Why deja vu happens, and why it's a good thing.
Our brains' built-in fact-checker
Read the full transcript of this episode.
When Chris Moulin went to New York City for the first time, his brain started playing tricks on him.
"When I was there I turned the corner and I had a massive sense of deja vu. I had this big feeling of familiarity, but I knew it was the first time I've been in New York so it wasn't possible that I'd been there before, so it wasn't possible that it was a memory. It was a very strange experience," he said.
Good thing, then, that understanding these strange experiences is his job. As a neuropsychologist at the Université Grenoble Alpes in France, he's dedicated his career to understanding how memory works in the brain, specializing in deja vu.
A little glitch
"Deja vu is caused by like a little glitch in the memory system where you have two feelings at the same time," Moulin told host Tai Poole in a recent episode of the CBC Podcast Tai Asks Why. "You have the feeling that you find something familiar. And at the same time is that you also know that that familiarity is false. In fact it couldn't possibly be true."
Déjà vu is French for "already seen," and it's just that - a sensation that something you're experiencing is something you've already experienced. Tai is 12 years old, which Moulin says, is the peak age for deja vu. Older people get less deja vu because they experience fewer novelty situations.
"They get less deja vu because they say, well, they've just done so much that it is it is quite possible they're confused because they did already do something very similar and I don't think that is deja vu," he said. "Deja vu is only that time when you're really saying it hasn't happened before and there's not that sort of confusion."
Our memories are constantly accumulating information to figure out what's useful and what isn't. And, Moulin said, deja vu is just your brain fact-checking that information.
"It's a sign that something's going on that's healthy. It's like a check saying hey hang on a minute," he said.
"It's something that checks the familiarity system doesn't run away with itself that things don't get too familiar or that you have strange sensations of familiarity when you shouldn't. If you didn't have deja vu and if you didn't have this fact-checking mechanism then you'd be in real trouble because you'd never know whether what you were remembering was a real memory or not."
The flip side to deja vu is something that Moulin calls jamais vu, which is french for 'never seen.'
"Have you had this sensation where you look at a word for a long long time and it starts to look strange? Like it might be spelt wrong, or did you like sometimes go to write a word and then think, 'hang on a minute, is it spelt like that? No that looks completely weird!'"
"And if you think about your life, what motivates you. And a lot of the good stuff is the new stuff... So we see that the people [who get jamais vu], they do kind of get a bit difficult to motivate and to find interesting things.- Chris Moulin
Both jamais vu and deja vu are normal signs of a healthy brain, but sometimes, they can go into overdrive, like a particular patient Moulin saw at a memory clinic he worked at in University.
"He complained that he had already been tested before in the memory clinic and his wife said that that was his problem: instead of being forgetful he complained that everything was a repeat of something he had already done."
"And if you think about your life, what motivates you. And a lot of the good stuff is the new stuff. You don't want to watch the last episode of your favourite show. You want to watch the new episode. So we see that the people like that, they do kind of get a bit difficult to motivate and to find interesting things just because they feel that everything is repeating."
Get the full story from Tai Asks Why Season 2 episode Deja Vu.