What exactly is déjà vu?

We all have some experience of a feeling that comes over us occasionally, of what we are saying and doing or things we have said and done before in a remote time. We think we know perfectly what will be said next as if we suddenly remember it! It is sudden and fleeting, leaving as unexpectedly as it came. While the experience is striking in its clarity and detail, it is difficult to recapture or recount. Generally, it is left unexplained and is described in a vague sense, often simply as "Wow, I just got the strangest déjà vu”. Because it is so difficult to research and seems to have no deleterious effects on daily and long-term nervous system functions, déjà vu has been left largely to the wayside of neurobiological investigations. In all of its ambiguity, déjà vu is still a perplexing phenomenon that has not yet been fully explained. 
 

What is déjà vu and how does it work? 

Déjà vu is considered a common phenomenon. Surveys show that about one-third of the population has had the most common form of déjà vu sensation. Due to the subjective and often indescribable nature of the associated feelings, it has been difficult to determine who is actually experiencing déjà vu. Déjà vu has been defined as "familiarity without awareness." While the situational cues of a déjà vu are familiar, there is a definite lack of awareness about the specific source of the memory. Arthur Funkhouser defines three types of déjà vu in an attempt to more clearly delineate between associated but different neurological experiences. These are déjà vecu (already experienced), déjà senti (already felt) and déjà visité (already visited). Déjà vecu is the most common déjà vu experience and involves the sensation of having done something or having been in an identical situation before and knowing what will happen next. These sensations are often felt through several senses: seeing, hearing, taste, touch and proprioceptive perceptions. The experience is often incredibly detailed and is usually connected to very normal activities. Although, the episode itself lasts from only a fraction of a second to several minutes. 
 

What causes a déjà vu episode? 

There are several possible explanations for what is occurring during a déjà vu experience. One possibility is simply the occasional mismatch made by the brain in its continuous attempt to create whole sensical pictures out of very small pieces of information. When the brain receives a small sensory input (a sight, a smell, a sound) that is strikingly similar to such a detail experienced in the past, the entire memory image is brought forward. The brain has taken the past to be the present by virtue of one tiny bit of sensory information. It is this mismatch of past and present sensory information that causes the sense of disconcertment and unease associated with a passing déjà vu. This theory provides a satisfactory explanation for the physical effects of déjà vu.
 
Another explanation for déjà vu is that there is a slight malfunctioning between the long and short-term memory circuits of the brain. Somehow, specific information shortcuts its way from short to long-term memory storage, bypassing the usual mechanisms used for storage transfer. The details concerning this shortcut are not yet well understood. When this new, recent piece of information is drawn upon, the person thinks that the piece is coming from long-term storage and therefore must have come from the distant past. A similar theory says that the error is in the timing of the perceptive and cognitive processes. Sensory information is rerouted on its way to memory storage and, so, is not immediately perceived. This short delay causes the sensation of experiencing and remembering something at the same time, which could be a very unsettling feeling. In combination, that accounts for the different experiences we call déjà vu.