Have you ever been overcome by the overwhelming sensation that you have experienced this exact situation before? This is the phenomenon of déjà vu – the two-fold feeling that something is familiar but the knowledge that it shouldn’t be.
Most people, approximately 2/3rds , report experiencing the fleeting sense of déjà vu, which is French for “already seen”. Interestingly, it is most common between the ages of 15-25 and is more likely to occur during times of stress or exhaustion. Despite the high prevalence, scientists do not understand the underlying cause. The feeling itself is hard to study because spontaneous déjà vu cannot be induced or created by researchers.
So, what causes this feeling? Research suggests it may be the result of a momentary hiccup in a basic memory process. The brain is constantly scanning the environment to identify familiar or new things. This process, termed recognition memory, depends on the brain’s ability to recognize something stored in our memory. Recognition memory is an essential function that helps us navigate and interact with the world. Recognition happens in two ways: familiarity, where we have a feeling that we have seen or experienced something before but might not remember when or where, and recollection, where we remember the precise details of a specific situation or event. For example, imagine visiting a coffee shop and realizing you’ve been there before. If you recognize the décor and layout but cannot remember when or why you were there, your recognition is based on familiarity (i.e., “you know this place”). In contrast, if you know right away that you were there on a certain date with a certain friend, your recognition would be based on recollection of that event (i.e., “you remember this place”).
Although controversial, research suggests that familiarity and recollection rely on two different structures within the temporal lobe of the brain, namely the rhinal cortex and hippocampus, respectively. One theory suggests that déjà vu is caused by abnormal activity of these memory related brain structures, resulting in an incorrect feeling of familiarity (e.g., “I feel like I’ve been here before”). This suggests that abnormal activity in the rhinal cortex, which is responsible for assessing familiarity, is likely involved in déjà vu. However, déjà vu is not only the feeling of familiarity. It is also accompanied by the feeling that this familiarity is wrong (e.g., “I feel like I’ve been here before, but I know I have not”), which may be based on recollection of our past experiences. What is not yet known is whether this recollection process is needed in order to experience déjà vu. In other words, does déjà vu rely on recollection, and the hippocampal structures that support it, to create the feeling of déjà vu?
Brain model with the temporal lobe, the region affected in temporal lobe epilepsy, highlighted in green. Image from: https://www.ucf.edu/pegasus/your-brain-on-music/
As déjà vu cannot be readily induced, figuring out how it works has eluded scientists, until recently. A possible window into the brain mechanisms of déjà vu comes from a disorder of abnormal brain activity: epilepsy. Specifically, patients with one type of epilepsy, called medial temporal lobe epilepsy (TLE), experience déjà vu immediately before seizures. Importantly, déjà vu is not experienced in all individuals with epilepsy. As different forms of epilepsy result from unusual activity in specific brain structures, investigating how individuals with different kinds of epilepsy experience déjà vu can provide unique insight into this complicated phenomenon.
Research at Western University, led by Dr. Stefan Köhler, has previously shown that patients with unilateral TLE (seizures that occur on one side of the brain) who experience déjà vu are more likely to have deficits in their ability to assess whether something is familiar or new (Martin et al., 2012). Further, they are more likely to have abnormalities in the rhinal cortex – the medial temporal lobe structure involved in familiarity recognition. These findings support the idea that déjà vu may involve dysfunctions in familiarity recognition and hint at the rhinal cortex as a key structure underlying this phenomenon. However, patients with damage to one side of the temporal lobe (unilateral TLE) may still have functioning recollection processes. While this study implicates the rhinal cortex and familiarity recognition in déjà vu, it is still unknown whether recollection (and the hippocampus) plays a role. In contrast to unilateral TLE, patients with bilateral TLE (seizures that occur on both sides of the brain) have more widespread damage to the temporal lobes and tend to have more difficulty with recollection.
In a new study published in the journal Memory, Dr. Köhler and his team investigated whether patients that experience déjà vu and have bilateral TLE show similar deficits to patients with unilateral TLE. Studying bilateral TLE patients allowed researchers to examine whether déjà vu could occur with impaired recollection. The authors reasoned that if the process of recollection was essential to the identification of false familiarity during déjà vu, patients that experience déjà vu should not have problems in recollection. On the contrary, if recollection is not essential in creating the feeling of déjà vu, patients that have deficits in recollection should still be able to experience déjà vu.
The authors analyzed recollection and familiarity using recognition memory tasks. For example, the Remember-Know task involved showing participants sets of pictures in an initial session and a second session and asking them whether the pictures were new or old and whether they specifically remembered it or if they just knew. The latter question aimed to assess whether they were working off recollection or familiarity, respectively. The authors also used magnetic resonance imaging (MRI), a technique that produces detailed images of the brain, to assess the volume of medial temporal lobe brain structures. Large reductions in volume can indicate dysfunction in specific regions.
They found that, in addition to deficits in familiarity, bilateral TLE patients also struggled with recollection memory and accordingly had abnormalities in the hippocampus – a brain structure in the temporal lobe region that is implicated in recollection. Importantly, despite the deficits in recollection memory performance, these patients still experienced déjà vu. Therefore, the researchers concluded that déjà vu likely does not depend on recollection, and by extension may also not involve the hippocampus.
While the results do not completely rule out a role for recollection in déjà vu, they strongly support the involvement of abnormal familiarity assessment, a process likely dependent on the rhinal cortex – another brain structure in the temporal lobe. The authors add that more research investigating the connections between different brain areas is needed to tease out the specific involvement of the hippocampus versus the rhinal cortex in déjà vu. So, the next time you experience the eerie sensation of déjà vu, remember that it is just a byproduct of your brain trying to help you process memories and figure out what is familiar in the world.
The study, “Relationship between déjà vu experiences and recognition-memory impairments in temporal-lobe epilepsy”, was published in July 2019 in Memory.
Original Research Article:
Martin CB, Mirsattari SM, Pruessner JC, Burneo JG, Hayman-Abello B, Köhler S. Relationship between déjà vu experiences and recognition-memory impairments in temporal-lobe epilepsy. Memory. July 2019:1-11. doi:10.1080/09658211.2019.1643891
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