interpret it as prescience—the sudden ability to predict the future, at least for a few seconds.
In 1942, a student named Morton Leeds recorded 144 instances of déjà vu that he experienced, noting that it occurred more often late in the day, and when he was tired or under stress. Survey reports suggest that education, income, travel experience, and a liberal viewpoint make people more likely to have déjà vu. People tend to have fewer déjà vu experiences as they get older.
Scientific Theories about Déjà Vu
Currently, there are four main scientific hypotheses about déjà vu:
1. Dual processing glitch. Memory may involve a set of neurons that signal when a stimulus is familiar, and another set that retrieves already stored data. Déjà vu might arise when the familiarity system functions independently of the retrieval system.
2. Seizures. Epileptic seizures are sometimes accompanied by a déjà vu experience, leading some researchers to suggest that all déjà vu might be caused by small brain seizures. There is also data demonstrating that déjà vu occurs when certain brain regions are electrically stimulated.
3. Missing context. It could be that déjà vu happens when the brain retrieves something from our vast memory bank of images and perceptions—without filling in the situational context.
4. Double perception theory. Déjà vu occurs when there is a slight interruption in our normal perceptual processing. When our attention returns a moment later, we feel that we’ve already processed this perception before.
Scientists have attempted to come up with studies to trigger or replicate déjà vu-like experiences. For instance, students at two universities were shown photos of the other campus and asked to quickly locate superimposed cross-shaped markers. After viewing more photos of the other campus three weeks later, including some that had appeared in the first task, 89% of the students thought that they had been to the other campus before and 50% reported feeling déjà vu. These results support the double perception theory. When they were intent on locating the cross-shaped markers, students were not paying attention to the other elements in the photos. When they looked at the photos later, they experienced déjà vu.
Déjà Vu and Memory
The methods by which the human brain processes, stores, and retrieves bundles of information that we call memories is still not fully understood. Neurobiologists and other scientists find it useful to study instances, like déjà vu, when memory appears to malfunction. By figuring out how and why the brain stumbles, scientists get a clearer idea of the way memory functions normally work.