It might just be the weirdest experience you'll ever have sober, but what exactly is deja vu?

這或許是每個人清醒時都曾有過的怪異體驗，那麼到底什麼是「既視感」？

One thing it definitely is, is common.

「既視感」是個普遍的現象。

Two-thirds of us have had it, with younger people, globetrotters, and film fans likely to get it more frequently.

約有三分之二的人都曾體驗過「既視感」，其中又以年輕人、環遊世界者和愛看電影的人最常發生。

Because of its inherent weirdness, deja vu was long thought of alongside paranormal events like clairvoyance and reincarnation.

因為「既視感」帶給人們怪異的感受，所以從以前就被視為一種像是超視覺或前世記憶的超自然現象。

In fact, it was parapsychologist Émile Boirac who first named the feeling in the 1870s using the French for "already seen."

事實上，「既視感」這個名字最早是由名為 Émile Boirac 的超心理學家於 1870 年代提出，在法語的意思是「曾經看過」。

The focus on the uncanny has persisted, and in films like "The Matrix," deja vu is a glitch in the computer simulation.

大家喜歡用神祕莫測的既視感作為創作題材，好比電影《駭客任務》中，就把既視感描述為因電腦模擬器故障而產生的現象。

So, what's actually going on?

那麼，究竟「既視感」是怎麼一回事？

The truth is, no one is 100 percent sure, but psychologists have suggested dozens of possibilities combining theories of memory, perception, and cognition.

老實說沒有人百分之百了解它，心理學家結合各種記憶理論和感官認知，進而提出多種可能的解釋。

One is divided perception.

其中一個說法叫作「分散性注意力理論」。

Maybe our brains process a situation in a quick and shallow way before we become fully aware of it, and then we get a jolt of having seen it before.

既視感之所以會產生，是因為大腦下意識快速地接收了片面情況，在尚未全面分析時進而使我們產生了似曾相識的錯覺。

Another is dual processing.

另一個說法為「雙重歷程理論」。

Incoming signals enter the temporal lobe from both hemispheres of the brain, one a millisecond later than the other.

某些資訊在傳遞至左右腦時，可能會出現些微的時間差。

And it's in this moment of delay that deja vu occurs.

而這個時候就會產生所謂的「既視感」。

Others speculate that errors around the hippocampus—the brain's librarian—are to blame.

也有部分學者認為起因為海馬迴，而海馬迴又被稱為大腦的圖書館管理員。

The problem with studying deja vu is that neurologists can't very well wait around for it to happen.

神經學學者在研究既視感時碰到的一大難題是無法準確抓到其發生的時間點。

One solution has been to look at people with temporal lobe damage.

有個解決辦法就是觀察大腦葉有暫時性受損的人。

Many find that they get chronic deja vu.

許多研究發現他們有慢性既視感。

Another way to study deja vu is to induce it under lab conditions.

研究「既視感」還有另一方法就是透過實驗去誘發它發生。

In 2012, one study used virtual reality to immerse people in different 3D environments, some of which were very similar in layout.

在 2012 年，有個研究利用 VR 模擬實境，讓實驗者沉浸在他們熟悉的情境中。

For instance, a doctor's waiting room and an aquarium, with furniture arranged in the same configuration.

例如，醫院的待診室或水族館，並特意把裡頭的傢俱擺放依照現實擺放。

People were more likely to report deja vu when they encountered environments that had a similar layout to previous, forgotten scenes, suggesting it's a memory phenomenon.

當人們看到之前遇過但暫時忘記的熟悉場景，發生「既視感」的機率越大，這代表了這是一種記憶現象。

A 2014 study had very different results.

不過，2014 年的研究有截然不同的結果。

Those who took part were shown a series of words with a secret common theme, words like bed, pillow, nap, dream.

實驗參與者接受到許多隱含共同主題的單字，例如：床、枕頭、午覺、夢。

The linking word, "sleep" never appeared.

不過「睡覺」這個字不曾出現。

Viewers were asked to keep note of any words beginning with S.

同時要求參與者記錄下 S 開頭的單字。

Those who took part were later asked if any words began with S, and sure enough, they said no.

接著，問參與者剛剛是否有出現 S 開頭的單字，他們全部人都回答沒有。

But many also felt that they had been shown the word "sleep."

但有很多人都覺得剛剛明明有看到 sleep 這個字。

For two thirds of people, this confusion was tantamount to deja vu.

三分之二的人都受到這種類似「既視感」的混淆。

Neurologists have used this method to scan the brain during deja vu.

神經學學者便利用此方法來研究「既視感」。

They found that rather than being a memory error in the hippocampus, deja vu involved the frontal areas of the brain responsible for decision making.

他們發現「既視感」並非在海馬迴出現的記憶錯誤，而是發生於大腦前葉的現象。

This led some to suggest that deja vu is a sign your brain's memory-checking mechanisms are actually working well.

這個研究結果使得部分人認為「既視感」是一種代表記憶檢查機制正常運作的徵兆。

But, if you're looking for something a bit more out there to explain deja vu, try quantum entanglement.

如果你要更進一步去解釋「既視感」，可以從量子力學的角度去研究。

Perhaps through the mysterious affinity of subatomic particles, deja vu might actually be a window into a parallel universe, or else a blip in time.

也許，透過原子粒子間的神秘力量，「既視感」可以成為觀察平行宇宙的窗口或是時空跳躍的機會。

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