She was given relaxant drugs like diazepam, but they didn’t work. In 2004, still in search of an effective treatment, the woman saw a neurologist who gave her 5-HTP, a naturally occurring amino acid that affects the central nervous system. It made the popping go away, but brought on a strange side effect.

In 1999, a 42-year-old woman went to the doctor for what she described as a popping noise in both her ears. The noise was so loud it had started to keep her up at night. The woman was diagnosed with Palatal Tremor, a movement disorder of some of the muscles at the back of the throat, in which they contract and cause a clicking sound.

We know déjà vu as a weird fleeting moment. You pause and bask in the strangeness, then move on—perhaps not to feel it again for months or years. We brush it off in our daily lives but déjà vu, says Adam Zeman, a clinical neurologist at the University of Exeter in the UK, can be a window into the many ways our brain regulates memory, familiarity, and other related processes. (From this particular patient, Zeman says, we may have learned that serotonin is involved in the genesis of déjà vu; the drugs she was taking were serotonergic, and they triggered the episodes.)

Déjà vu is French for already seen. It’s thought to be very common, having occurred in between 30 and 96 percent of the population, and usually lasts only seconds. It can be triggered by fatigue or emotional stress and is most frequent in our 30s, tapering off after that. People report more déjà vu the more years of education they’ve had. It’s more common at night than in the morning. People who travel regularly and people who remember their dreams are more prone to déjà vu**.**

“Since the doctor did not think it was the pills, I decided to try [them] again,” she wrote. “I had the same feeling of having seen and done all of this before. I did not have any eerie type of feeling, I knew I couldn’t know these things but I felt like I did.”

But her sister hadn't told her that before, and there hadn't been a power outage at the school a few days earlier. Instead, the woman was having an extreme, and long-lasting, episode of déjà vu.

“When I watched TV, I felt I was watching repeats, although I knew I wasn’t, as it was the news,” she wrote at the time (her account was later documented in a 2007 case report ). “I then got a phone call from my sister to tell me the kids were being sent home as there was a power cut at school. I asked her why she was telling me this again as she had told me this several days before.”

Zeman says that because scientists are sometimes able to record directly from the brains of people with epilepsy—they have more reasons to undergo neurosurgery, after all—we’ve been able to determine which parts of the brain are associated with déjà vu. In the late 1950s , researchers found through electrical stimulation and recording of seizures that the temporal neocortex was primarily involved. In the late 1970s , it was shown that you could provoke déjà vu through electrodes in the medial temporal lobe, and more recent work has found with even more specificity which parts of the medial temporal lobe are associated with déjà vu.

But déjà vu has also long been recognized as an aura of temporal lobe epilepsy , or the beginning of a seizure. For a person with epilepsy, a feeling of déjà vu can mean they will soon start seizing or lose consciousness. Zeman collaborated on a study a few years ago that asked whether there was anything about an epileptic’s déjà vu that was different from déjà vu experienced by a healthy brain; the answer seems to be no. It might be slightly more drawn out or occur more frequently in epileptic people, but it's otherwise the same.

With déjà vu, we get an eerie sense that our surroundings or current experiences are familiar, but we simultaneously recognize there's something “slightly bogus” about that familiarity, Zeman tells me.

Zeman says that even though we tend to lump déjà-vu-like experiences together, researchers are now distinguishing between different kinds of déjà vu. Some are so intense you might realize you have never experienced them, even if you consider yourself a déjà vu veteran.

“Maybe you’re sitting in a café where you've never been before, you get activity in the familiarity area, and suddenly the whole experience of sitting there seems intensely familiar, but you have no way of justifying that because you can't,” he says. “Because you have never been there before. You can't fish out a memory. That's why this strange combination leads to the feeling that the familiarity is false.”

The current theory is that epileptic déjà vu is caused by abnormal discharges of electricity in that familiarity region. When that area is hyperactive, you feel familiarity that's not accompanied by recollection—which is why it feels so weird.

“Say your boss comes into the room and you look up—you have an immediate familiarity response,” Zeman says. That’s a little different from recollection, which would be like you asking yourself when you last saw your boss.”

I had always assumed that déjà vu was a phenomenon of the hippocampus, our main memory center. But Zeman says that it more likely involves an adjacent area, called the perirhinal cortex, which allows us to recognize when things are familiar.

The medial temporal lobe includes the hippocampus and the parahippocampal gyrus, both of which are important in memory. The famous patient HM had epilepsy in both of his medial temporal lobes, and they were removed to stop his seizures. Back then, it wasn’t known how crucial those brain regions were, and HM’s life essentially came to a grinding halt. He was unable to form any new conscious memories from then on, not remembering the doctors and others who came to treat or visit him.

How can this warning bell for a seizure manifest in people who don't have epilepsy? Zeman says we don’t completely know. One theory is that ordinary déjà vu is a kind of seizure that healthy people experience. (Wilder Penfield, a famed Canadian neurosurgeon even called déjà vu “little seizures.”)

I tend to enjoy getting déjà vu. It has a way of making even the most mundane details take on a magical air. But Zeman says that people with epilepsy can be bothered by it. Not only is their experience of it heightened, and often longer, it can also be accompanied by fear and anxiety, since it is usually followed by a full-blown seizure.

In Capgras syndrome, it’s actually the amygdala, which is involved in emotional judgments, that goes awry, he says. You can recognize someone as familiar, but not have that supported by the right emotions, the ones you would normally feel looking at your husband, wife, or friend. “It might lead you to the conclusion that your relative must've been replaced by somebody else,” he explains. “They look as they should, but they don't feel as they should.”

I tell Zeman that it sounds to me like the opposite of Capgras syndrome, or imposter syndrome—when you think that people who you know, like your spouse or best friend—are actually imposters. But Zeman corrects me again, and reveals how subtly varied these concepts are in the brain—even seemingly basic tasks like seeing and recognizing people.

Moulin, a cognitive neuropsychologist at Université Grenoble Alpes, is now one of the leading researchers in déjà vu today. He became interested in déjà vu when, after meeting this man, he consulted his handbook of memory disorders and found no details on it, aside from a small mention of temporal lobe epilepsy.

He wasn’t forgetful; in fact, he claimed that everything he did he had already done: He wouldn’t watch TV because he said he had seen all the shows before; he refused to read the newspaper because “even the news seemed the same,” Moulin remembers. The man said he had already met Moulin before, too. He had already done all the tests Moulin was doing on him, had been asked all of the questions Moulin was asking him. He appeared to be stuck in some kind of time loop.

“But I think that the truthful answer," he says, "is that we still don't know what the underlying mechanism of normal physiological déjà vu is."

But that idea isn't widely accepted by many neuroscientists, and it's hard to find any evidence for it because most healthy people don’t get intracranial brain recordings. One study that Zeman collaborated on found that there might be a subtle reduction in volume in areas like the medial temporal lobe in healthy people who experience déjà vu, compared to those who said they'd never had it. There's some thought that a change in the structure of their brain might explain how a person without epilepsy could have déjà vu.

The confabulations of the man with Alzheimer’s went like this: When his wife would bring him the paper in the morning, and he wouldn’t read it, he would say, "When you were asleep in bed, I actually got up, went to the newsstand, and they were unloading the newspaper from the newspaper shop, and that's how I already read it. Then I snuck back in the house, came back to bed, and then went back to sleep.” Of course, none of it was true. It was a way for him to explain to himself how the paper seemed so overwhelmingly familiar.

“Typically, when we have déjà vu we know that there's something at fault,” Moulin says. “We know that we're finding something familiar but it soon passes and we think, ‘Oh, that was strange.’ There's a conflict in our interpretations. But some people are seduced by their feelings of familiarity so they really do think that they had the conversation before. Then they justify reasons for how they could have had the conversation before, or read the newspaper before, and they invent stories to justify this strong belief. This is a memory disorder called confabulation.”

Moulin says this man represented an extreme case. Now that he’s been studying déjà vu for a while, he makes the distinction between those who know that their sense of familiarity can’t be right, and those who think they truly have experienced a given moment before.

“That started a kind of accidental career in déjà vu,” he tells me. “I am a memory researcher, and do other things. I used to say that was my day job, and my hobby was déjà vu, but now the déjà vu is slowly taking over.”

That bit of it—the awareness part—led Moulin and others to think the prefrontal cortex is involved too, as a kind of control mechanism that helps monitor and organize the entire memory system. In the case of déjà vu, the prefrontal cortex is watching what's going on in the memory regions, and then detects a conflict between what's logically possible and what the memory system is saying is familiar. “That again fits in with this story that normal déjà vu is a healthy thing,” Moulin says—because your prefrontal cortex is astute enough to notice that this over-activity in the memory system isn’t quite right.

Moulin tells me that the latest theories on déjà vu involve other parts of the brain aside from just the familiarity regions. He says that the temporal-lobe hypothesis came about largely because you can artificially induce déjà vu by stimulating those areas. But Moulin thinks that might not be enough. Over-activity just in the familiarity areas would lead only to a feeling of familiarity—like thinking you know somebody on the street when you don’t. But the typical déjà vu isn’t only familiarity but also the self-awareness that that familiarity is false.

In general, Moulin thinks that there is probably a spectrum of déjà vu, from the intensity of the experience itself to how much you know it to be false. In healthy people, he now considers being able to recognize when you have déjà vu a healthy symptom. If you feel that something is overly familiar, but you know that’s not right, it’s a sign that you're highly cognizant of what's going on in your brain.

These two different experiences might implicate different brain regions: While déjà vu is coming from the perirhinal cortex, or the familiarity region, déjà vécu might be involving the hippocampus, because it’s more like an actual false recollection.

The second kind, Moulin calls déjà vécu , which translates to already lived. People with déjà vécu don’t only feel as if something is familiar, it really seems that they have lived that moment before, and that they know what will happen next—but still with an overarching knowledge that those memories can't be real. (This is unlike confabulation, which has no self-awareness that the experiences a person remembers never happened.) Déjà vécu can include recalled fragments of memories, “as if you're on the verge of actually pinpointing the déjà vu and it may be combined with a feeling that you can sense the future," he says.

Moulin distinguishes between two kind of déjà vu’s, which have been accepted by other déjà vu researchers. One is the classic déjà vu—a sense of familiarity with your current experience that you know is false .

“Young people get it more than old people get it,” he says. “I think as you get older, you lose the precision in the memory system and the fine-grained control of what's happening in that system. One journalist I talked to described it as a fact-checking system, and I really like that idea: That déjà vu is just a sign that there's something to stop you getting carried away with your sense of familiarity.”

When she recovered from her déjà vu, though, she could no longer recognize faces, a condition called prosopagnosia. Zeman says they found a vascular abnormality in the part of the brain where facial recognition takes place. That abnormality had initially manifested as déjà vu before emerging as prosopagnosia. When Zeman met her in 2000, she couldn’t recognize famous people on TV and her relatives had to introduce themselves by name each time they came to see her.

When she was examined, they found that she had had epilepsy when she was younger. Through EEG they noticed abnormal activity in the right side of her brain and diagnosed her with nonconvulsive status epilepticus, or epilepsy without seizures. After being treated for her epilepsy, the déjà vu went away. “Shona was greatly delighted to find the world restored to its only roughly familiar, obligingly unpredictable old self,” Zeman writes.

After several days of constant déjà vu, Shona got medical help, after first being sent to a psychiatric hospital. She had also started to have “peculiar bodily feelings,” like “tingling in the left side of her face, a feeling of floating in the air looking down on her body, and a feeling of compulsion to do things.”

“She felt she had lived just these same moments, just this same day before,” Zeman describes in his book, Portrait of the Brain . “She was mysteriously caught up in a repeat performance, point for point: throughout the day she had the sense of knowing precisely what would happen next.”

Zeman says he has seen several people whose experiences fit the description of déjà vécu more than déjà vu, because they are so overpowering. Like Shona, a woman in her mid-twenties who woke up one morning, ate breakfast, got ready for work, but felt she was "acting in a film that she had seen before.”

Moulin cautions that sometimes people use the phrase déjà vécu to describe patients that have memory delusions, the confabulation that everything is repeating—like his first patient. But in déjà vécu, self-awareness is key. In fact, Zeman's first patient, the one who wanted her loud popping noise to go away, was an apt example of déjà vécu—hers was a stronger, more persistent experience than déjà vu. But she didn’t actually believe her life was repeating, or that they were actual memories she was retrieving. She didn’t come up with stories to explain what was happening. “It felt to her very much like an illusion and a strange sensation,” Moulin says.

There may be a need for even greater refinement of the many déjà-like experiences, to separate them out, especially if they have different underlying mechanisms. (There’s even a phenomenon now, considered separate, known as déjà-rêvé, which is an intense recollection of one’s dreams. It too has been shown to be provoked through electrical stimulation in the brains of epileptics, and has been reported by people with epilepsy during seizures.)

I am a person who gets déjà vu regularly. Not enough to qualify as persistent, but I think more than most. I ask Moulin what he thinks this means. Has my in-brain fact checker had too many cups of coffee? Is my brain really good at noticing even the slightest errors in over-familiarity?

Moulin says that people with déjà vu could be more in touch with the signals being given off by their cognitive systems, or especially sensitive to how their familiarity systems are working. But Moulin says we don’t know why some people get it more than others. He says he’s met extremely bright people who have never experienced it, so unfortunately for me it’s not necessarily a mark of intelligence or an exceptional brain.

He does wonder if people who don’t get déjà vu have the same relationship with their memory than those who have it a lot. "People who experience déjà vu might use more of their reflective abilities to think about what's going on in their memory system,” he says. “Maybe they have this sort of memory they don't trust all the time. Whereas people who don't have déjà vu, they just explain everything away once they find something familiar. They try and find a reason for why they find it familiar and they don't enjoy the same mysterious relationship with their own memory.”

Moulin says that he used to have déjà vu more than most, even before he started his research. “I think that's quite consistent with how I was always interested with how things were working,” he says. “You can't really create déjà vu, but you can cultivate the way of thinking about what you're thinking about. That might make you a bit more likely to notice it than other people.”

Moulin asks me if I remember my first time experiencing déjà vu, saying that in most people, it starts around the age of ten, but not earlier. “I remember my first. I don't know if you remember your first,” he says.

I do remember my first—I was nine or ten and on the playground at my elementary school. I never forgot it because it was so powerful and strange. It’s also odd to remember it—a real memory of a feeling of false familiarity.

“Yeah, then you get into all kinds of recursive Donnie Darko kind of loops,” Moulin laughs. But the fact that déjà vu doesn’t happen commonly at much younger ages supports the “fact-checker” theory that Moulin holds. “Before that age we don't have those reflective capacities,” he says. “We haven't learned yet to mistrust or evaluate our memory system and we don't really coordinate and think about our memory in the same way that we do as adults.”

And for people who have never had it, it's a hard thing to explain. It's a powerful feeling, but subjective, one of the many challenges that comes with studying it—even if it can reveal the intricacies and mechanisms of memory, recollection, and familiarity in potentially new ways. Moulin says that he's met researchers or academics who have never experienced it themselves, and can't be sure it exists.

"To them," he says, "I might as well be researching ghosts."