Cognitive Science: An Introduction/Psychology of Time

Introduction to Déjà vu
French for "already seen", déjà vu recounts the eerie sensation that an individual has already experienced something, even when the individual believes they never have. The events of déjà vu are accounted for in approximately 60 percent of individuals and are defined as a dissociative encounter and a peculiar memory illusion for those who have experienced it. Regardless of investigators' substantial curiosity in the déjà vu experience, there has been an insufficiency of studies conducted by researchers. The events of déjà vu fall short of an evoking stimulus, rendering it an event that happens without warning and one that lasts for a limited amount of time, making it challenging to be readily investigated by researchers, specifically in healthy subjects. Contrarily, déjà vu has been termed to be "pathological" in circumstances of epilepsy, making it easier to study in those with the disorder as déjà vu can be an initiating event prior to an epileptic episode, otherwise known as a seizure. Despite the difficulties in studying déjà vu, theories to understand this phenomenon have been established, demonstrating progress in this area of research. Since the 19th century, innumerable theories have been suggested to elucidate the events of déjà vu, varying from memories of a past life to delayed neural transmission and mismatches in these neural pathways. This paper is restricted to exploring only a few possible theories responsible for explaining déjà vu due to the vast number of theories.

History of Déjà vu
Déjà vu has been a popular term used in the mouths of many individuals, yet, the exact theory that underlies it is still substantially unknown and investigative due to its difficulty in studying. The proper definition behind déjà vu took almost a century to establish. In the 1990s, different investigators believed different psychologists were responsible for "discovering" the term déjà vu. By extension, Berrios (1995) believed that French psychologist Arnaud was the first to use déjà vu in 1896, whereas Sno (1994) declared that French psychologist Boirac was the earliest individual to apply déjà vu in his work "The Future of Psychic Sciences". Many speculations and uncertainties amidst the founder of déjà vu existed and the century of the mid-1800s to the mid-1900s also provided many discrepancies in the terms used to delineate the events of déjà vu. Many researchers suggested different definitions of déjà vu, believing that the phenomenon had to do with memory dysfunction or the fallacy of memory. The number of suggested terms over the centuries was astronomical; thus, this paper is limited on articulating the final, accepted term of déjà vu. After decades of proposing innumerable definitions, the final interpretation was suggested by Neppe (1983) and is now the most widely used term dictating that "any subjectively inappropriate impression of familiarity of a present experience with an undefined past" is what occurs during déjà vu.

Theories Underlying Déjà Vu
The events of déjà vu are experienced by approximately 60-80 percent of healthy subjects, being increasingly popular in young adulthood and decreasing with age as the younger brain is more susceptible to excitability, delineating that their occurrences are prevalent. However, déjà vu has also been investigated to transpire in clinical circumstances, specifically in those with temporal lobe epilepsy (TLE). In both healthy and pathological subjects, there have been abundant theories to suggest the underlying mechanism of déjà vu, serving as a point that the area is still widely investigated. This paper is limited to outlining only a few theories that have been suggested due to the wide variety of propositions that investigators have made. The theories of déjà vu that will be discussed are the dual processing, neurological, memory, and attentional theories, in addition to alternative abstract theories such as reincarnation and dreaming.

The Dual Processing Theory
The dual processing theory refers to the interruption in the normal workings of two unrelated but usually synchronous cognitive processes. Throughout each type of dual-processing theory, two mnemonic operations that typically work together may become unattached, or one process can work without the other, which is a theory thought to underlie déjà vu, possibly.

Familiarity and Retrieval
One type of dual processing is familiarity and retrieval. Gloor (1990) suggested that these are two unconventional cognitive functions that work in a coordinated way because memory recall goes along with a sense of familiarity. Nonetheless, these two processes may work without each other, indicating that an individual can retrieve information about memory without familiarity, leading to a familiar setting becoming quickly unfamiliar, otherwise known as jamais vu. On the other hand, one may be familiar with a particular situation without having retrieved the memory, known as déjà vu.

Perception and Memory
Another kind of dual processing is perception and memory, the notion that perception and memory happen synchronously where memory is not posterior to the emergence of perception; it is synchronous with it. This hypothesis epitomizes that when an individual perceives something, the memory of it is created at the same time. The perception of an event can be interrupted through distraction, inattention, or fatigue, meaning that memory and perception are no longer walking hand in hand. If information is stored only sometimes when the moment happens, and not all the time, it could lead to déjà vu.

Dual Consciousness
The final type of dual processing discussed in this paper is dual consciousness. This theory dates back to Hughlings-Jackson (1888), proposing that individuals have two types of consciousness: the normal, which processes incoming information from the outside world, and the parasitic, which overlooks thoughts and reflections from the inner, mental world. Distraction, fatigue, or TLE can impact the expected behavior of consciousness, lessening awareness, indicating that to perceive any familiarity of incoming sensory experiences, it must come from primitive internal consciousness that works on a mentally made image. Altogether, this contributes to a quick misinterpretation of a new experience as old, eliciting déjà vu.

Neurological Theory
Contrasting to the dual processing theory where two synchronous processes momentarily become asynchronous, the neurological conjecture proposes that the events of déjà vu arise due to a neurological problem including TLE/seizures or an alteration in the rate of regular neural transmission.

Temporal Lobe Epilepsy (TLE)/Seizures
The temporal lobe consists of different substructures responsible for memory acquisition and recognition of faces, objects, and places that seem familiar. On account of a lack of studies, it has not been entirely justified that déjà vu is unequivocally connected to the temporal lobe; however, some prior investigations and hints have pointed investigators towards this relationship. A type of neurological theory that may underlie déjà vu and that is responsible for pointing investigators towards a possible relationship between the temporal lobe, and déjà vu is TLE. TLE is a chronic disorder of the nervous system, consisting of recurrent, unjustified focal seizures that arise from the temporal lobe. TLE is considered the most common form of focal seizures, a type of epileptic event that arises in a specific lobe of the brain. Prior to a TLE seizure, individuals have reported various symptoms, amongst them, the déjà vu experience. Until this day, there has not been a precise statistic that stipulates the exact percentage of those with TLE who experience déjà vu prior to a seizure. However, sufficient studies have explored and associated déjà vu and TLE to report that a link may be undeniable. The concept that déjà vu is a pre-seizure warning in those with TLE has sparked investigators' curiosity into believing that déjà vu could arise from a small temporal lobe seizure in healthy subjects. However, insufficient research on this topic leads to an unresolved conclusion.

Neural Transmission Delay
The second type of neurological theory that has been hypothesized to underlie déjà vu is neural transmission delay, the idea that the events of déjà vu arise due to a brief delay in neural transmission from one perceptual organ to their higher-order processing regions in the brain. One type of neural transmission delay includes a slight elevation in the usual time taken to communicate the message to the higher-order processing centers from the perceptual organ on account of synaptic malfunction. This slight delay in communicating the message to the higher-order processing centers demonstrates that the information is old. An older investigation conducted by Anjel (1896) exemplified that déjà vu is often associated with exhaustive circumstances, possibly substantiating that these circumstances are responsible for the neural slowing, which lengthens the time taken between perception and transmission of the message. The reasoning behind this theory is minimally investigated and is therefore limited on account of being outdated.

Another kind of neural transmission delay consists of two perceptual pathways. The primary pathway is delayed, and the secondary pathway is not, meaning the information from the second pathway will appear somewhat before the information received from the primary pathway. As information from the primary pathway is an individual's primary perception of their surroundings, the information arrives following the secondary pathway information meaning it is perceived as a memory as a "memory match" has already been established.

Memory Explanations
The salient theme of the déjà vu encounter is the feeling of familiarity, which lacks any objective reference, like false memories. People may believe that situations have occurred previously or have been forgotten and then recovered subconsciously throughout the events of déjà vu. However, a better way to examine false memories as a theory of déjà vu is that only parts of these events have been experienced before where the sense of familiarity to a specific aspect of memory can be misrepresented to the whole memory that a person is experiencing déjà vu with.

Single Element Familiarity
The single element familiarity is one of the most sought-after theories of the déjà vu experience. This theory exemplifies that any single element in the memory experience during déjà vu may be impartially recognizable but is not deliberately recalled and leads to the déjà vu occurrence. For instance, one goes to a restaurant with friends where they see tablecloths lining the tables, identical to one of their friend's or relative's tablecloths from one of their parties. They feel an immense sense of familiarity but cannot recall whose tablecloths they were and when the party was held; thus, ascribing this single element of familiarity to the entire situation.

Multiple Element Familiarity
A similar but contrasting theory to the single element familiarity is the multiple-element familiarity. These theories are similar in eliciting the déjà vu encounter but differ in the sense that multiple individual elements elicit déjà vu as opposed to a single element.

Gestalt Familiarity Theory
 Alike to the multiple element familiarity theory, the gestalt familiarity theory requires multiple elements to evoke the déjà vu response; however, it is not the elements that lead to déjà vu but rather the organization of the multiple elements that are being recalled. By extension, if a grocery store is organized similarly to the one an individual has seen before, with fruits and vegetables at the front and meats at the back, the familiarity of this organization could lead to a familiarity of the whole experience.

Attentional Theories
The final theories that have been investigated to underlie déjà vu are the attentional theories. They elucidate that déja vu is a momentary hiccup in an individual's perception, where an individual's perception is broken down into two distinct perceptions due to distraction or inattention. For example, an individual perceives a specific situation with reduced attention but quickly perceives the same situation with full attention moments later. The second perception is the same as the first perception that occurred under minimal attention; however, the individual does not know the prior perception as only moments ago but instead recalls it as happening a while ago.

Perceptual occlusion
Perceptual occlusion is a type of attentional theory that exemplifies the déjà vu theory. It indicates that an individual has an occluded perception of a specific event followed by perceiving it even with complete comprehensibility moments later. Technically, only one perceptual experience occurs, the insight of focal stimulus; however, the change from the occluded perception to the understood perception makes the experience feel like it happens twice, leading to familiarity.

Reincarnation
Reincarnation may seem like a far-fetched theory to explain déjà vu; however, it has been questioned by individuals to possibly underlie the events of déjà vu. Reincarnation describes that individuals lived as someone else in a previous life before being born into this life. While some individuals have had intense recollections of past lives, believers of reincarnation postulate that most individuals move into their new life with no memories of their past life. People who believe in reincarnation as the theory underlying déjà vu postulate that déjà vu occurs in an unusual state of consciousness, meaning that reincarnation would describe the occurrence by attributing the moment to a signal from a previous life. By extension, people may perceive a specific smell or image from their previous life but cannot place where they saw this sound or image, eliciting déjà vu.

Dreams
Aside from the scientific theories that have been postulated to underlie déjà vu, individuals have delineated that their déjà vu encounters imitate a prior dream they have had. Zuger discovered that those with no dream recall had no déjà vu encounters but those who had stipulated dream recall had déjà vu experiences. Therefore, when people experience déjà vu, it could simply be that they recall something that they have previously dreamt about. However, there exists no neuroscientific proof to substantiate this claim.

Summary
The déjà vu experience is a universal phenomenon whose popularity has sparked much confusion and investigation since the 19th century. This paper was limited to explaining a portion of the theories, such as the dual-processing, neurological, memory, and attentional theories, in addition to déjà vus' more abstract conjectures of reincarnation and dreaming. The old age of these studies indicates that the research on the possible theories underlying déjà vu may be old, with newer theories to possibly look at. However, these four theories are the ones that researchers remain to still consider when investigating déjà vu.