Most people have had the common experience of feeling as if we have been someplace before, or that events that are occurring in real time have happened before. Sometimes we feel as if we know what is going to happen next – and then it happens. Unsurprisingly some have interpreted these phenomena as evidence for some type of extrasensory perception. Something weird certainly seems to be going on.

Neuroscientists have been extremely successful in at least partially explaining many such weird experiences. What is uncanny is that our experience of reality is a constructed illusion, and occasionally we experience the glitches in this construction. The most obvious examples of this are optical illusions. We marvel at how our visual construction can be deceived, or can flip between different states.

But everything, not just vision, is a similarly artificial neurological construction subject to illusory effects. That includes memory.

That déjà vu is a memory glitch is old news. But neuroscientists have been teasing apart the phenomenon in more detail, revealing some aspects of how our memories work. A recent study adds another bit of information to our understanding, which we can use as a jumping off point to review what we know.

Anne Cleary, a cognitive psychologist at Colorado State University, has been researching déjà vu and related phenomena. She believes that déjà vu is a manifestation of a memory phenomenon known as familiarity.

First we need to recognize that there are many types of memory. Psychologists researching memory have identified many different components and features of memory. Neuroscientists have also been trying to correlate different memory phenomena to specific neuroanatomical structures with some success. The type of memory relevant to déjà vu is called recognition – recognizing that we have a memory of a prior experience. Recognition, in turn, results from two phenomena: recollection and familiarity. Recollection is the memory of specific details. Familiarity is more of a vague sense that we have experienced something before.

Cleary believes that déjà vu primarily results from the phenomenon of familiarity. This is a largely subconscious process of pattern recognition, something at which our brains excel. We are confronted with a pattern, our brains automatically search for similar patterns, and when it finds a near match we have the subjective sense of familiarity. We may then try to recall details, to make a more specific match.

Cleary also relates this same process to the subjective sense of having a word on the tip of our tongue. You know that you know the word, the pattern is there as is the sense of familiarity, but you have not yet made a specific pattern match or recalled the detail.

Sometimes our brains make an incorrect pattern match. We confuse one actor for another, for example,  because they evoke a sense of familiarity and then we match them to the wrong specifics.

To test the relationship between the phenomenon of familiarity and the sense of déjà vu, Cleary conducted previous experiments in which she had subjects go through a virtual reality game that they constructed. In later parts of the game the virtual scene mapped exactly to a previous scene in the game, but with different details and in a different context. Subjects were more likely to report déjà vu when they were experiencing a scene that mapped to an earlier one.

As an interesting aside, déjà vu often feels subjectively “creepy”. Cleary thinks this is because we are experiencing a sense of familiarity in a place we have never been, and the disconnect is disconcerting. This reminds me of other neurological phenomena in which subjective feelings do not match expectations, even when we are not consciously aware of those feelings or their absence.

For example, Capgras syndrome occurs when the connection between parts of our visual cortex and the limbic system (which produces emotions) is broken. Normally those objects our brains categorize as having agency are assigned an emotional significance. We feel something about them. So when you see and recognize a family member, you would normally feel love toward them. In Capgras syndrome, that feeling is missing. There is a disconnect between your recognition of the person and the absence of expected feelings. This is very unsettling and often leads patients with this disorder to conclude that the loved-one is actually an imposter, even though they can’t say exactly why.

So with déjà vu we have a sense of familiarity when we shouldn’t. The veil of the neurologically constructed illusion breaks momentarily, we get an uncanny sense, a sense that the mundane material world is more than we think. I think The Matrix is a perfect allegory for these types of experiences, and was one of the compelling aspects of that movie.

In the recent experiment Cleary tackles another aspect of the déjà vu phenomenon, the sense by some that they are not only experiencing the familiar, but that they can predict what is happening next. To test this she repeated her virtual reality setup, but toward the end of the scene meant to provoke familiarity and therefore déjà vu she asked subjects if they felt they knew what was coming next. About have the subjects reported a strong sense of premonition, and predicted what the next turn would be. Those who had the sense of premonition, however, did not better than chance at predicting the next turn, even when they were highly confident.

This false sense of premonition likely represents another memory phenomenon related to hindsight bias. Once something happens we may have the feeling that it was inevitable, or that we knew it was going to happen. In the moment of déjà vu we may have the vague sense of premonition, and then as soon as the next thing happens we feel as if that was what we thought was going to happen. Our memory of the premonition is immediately altered to incorporate actual events, and we are left with the memory that we knew what was going to happen. But if you make people state their premonition before the experience, it is random and not related to later events.

The takeaway from all this, as is often the case when I write about neurological phenomena like memory, is that our brains construct our experience and memory of reality, and this construction is glitchy and imperfect. There is no doubting this basic truth, as any experience with optical illusions proves. The only question is – how deep does this neurological rabbit hole go. So far the evidence suggests that it goes all the way.