Other phenomena might also help explain déjà vu, such as inattentiveness. Because we often navigate the world on autopilot, we take in much of our surroundings on an unconscious level. People who text on their cell phones while walking are only superficially aware of the shops and pedestrians they are passing. Perhaps an episode of déjà vu begins during such a moment. When we emerge into full awareness, we might do a perceptual double take. We are struck by a strange sense of familiarity because we saw the scene just moments before, unconsciously.
In a recent study, Elizabeth Marsh of Duke University and I investigated this idea. We showed participants dozens of unique symbols. Some of them were flashed too quickly for participants to consciously detect before they were revealed for longer viewing. Our participants were significantly more likely to identify a novel symbol as familiar if they had subconsciously glimpsed the image before.
A third possibility is that we have forgotten the prior experience. The psychology literature is replete with stories of adults visiting a notable place, such as a castle, and becoming overwhelmed by an uncanny sense of having been there before. Their parents, however, clued them in: they had been to the castle as a very young child. Similarly, television and photographs can breed a false sense of familiarity later on. For example, having watched a documentary on a castle a decade ago might lead to a sense of déjà vu when you visit it.