Most of us have experienced it—the fleeting, slightly spooky moment when the present, suddenly strangely familiar, feels like something we remember from the past.

It’s called déjà vu, which is French for ‘already seen,’ and despite the almost universal nature of this quirky memory glitch, its true nature has been remarkably resistant to research, analysis or understanding.

Century-old theories are still in circulation, offering various explanations including the contradictory ideas that fatigue or being well-rested may disrupt the way we process sensation and perception.

Déjà vu is also associated with some psychiatric disorders and temporal lobe epilepsy.

“Our view is that déjà vu is caused by disruptions to brain functioning in the temporal lobe, the area of the brain behind the ears. We know this area to be important for memory function, and our view is that déjà vu is just a brief memory error. The exciting thing about déjà vu is that it is the result of two opposing evaluations, the knowledge that something hasn’t happened before, but the feeling that it has. We believe that one of the functions of the temporal lobe is to produce these feelings of memory. Sometimes, when this area is overactive, or ‘tricked’ into thinking something is familiar, we get a brief sensation of feeling something is old, but knowing it is new,” explains Dr. Chris Moulin, lecturer in Cognitive Neuropsychology at the University of Leeds in the UK.

Dr. Moulin’s views are supported by research involving a rare memory disorder known as chronic déjà vu, which is caused by cell loss in the brain leading to almost constant déjà vu—especially when it comes to “novel events,” he says.

“Of course, this is very different from the fleeting sensation of déjà vu we get. These patients withdraw from activities because they feel they’ve done them before—we don’t do that with ‘healthy’ déjà vu. But the idea is broadly the same—there are two separable elements to memory function: knowledge or memory content, and the subjective experience of that—a conscious state of remembering, or a feeling.”

This ongoing pattern of false remembering can inflict a toll on patients and their families and the normal conduct of everyday life.

People suffering from chronic déjà vu share certain commonalities—almost all have withdrawn from watching television, for example, insisting they’ve previously seen every program, even if it’s new. Similarly, they will claim familiarity with places they clearly have never visited in the past.

Differences do exist however.

“We’ve seen equal numbers of men and women, and a range of ages from the early 60s to the 90s. Broadly speaking, these people have some sort of memory disorder, they are usually forgetful and vague about memory, but a definite diagnosis isn’t always possible. As a very crude generalization, these people seem above average in terms of intelligence, and all are living independently. We do view this as pathology but not a new diagnostic entity. It seems to be another way in which memory can be disrupted by the aging process, albeit very rare. It isn’t necessarily indicative of any diagnosis, but where it does occur, it is certainly very disruptive for the carer and the patient, and leads to large changes in everyday life,” says Dr. Moulin.

“Normal déjà vu, is, just that, entirely normal. It actually can be fairly pleasant, a mildly peculiar sensation which gives you a little window into how the mind works. But our patients don’t have this awareness. They are not conscious of the fact that their mind is playing this awful trick on them.”

The normal experience of déjà vu, a memory error, is typically associated with youth, unlike most other forms of memory error, although experts aren’t certain why.

“My favorite explanation is that as you get older you are slightly, but definitely, more forgetful—so that you just forget the déjà vu experiences you may have had. But there’s other explanations, some more based on common sense—like as you’re older you visit less novel places –and novelty, ironically, is somewhat critical for having déjà vu. Everything is likely to be more familiar for older people. Other explanations are more scientific and need more support from experiments, for instance, older adults have a lack of ‘recollection,’ related to slight reductions in temporal lobe activity, so they have a lack of the conscious feeling associated with memory, and thus they are less likely to have disruptions to it,” notes Dr. Moulin.

According to Dr. Moulin, temporal lobe epilepsy helps researchers converge on the importance of the temporal lobes in feelings of recollection and remembering because it supports the view that déjà vu is a temporal lobe phenomena related to over-activity of this brain area and a malfunction of normal memory processes.

His research is designed to elucidate the mechanisms behind this disturbing symptom and to accurately gauge prevalence and prognosis.

“Our ultimate aim is to be able to understand this problem to the extent that we can offer informed care and support, and hopefully treatment. Progress is somewhat slow because we have a lack of people with this condition, and we cannot keep researching the same people over and over, because of the nature of their difficulties, anyway. Of course, as well as helping sufferers and carers, this work throws up very interesting perspectives on the mind and how consciousness and memory works, and the knowledge that we gain in that area can be applied to many different situations and conditions.”

He points out that many sufferers are reluctant to come forward to talk to their doctors about what they consider to be embarrassing symptoms, suggesting to him that chronic déjà vu may be more common than originally thought.

“By publishing work on this condition we also hope to remove some of the stigma attached to this issue…It is much easier to talk about forgetfulness, which doesn’t have nearly such a taboo attached to it. People who have this condition, and their carers are likely to be worried that they are going mad, which certainly isn’t the case. Our research suggests that this is just a very unsettling, but peculiar memory problem.”