Seizures, or fits, occur after an unanticipated electrical discharge in the brain. They are usually preceded by something called an ‘aura’, a sort of minor foreshock lasting anything up to a couple of minutes before the main event begins. The nature of this aura differs greatly from patient to patient. Some people experience synaesthesia, extreme euphoria and even orgasm at the onset of a seizure. My own aren’t nearly as exciting-sounding, being distinguished by sudden shifts in perspective, a rapidly increased heart rate, anxiety, and the occasional auditory hallucination.

Pioneering English neurologist John Hughlings Jackson was the first to define the epileptic aura, observing in 1898 that its hallmarks included vivid memory-like hallucinations, often alongside the feeling of déjà vu. “Old scenes revert,” one patient told him. “I feel,” said another, “in some strange place.”

By far the most significant trait of my aura is the striking sense of having lived through that precise moment before at some point in the past – even though I never have. During my most intense seizures, and for a week or so afterwards, this feeling of precognition becomes so pervasive that I routinely struggle to discern the difference between lived events and dreams, between memories, hallucinations and the products of my imagination.

I don’t remember déjà vu happening with any kind of regularity before the onset of my epilepsy. Now it occurs with varying degrees of magnitude up to 10 times a day, whether as part of a seizure or not. I can find no pattern to explain when or why these episodes manifest themselves, only that they usually last for the length of a pulse before vanishing.

Many of the estimated 50 million people in the world with epilepsy experience long-term memory decline and psychiatric problems. And it’s hard for me not to worry whether the blurring of fact and fiction that I experience might one day engender a kind of mania. By trying to understand more about déjà vu, I’m hoping to make sure that I never lose my way on the path back to reality from that same ‘strange place’.

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In Catch-22, Joseph Heller described déjà vu as “a weird, occult sensation of having experienced the identical situation before in some prior time or existence”. Peter Cook put it his own way in a magazine column: “All of us at one time or another have had a sense of déjà vu, a feeling that this has happened before, that this has happened before, that this has happened before.”

Taken from the French for ‘already seen’, déjà vu is one of a group of related quirks of memory. Research from 50 different surveys suggests that around two-thirds of healthy people have experienced déjà vu at one time or another. For the majority, it is dismissed as a curiosity or a mildly interesting cognitive illusion.

While déjà vu is instantaneous and fleeting, déjà vécu (already lived) is far more troubling. Unlike déjà vu, déjà vécu involves the sensation that a whole sequence of events has been lived through before. What’s more, it lacks both the startling aspect and instantly dismissible quality of déjà vu.