Have you ever had that sensation where you feel like you’re reliving a moment from your past, like you’ve “been here” or “done this” before? The chances are that you have, as 60-70% of people have experienced déjà vu: the phenomenon of thinking you’re in a familiar situation which in fact may or may not have happened before. This surging wave of familiarity can knock you a little off guard and can be quite disconcerting to some, but rest assured it’s totally harmless. Most experiences of déjà vu are over within seconds and you’re fully aware that’s it’s actually déjà vu.
The majority of déjà vu cases occur in healthy people with no strong links to medical conditions. It can, however, occur with increased frequency in patients with temporal lobe epilepsy, with which it has a strong association. This form of epilepsy is concurrent with hippocampal neuronal and interneuronal loss which may give rise to the over-excitability and hyperactivity of misfiring epileptic neuronal networks. Studies have shown that most of the neuronal alterations that occur as part of natural, experimentally induced, or epilepsy-related déjà vu happen in the medial part of the temporal lobe, and that cortical stimulation of the entorhinal cortex housed within the medial temporal lobe is sufficient to stimulate a déjà vu experience.
A paper from 2012 found that synchronous firing between the entorhinal cortex and the hippocampus where upregulated when déjà vu-inducing stimulations were increased implying that paired firing within the medial temporal lobe could trigger aberrant memory recall.
So alterations in memory function may make a person think they’ve lived a particular moment before. In some situations, the triggering experience overlaps with a separate stored memory. You might be led to believe that this would just manifest as a new experience and therefore form a new memory, however the person does not know what has triggered the sensation so experiences déjà vu, i.e. you recognise the situation but can’t identify where you know it from.
So for a sensation of déjà vu, you need to recognise and recall the situation, and feel like it’s familiar but know that it’s false. For déjà vu to happen, the recalled “memory” is initiated by a very small amount of incoming sensory information (e.g. a single object or a smell). One theory suggests that there is a miswiring between the input (stimulation) and output (memory) pathways. Another theory suggests that it is not the sensory input pathway, but the long-term and short term circuits that are derailed. In this way, an encounter or occasion may enter the long term memory without proper transferral via rehearsal or repetition. Therefore when we encounter a familiar situation it seems as though it’s being recalled from the long term memory, i.e. way back in the past. Long-term memory also comes into play in a seperate hypothesis that states that when we encounter a moment, it’s being simultaneously entered into long term storage such that its automatically thought of as a memory.
In essence, a strong “feeling of knowing” presents itself as déjà vu due to an inappropriate overreaction of the memory centres of the brain to a seemingly familiar situation.
So the next time you get that surge of familiarity, don’t forget to check back here to remind yourself what’s really going on!
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O’Connor AR, & Moulin CJ (2013). Déjà vu experiences in healthy subjects are unrelated to laboratory tests of recollection and familiarity for word stimuli. Frontiers in psychology, 4 PMID: 24409159
Bartolomei F, Barbeau EJ, Nguyen T, McGonigal A, Régis J, Chauvel P, & Wendling F (2012). Rhinal-hippocampal interactions during déjà vu. Clinical neurophysiology : official journal of the International Federation of Clinical Neurophysiology, 123 (3), 489-95 PMID: 21924679