One drab afternoon a few years ago something very unusual happened to me.

I was lounging under a tree in a packed east London park when I experienced a sudden feeling of vertigo, followed immediately by an overwhelming and intense sense of familiarity.

The people around me vanished and I found myself lying on a tartan picnic blanket amid a field of high golden wheat. The memory was rich and detailed. I could hear the sway of the wheat ears as a gentle breeze brushed through them. I felt warm sunlight on the back of my neck and watched as birds wheeled and floated above me.

It was a pleasant and extremely vivid recollection. The problem was that it never actually happened. What I was experiencing was an extreme form of a very common mental illusion: déjà vu.

We view our memories as sacred. One of the most fundamental doctrines of Western philosophy was established by Aristotle. He saw a newborn baby as a kind of empty ledger, one that is gradually filled as the child grows and accumulates knowledge and experience.

Whether it’s how to tie a shoelace or recalling your first day at school, memories make up the autobiographical map that helps us navigate the present day. Jingles from old television adverts, the name of the second-to-last prime minister, the punchline to a joke: memories are the constituent parts of individual identities.

Most of the time memory systems run quietly in the background as we go about the business of everyday life. We take their efficiency for granted. Until, that is, they fail.

For the past five years I have been suffering epileptic seizures resulting from the growth and eventual removal of a lemon-sized tumour from the right-hand side of my brain. Before my diagnosis I appeared fit and healthy: I was in my mid-30s and displayed absolutely no symptoms. Until, that is, the afternoon that I woke up on the kitchen floor with two black eyes after suffering my first recorded seizure.

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.”

© John Gribben at Kiosk

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 ten 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’.