It's an eerie experience that just about everyone has had more than once: you walk into a room or find yourself in a conversation, and suddenly you have the overwhelming sense--even though you know it's impossible--that you've been here before. Psychologists call it déjà vu--"already seen," in French--but despite the phenomenon's universal familiarity, no one has offered a convincing explanation for why it happens.
But the mystery may have been solved, by a team of neuroscientists at MIT's Picower Institute for Learning and Memory. Researcher Thomas McHugh and several colleagues have uncovered a specific memory circuit in the brains of mice that is probably the cause of this weird sensation, which turns out to be a sort of memory-based analogue of an optical illusion. Although neuroscientists have realized for some time that memory is made up of many different components--long and short term, episodic (that is to say, memories of events) and fact based, and that it takes place in different parts of the brain--McHugh's research, first reported in the online edition of Science, adds another intriguing clue to the phenomenon.
McHugh and his team were trying to untangle the neurological circuitry of the hippocampus, a region of the brain where new memories are formed. Neuroscientists know memories are actually groups of brain cells linked by especially strong chemical connections; recalling a memory involves finding and activating a specific group. It's important for the brain to know some memories are similar to each other--the pleasure of eating raspberries is much like that of eating strawberries, for example. But it's also important to be able to distinguish memories that are similar but not identical--eating another kind of red berry could make you sick. This ability is known as pattern separation.
McHugh's senior colleague Susumu Tonegawa, a Nobel laureate for his work on the genetics of immunity, had uncovered a related mechanism, called pattern completion, several years ago. That enables you to retrieve complete memories based on just a single cue--for example, the question "Did we go to school together?" He and McHugh suspected, based on this earlier work, that they could identify the specific gene that regulated pattern separation.
So they used genetic engineering to create a mouse without this crucial gene and devised an experiment to test the hypothesis. The mice were guided into a box where they would get a mild foot shock; they would react by freezing. Then they were guided into a very similar box with no shock. The altered mice would freeze in the safe box as well, and it took them a long time to figure out the difference. Normal mice figured it out pretty quickly.
It's also this circuit, the scientists are convinced, that explains déjà vu. Every so often, they believe, the pattern-separation circuit misfires, and a new experience that's merely similar to an older one seems identical. "It doesn't happen very often to most people," Tonegawa says. Intriguingly, some people with epilepsy have this experience all the time. "Epileptic seizures involve random firing of neurons in the temporal lobes, which include the hippocampus," he says, and that could scramble the circuit.
But people with epilepsy-induced déjà vu usually don't experience the same disturbing eeriness that's so common in others. And that difference supports McHugh and Tonegawa's theory as well. "We suspect that the strange feeling comes from a conflict between two parts of the brain," Tonegawa says. "The neocortex is aware of the fact that you've never been in a situation before. The hippocampus is telling you that, yes, you have."
As basic research scientists, Tonegawa and McHugh don't claim that their work will lead to a drug or therapy--not yet. And if it does, nobody is likely to focus on déjà vu, a mere side effect of memory. But a fuller understanding of how the hippocampus works could lead to the creation of a drug that strengthens the pattern-recognition circuit, which could help people overcome fearful memories that are triggered by associations with a familiar-seeming place (like a dentist's office). Of course, if you strengthen the circuitry too much, you might get the opposite illusion: jamais vu, in which you get the eerie feeling that you've never been in a situation before even though you know otherwise.