Have you ever had that eerie feeling that you're reliving a past experience?

你是否也曾經有過那個詭異的感覺，好像重活了一次過往的經歷？

Maybe you're traveling to New York City for the first time and it feels strangely familiar.

也許你第一次去紐約，但卻覺得異常的熟悉。

Or maybe you're experiencing it right now, when you think you've already watched this video before.

或許你在看這部影片的當下也有這種感受，覺得自己好像已經看過了一樣。

Ooh. Spooky.

噢，有點可怕。

This phenomenon is called "déjà vu", which is French for "already seen".

這種現象叫「似曾相識」（註：學名為「既視感」）在法文中是「曾經看過」的意思。

Historically, déjà vu has been linked with epilepsy and seizures in the brain.

在歷史上，似曾相識被認為與癲癇及腦癲癇有關。

But that doesn't explain why people without epilepsy experience déjà vu.

但這無法解釋為什麼沒有癲癇症的人也會感受到似曾相識。

After all, about two thirds of the population have experience it at least once in their lives.

畢竟有全球三分之二的人們一生中都至少經歷過一次。

So there has to be something else going on here.

所以一定有別的解釋。

The exact cause of déjà vu is unknown, but there are a lot of theories attempting to explain it.

造成似曾相識的確切原因仍未知，但有很多相關的理論。

For simplicity's sake, let's narrow it down to three main categories.

我們將原因簡化為三個。

The first of these is called "Dual Processing".

第一個叫「雙系統理論」。

This is when two normally coordinated cognitive processes become momentarily out of sync.

這是當兩個原本協調一致的認知系統暫時產生不同步的現象。

For example, if I feel familiar with something, it might be because my brain has retrieved a memory similar to that one that I'm currently experiencing.

舉例來說，如果我對某事感到熟悉，可能是因為我的大腦自動找到了一個與目前狀態相似的記憶。

Retrieving a memory always comes before the familiarity.

通常尋回的記憶一定會比熟悉感來的強烈。

Well... except in déjà vu.

除了在經歷似曾相識的時候。

In that case, I would have a feeling of recalling something without my brain actually doing any retrieving.

在這個情況下，大腦完全沒有找尋記憶，我們卻能有熟悉的感覺。

I could feel like I recognize something on that street corner in New York City without ever having been there before.

我也許會覺得我認得紐約轉角的事物，即便我從來沒有去過。

The next theory is related to the nervous system not functioning properly.

下一個理論與不當運作的神經系統有關。

Déjà vu can either be caused by a spontaneously firing of a neuron, or from a delay in the neuron's transmission speed.

似曾相識可能是因神經元自動發射訊號，或是因為神經傳送速度的延遲造成。

To demonstrate the second one, let's take a look at this alpaca walking in high heels.

為了證明第二點，我們來看看這隻穿著高跟鞋走路的羊駝。

Now you've more than likely never seen an alpaca do that before, if you had, I'm very jealous of you.

正常來說你應該沒有看過羊駝做這種事，如果你有，那我很嫉妒你。

But if you were experiencing déjà vu, one explanation is that the information passing through your eyes into your brain is reaching in a different time.

但如果你剛才有體驗到似曾相識的感覺，其中一個解釋是，資訊從你的眼睛進到腦袋的到達時間不一致。

Let's say the information gets from you left eye to your visual cortex just before the information from your right eye gets there.

我們假設左眼的資訊先進到你的視覺皮層，才換右眼的資訊。

These two events happen in such a short amount of time that the left eye's information has not stored it's memory yet.

這兩件事發生得如此迅速，導致左眼的資訊根本還沒存進記憶裡。

But since your brain received the information twice, it feels familar nontheless.

但因為你的大腦接受了兩次訊息，所以會感到熟悉。

The last theory's a bit more intuitive and states that if you have a false sense of familiarity with something new, it's because some elements of the situation are familiar.

最後一個理論比較直觀，它認為如果你對於新的事物有熟悉感是因為在這個狀況內的某些元素真的是你遇過的。

For example, if you were to go to a new friend's home for the first time, and they have the same kitchen table as your great aunt, you may have a strong feeling of familiarity without realizing where you've seen it before.

舉例來說，如果你第一次去一個新朋友家，他跟你的姑婆有一樣的餐桌，你可能會有強烈的熟悉感，卻想不起是在哪裡見過。

In your confusion, you mistakenly think that you must have experienced the entire situation already, instead of just the table.

而你會混亂的以為你已經經歷過整個場景了，而不是只有那張桌子熟悉而已。

Well, then why don't we experience déjà vu every time we subconsciously recognize something?

那為什麼我們不會在每次潛意識認出東西時，就經歷似曾相識的感覺呢？

Some theories say that there may have to be multiple elements that are familiar to you.

有些理論說，也許周圍需要有多樣對你而言熟悉的元素，

Or maybe the room just has to be organized in a similar way as the one you recognize.

或是房間的規劃必須跟你認出的那個很相近。

Either way, something is familiar and you just haven't realized it yet.

不論是哪種說法，就是有東西是熟悉的，你只是還沒察覺罷了。

Have you ever experienced déjà vu?

你有過似曾相識的感覺嗎？

And tell us out of all these, which theory sounds the most convincing to you.

告訴我們你覺得哪個理論比較能說服你吧！

If you want even more Life Noggin, check out this great video we did on why our hair turns gray, and make sure you're following us on Facebook and Twitter.

如果你想收看更多的 Life Noggin，可以看看這部解釋為何我們頭髮會變白的影片，也請確認你有追蹤我們的臉書及推特。

We post some pretty cool stuff there, too!

我們在那也會發布一些滿酷的東西！

I'm Blocko. This has been Life Noggin. Don't forget to keep on thinking.

我是 Blocko，這裡是 Life Noggin。別忘了要繼續思考喲！