‘I Have Experienced This Before’: What Actually Happens in the Brain During Déjà Vu?

Many people are familiar with the brief but intense feeling that the present situation has already been experienced at some point in the past. Déjà vu usually lasts only a few seconds, yet in that short time, it can disrupt our usual sense of time, memory, and reality. Although déjà vu is often presented in popular culture as a mysterious phenomenon, cognitive research helps demystify it by offering an empirically grounded yet no less fascinating explanation.

The term déjà vu originates from French and literally means ‘already seen’. However, according to Vytautas Jurkuvėnas, Associate Professor at the Institute of Psychology at the Faculty of Philosophy, Vilnius University, the term may be somewhat misleading: ‘In cases of déjà vu, we have not necessarily seen or experienced something before; rather, we experience a sense of familiarity without an actual memory. A feeling arises that the situation is familiar, yet at the same time we clearly understand that we do not remember the event itself.’

An evolutionary compromise, not a system error

Assoc. Prof. Jurkuvėnas explained that déjà vu is neither an echo of past lives nor a ‘contact’ with a parallel reality. It is a brief mismatch between several brain systems that work together every day to construct the reality as we experience it. Most of the time, this cooperative process goes unnoticed; however, occasionally different mechanisms generate conflicting information. It is precisely at such moments that we experience déjà vu.

‘Déjà vu is often described as a ‘memory error’, but that characterisation is overly simplistic. Our brains are not designed to function like a computer that precisely records, stores, and retrieves information’.

Human memory is imperfect, the researcher explained, but it was never meant to be perfect. ‘Memory evolved as a system whose primary goals are efficiency, speed, and the ability to predict. Therefore, our cognitive system operates probabilistically: it is usually highly reliable, yet occasional low-probability mismatches are unavoidable.’

‘In this context, I would not be inclined to call déjà vu an error or a flaw. Rather, it is a feature of our cognitive system. It reveals not a weakness of memory but the complexity of the entire system. If our memory functioned the way computer memory does, it would require far too many resources. Over the course of evolution, what developed was not maximum accuracy but a more flexible mode of operation,’ added Assoc. Prof. Jurkuvėnas.

The predictive brain and excess familiarity 

This probabilistic principle of operation becomes especially clear in another key feature of the cognitive system – constant prediction. According to the psychologist, the brain is continuously trying to anticipate what will happen in the next moment, in a few seconds, or even in the more distant future. This occurs both consciously and subconsciously and serves a highly practical function: it allows us to conserve biological resources and adapt to our environment more efficiently.

This mechanism is particularly evident in language. When listening to a sentence, we do not passively wait for the next word; instead, we constantly predict it. The researcher notes that we anticipate everyday situations in the same way – images, human behaviour, and surrounding circumstances. As we recognise our environment, we continually generate expectations about what information will reach the brain next and, on this basis, we shape our perception.

Déjà vu may occur when this predictive system generates a powerful signal of familiarity. The situation feels as though it has already been recognised by the brain, even though it has not been experienced before. As additional information becomes available, the brain detects that the prediction does not match reality – a mismatch arises between what was expected and what is actually happening.

Importantly, in such cases, another cognitive system rapidly evaluates this discrepancy. In the absence of other neurological disorders, the brain usually quickly recognises that the initial prediction was inaccurate and adjusts perception accordingly. For this reason, déjà vu is typically very brief and has no long-term consequences.

When one system says ‘I recognise this’ and another says ‘I do not remember it’

An essential aspect of explaining déjà vu lies in how different brain systems interact. Assoc. Prof. Jurkuvėnas emphasises that our cognitive system is not centralised. There are distinct mechanisms involved in recognition, the retrieval of specific episodes, verification, and the integration of information into a coherent picture of the world.

‘These systems operate relatively autonomously, and that is a major advantage. This principle is comparable to the separation of powers, in which state authority is distributed among different institutions to limit governmental power and preserve freedom and equality. Just as it is beneficial to a state when decisions do not come from a single centre but emerge from the interaction of different institutions, we also benefit from the absence of a single ‘central controller’ in the brain that dictates one absolute truth,’ the researcher explained.

Such a division ensures flexibility and allows systems to compete with one another, resulting in better decisions. However, this autonomy also means that systems may occasionally disagree. Déjà vu is precisely that brief moment when one system says, ‘I recognise this,’ while another says, ‘I do not remember it’.

This is not a hidden memory

Neuroscientific research shows that the medial temporal lobe, including the hippocampus – a structure that plays a key role in forming memories and generating familiarity signals – plays a vital role in déjà vu. However, the researcher points out that it would be misleading to explain déjà vu simply as a ‘mistake’ of the hippocampus.

The feeling of familiarity and the retrieval of a specific memory are not the same. Assoc. Prof. Jurkuvėnas explains that the experience can induce a sense of familiarity while at the same time making us aware that no specific episode is stored in memory. This mismatch occurs because different brain mechanisms are responsible for generating the signal of familiarity and for retrieving actual memories. As a result, we can feel that something is familiar even when there is no actual memory to relate it to. These processes operate at an additional level – metacognitive systems that allow us to evaluate the mismatch itself. These systems help us understand that the feelings we are experiencing do not correspond to reality; they enable us to interpret déjà vu as an unusual, but not necessarily problematic, phenomenon.

It is essential to distinguish déjà vu from other memory phenomena sometimes mentioned alongside it but operating differently. One such phenomenon is the so-called Proust effect, described in Marcel Proust’s novel ‘In Search of Lost Time’, in which a scent, taste, or another sensory stimulus suddenly evokes a vivid memory from the past. In these cases, memory retrieves a specific episode, even if the recollection occurs involuntarily. 

‘The Proust effect is an actual memory; it simply emerges subconsciously. In the case of déjà vu, a sense of familiarity arises, but there is no memory that can be retrieved. This shows that déjà vu is not a hidden memory,’ Assoc. Prof. Jurkuvėnas concluded.

Cognitive noise and its regulatory mechanisms

Although déjà vu can occur in entirely ordinary situations, its likelihood increases under certain conditions. Assoc. Prof. Jurkuvėnas explained that tiredness, stress, or lack of sleep amplify what is referred to as ‘noise’ in brain systems – the unavoidable inaccuracies in information processing inherent to biological systems.

Neurons never transmit information with perfect precision; however, under normal conditions, these minor discrepancies are constantly corrected. A large part of the cognitive system’s work is specifically devoted to controlling this noise: suppressing irrelevant signals, correcting errors, and structuring information. Sleep plays a crucial role here – one of its functions is to optimise memory systems and reduce the noise accumulated during the day.

When a person experiences insufficient sleep or prolonged stress, the efficiency of these mechanisms declines. The system of suppression or inhibition that coordinates the activity of different brain processes becomes weaker. As a result, mechanisms responsible for familiarity, memory retrieval, and verification may more easily fall out of synchrony, increasing the probability of experiencing déjà vu.

This phenomenon also highlights how many mental processes occur subconsciously. According to the psychologist, consciousness is not the primary driver of cognition – it functions more as an observer, receiving information that has already been processed. Déjà vu is a moment when this usually invisible work briefly emerges into awareness.

A signal that reality is constructed 

In popular culture, déjà vu is often explained as a connection with parallel realities or alternative versions of the self. Assoc. Prof. Jurkuvėnas asserts that such stories reveal more about our imagination and cultural narratives than about the way the brain functions in reality. However, they can still be meaningful as metaphors.

In this context, it is worth recalling Jorge Luis Borges’ short story ‘The Garden of Forking Paths’, in which time is depicted as an ever-multiplying array of possibilities. The brain is constantly choosing among different interpretations: how to understand a situation, what to expect, and how to relate new information to prior experience. Without this constant selection process, we would simply become lost in the abundance of information.

From this perspective, déjà vu can be understood as the moment when one of the possible interpretations briefly ‘outpaces’ reality. This does not imply that a different reality exists; rather, it reminds us that the reality we experience is not given, but is constantly being constructed. Ultimately, déjà vu is neither a memory defect nor a hidden signal from another world. It is a brief moment when different brain systems fall out of sync. This mismatch fades quickly, yet it offers a glimpse into the complex and dynamic functioning of our cognitive system.