What epilepsy taught us about déjà vu

I walked into the building as if I lived there.

I climbed the staircase, turned the right corner on the right floor, and pushed open the door of my old apartment without a single hesitation, the way you open a door when you're coming home from the office, although I was visiting there after two months.

I knocked on the door, and the door was open as usual. And there she was, my ex-roommate. She was shooting her outfit for the day reel as usual. I walked to my old room. The one who had technically stopped being mine two months ago. I dropped onto the bed. And stared at the ceiling. She went for a shower. Only after asking me whether I wanted to have tea. Of course, I was never allowed to make tea in her presence. Her belief was that she was a tea expert, and no one could make a better tea than her.

It was our ritual. Every evening, always. I had moved out because my office location changed, and honestly, because I needed a space that was only mine, a quieter kind of solitude. But that day, I realised I was enjoying living with her more. The two months hadn't felt like two months. It felt like an ordinary day. Like I had just walked back from the office.

I told her: "I don't feel like I've come back after two months. It feels like just another day. Not for once."

“There is a specific term for this feeling”, I said, and I started recalling…

She laughed and said, "You're experiencing déjà vu. It’s called déjà vu, my babe"

I had heard the phrase before as well. The way most people have loosely, casually, as a label for something uncanny. But I had never been so curious about it. That day, the feeling was so strong, so I looked it up.

What I found stopped me.

The Word That Means "Already Seen"

Déjà vu is French for "already seen." In scientific literature, it is defined as any subjectively inappropriate impression of familiarity for a present experience, a moment your brain decides it recognises something it cannot possibly have encountered before, or not in the way it's being encountered right now.

It lasts, on average, about two seconds.

And yet those two seconds have puzzled neurologists, psychologists, and philosophers for over a century. What is happening inside the brain during that brief crack in time? To answer that, researchers had to study the people who experience déjà vu not as a flicker, but as a flood, people with temporal lobe epilepsy.

What the Epileptic Brain Revealed

Temporal lobe epilepsy is a neurological condition where seizures originate in the temporal lobe, the part of your brain most involved in memory and sensory experience. For decades, neurologists noticed something strange in their patients: many of them reported experiencing déjà vu just before a seizure arrived. It came like a herald. Eerily calm, disturbingly familiar, and then gone.

Clinical reports show that patients with temporal lobe epilepsy experience déjà vu almost as a warning sign before a seizure event; the déjà vu arrives first, and the seizure follows. This was not just a passing observation. It provided researchers with a population of people who experienced déjà vu in a predictable, repeated, and intense manner that allowed for study.

They found a specific structure deep in the brain called the parahippocampal gyrus.

The parahippocampal gyrus sits in the medial temporal lobe and is responsible for a very particular kind of knowing, not full recollection, but familiarity. The feeling that something is known, even when you can't quite say why or how. In temporal lobe epilepsy patients who experience déjà vu, PET scans have revealed hypometabolism, reduced activity in this exact region, particularly in the parahippocampal and perirhinal cortex.

In simple terms: the brain's familiarity detection system misfires. It fires without any real memory to back it up. The sensation of recognition floods in, but when the rest of the brain- the hippocampus and the prefrontal cortex- goes looking for the actual memory? There's nothing there. The alarm rang, but no one broke in.

This is what's called the decoupled familiarity hypothesis. Déjà vu happens when the sense of familiarity becomes unhooked from the process of actual recollection. The brain says: I know this. But the evidence says: No, you don't. The two conflict, briefly, and in that conflict lives the uncanny feeling of déjà vu.

It's Not Just Epilepsy. It Happens to Most of Us.

The part that surprised me is that studies suggest that somewhere between 60 and 80 percent of people have experienced déjà vu at least once in their lives. It is remarkably common and almost exclusively harmless.

The same misfiring that occurs dramatically in epilepsy can happen, at a tiny scale, in a completely healthy brain. Electrical stimulation of the entorhinal and perirhinal cortical regions inside the medial temporal lobe has been shown to elicit déjà vu more reliably than stimulation of the amygdala or hippocampus alone. This suggests déjà vu isn't some mysterious aberration. It is a low-level version of what epileptic brains experience in a larger way.

Another popular theory suggests something slightly different: a dual-processing delay. Your brain is always running two pathways: one scans sensory input, and one processes context a fraction of a second behind. If the second signal arrives just a little late, your brain can interpret the same single experience as having occurred twice: just now, and just before now. The present moment becomes its own echo.

What causes these feelings in a healthy individual? Fatigue and stress are key factors. This little realisation really opened my eyes to my own journey: the amazing way that travel can change us for the better!

Why People Who Travel Experience It More

Research has confirmed that people who travel frequently report significantly higher rates of déjà vu.

One study found that only 11% of people who didn't travel had ever experienced déjà vu, compared to 32% among those who travelled up to five times a year.

Déjà vu is also most common in younger adults, particularly between ages 15 and 25, and declines steadily with age. Across more than 30 separate studies, this age effect has held consistently.

The reasoning makes sense when you think about what déjà vu actually is. When I think about my brain, it has encountered many new places, new rooms, new routines; it is also a brain that is constantly forming new memory fragments, partial scenes, sensory impressions, and spatial layouts. If enough of those fragments accumulate, the chances of a partial overlap of this new hallway vaguely echoing the architecture of something stored somewhere go up. The familiarity alarm can be triggered by resemblance, not repetition.

No wonder this year I was jumping from Kolkata to Haryana to Hyderabad to Chandigarh to Maharashtra and then back to Haryana. It was not done here, so I was commuting home almost every weekend. In twelve months, I had lived in five states. My brain had been collecting fragments of rooms, staircases, rituals, and neighbourhoods at a pace that most people don't. Every two months, a new normal had to be built from scratch.

Is it any wonder that when I walked into my old apartment, something misfired?

What I Think Actually Happened

When I walked back into that room, it wasn't new. But my brain couldn't find the right file for it either. It didn't have a slot labelled "coming back after two months." What it had was a much older slot, one built over months of habit, the ritual of the evening, my roommate's reel shoots, the particular sound of her shower, the request for tea. All of it was stored not as memory but as a pattern. The parahippocampal gyrus doesn't care about timestamps. It recognises shapes.

And the shape of that evening matched the shape of hundreds of evenings before it.

So the brain said: already seen. Even though it wasn't. Even though two months had passed, and I was no longer living there, everything had technically changed.

There was something else, though. Beneath the neurological explanation, something more personal was happening.

The Realisation Inside the Feeling

I have been thinking about this since. There is a theory that déjà vu can sometimes be triggered not just by a sensory match, but by an emotional one, a moment when the brain recognises a pattern of feeling, not just a pattern of space. And I think that's what happened in that room.

I was happy there. Not in a dramatic way, just in the ordinary, accumulating way that good routines make you happy without you ever consciously deciding to be. Evening tea. Someone is getting ready nearby. A room I had made mine. I had moved out and built my own space for good reasons. But there, sitting on that bed, some part of my brain said: this is known. This is right. This has happened before.

Not because it was the same day. But because the emotional frequency was identical.

And then, quieter than that: I was not going back. This was not another Thursday. This was a visit, and visits end. The déjà vu felt so strong to me, I think, because it was the last time that pattern would repeat. My brain was saying already seen precisely at the moment that part of my life was ending.

The Three Explanations That Might All Be True

Science calls déjà vu a memory misfiring. Philosophy calls it proof that the self is unstable. Religion calls it the soul remembering. Maybe it's all three.

I have spent enough time with this now to believe that, yes, maybe it's all three.

Science is right that the parahippocampal gyrus misfires. That's real. That's measurable. But the science doesn't explain why that misfire arrives loaded with meaning. Why do we feel, in that two-second crack, something more than confusion? Why does it feel like recognition of something that matters?

Philosophy is right that the self is unstable. I was not the same person who had lived in that room. I had changed cities, changed routines, changed what I needed. And yet something essential, some version of me built in that room over those months, was still there. Déjà vu is what happens when you catch the ghost of a former self walking through the present.

And maybe the spiritual instinct isn't wrong either. Not in a doctrinal sense, but in the way that some moments arrive pre-weighted as if they have already happened somewhere, in some earlier register of the self. The soul, if you believe in one, might not track time the way the brain does.

What Two Seconds Can Hold

Déjà vu lasts about two seconds. In those two seconds, according to the research, your parahippocampal gyrus fires out of sync, your prefrontal cortex notices the mismatch, and your hippocampus comes up empty. The result is a sensation that something is both new and known, a moment of double exposure.

But for me, standing in that apartment that evening, holding a cup of tea my roommate had made me, those two seconds held the weight of an entire chapter of my life. The staircase. The reel outfits. Five states in a year. The need for space and the grief of finding it. All of it, compressed into a feeling so familiar it made the present moment flicker.

I experienced déjà vu for what I think might be the last time in that place. And I was glad for it. Not despite its strangeness, but because of it.

For two seconds, time cracked open. And I caught my own consciousness in the act of remembering what it was like to be there before I left for the last time.

If this found you at the end of something, or the beginning of something that feels suspiciously like it's happened before, you're not imagining it. Your brain is just doing what brains do: looking for the shape of home inside everything new.

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