I'd heard a number of explanations of the phenomenon -- that it was memory being triggered subliminally by another sense, or that it came from the fact that our sensory processing and cognitive processing were running at different speeds, so the by the time everything was integrated it created a false memory of an experience that had already occurred. Neither of those has ever sounded all that convincing to me.
[image courtesy of the Wikimedia Commons]
Now, cognitive neuroscientists Josephine Urquhart and Akira O'Connor of the University of St. Andrews (Scotland) have devised an experiment that gives us at least a window on what might be going on -- by creating a situation where déjà vu can be induced.
The setup is simple and elegant. You give your test subjects a list of words to memorize, and include several that have to do with sleeping -- bed, blankets, dreams, pillow. "Sleep" itself is not included. After studying the list, you ask the subjects if there were any words on the list beginning with the letter "s" (there weren't). Afterwards, you ask them if the word "sleep" was on the list.
They know it couldn't have been, because they all answered in the negative regarding there being words beginning with "s" -- but when asked the question, most of the test subjects experienced an eerie sense of déjà vu, that the word "sleep" actually was on the list -- or, perhaps, on another similar list they'd seen before, somewhere else. Urquhart and O'Connor write:
Déjà vu is a nebulous memory experience defined by a clash between evaluations of familiarity and novelty for the same stimulus. We sought to generate it in the laboratory by pairing a DRM recognition task, which generates erroneous familiarity for critical words, with a monitoring task by which participants realise that some of these erroneously familiar words are in fact novel... The key omission in [prior] déjà vu generation procedures... is the provision of information allowing the participant to make an evaluation of unfamiliarity or novelty to clash with the experimentally-generated familiarity. In these procedures, there was no objective standard by which participants could verify that the stimuli provoking familiarity had in fact not previously been encountered.Interestingly, when the subjects were being tested, they were simultaneously being monitored by an fMRI scanner -- and when the feelings of déjà vu were the most intense, the areas in the brain involved in memory (such as the hippocampus) were not very active. Instead, the frontal cortex -- the part of the cerebrum responsible for decision-making -- was lighting up like mad.
O'Connor and Urquhart believe that the explanation for this is that déjà vu comes from our memory's error-checking procedure. When we are forming memories, the frontal cortex is doing a continual spot-check to make sure that what is being placed into memory is accurate. When an error is noted, it's brought to our attention. Most of the time, the error is something that can be resolved quickly -- with a conclusion of "okay, that's not the way it happened." But when the memory being analyzed is close in content to something else, especially something that the conscious brain knows can't have occurred, it generates a conflict that is what results in the sensation of déjà vu.
This is still a tentative finding -- there is a great deal we don't understand about memory and sensory processing, so concluding that the phenomenon of déjà vu is explained is probably premature. But to my thinking, this is a hell of a lot better explanation than anything else I've heard. O'Connor and Urquhart are going to continue trying to explore the phenomenon. As a mysterious sensation that is nearly universal to all humans, it certainly begs explaining. But look for more studies coming down the pike. And don't forget: you heard it here first.