why does it occur? You know, those moments where the current situation feels like it's

既視感又是怎麼產生的？有時候我們會有種感覺，好像當下正在經歷的事情，

happened before? You're certain it has, but you don't know when, or how it became so familiar.

其實早就發生過了。你覺得一定有這麼一回事，卻想不起來何時發生，也不知道為何如此似曾相識。

It's difficult to scientifically study Deja vu because there's no reliable way to cause

要以科學的角度來證明既視感如何發生其實有難度，因為目前我們沒有確切的辦法

it to happen in people's heads in a laboratory.

來蓄意使大腦產生既使感，以致於無法做實驗。

But here's what we do know: humans don't seem to experience Deja vu until they're at least 8 or 9 years old.

不過有件事情我們可以確定：人類至少要到八至九歲才會開始產生既視感，

It's most frequent in your teens and twenties, and then tapers off as you get older.

青春期到二十來歲的時候既視感發生的最頻繁，接著隨著年齡增長而慢慢減少。

So, it might have something to do with brain development. In fact, we should probably go get ourselves a brain.

由此推斷，既視感應該和我們大腦的發育有關，我們應該去弄顆腦袋來研究。

That was easy. Now, like a stomach ache,Deja vu may be a symptom with many possible causes.

輕鬆入手。就像胃痛一樣，

A lot of the popular theories about what causes

既視感其實可能是多種因素產生的症狀。在許多有名的理論中提到既視感的成因是大腦深處

it involve a disconnect that may be occurring between the deep structures of the brain that

無意識記錄下我們的各種經歷的部位

process our experiences unconsciously, and the parts of the brain that are conscious

和大腦中有意識到我們所經歷且即時傳達訊息的部位

about what they experience, process it, and then tell us what we're seeing.

在串連時有些地方接續不起來而產生的錯覺。

For the sake of simplicity, let's begin with the visual system. Now, the brain sits in

從我們的視覺系統下手可能會更好解釋。如果大腦在你的頭殼中

your head like this, and your eyeballs are over here seeing things, sending that signal

位置在這，那你的眼睛則會在前方接收影像，並傳送訊息

to the back of the brain, the Occipital lobe, where visual cortex is. That's where the image

回到大腦後方的枕葉，也就是我們的視覺中樞所在位置。透過這裏，

is processed and we become aware of what we're seeing. But visual cortex isn't the first

眼睛接受到的影像才能轉換成我們意識中看到的東西。不過視覺中樞並不是影像處理的第一站。

stop that information makes. Instead, we know that it stops in lots of other places. For

其實在我們處理的過程中還要經過很多部分，舉例來說，

instance, the amygdala, where it may be processed at an involuntary emotional level, and this

像是我們的杏仁核，大腦中自動產生情緒的部分系統；以及你看到的

fun little part right here, #31, the tectum. It's involved in preliminary visual processing

三十一號區塊，頂蓋（又稱四疊體），負責影像處理的初步過程

and helps control eye movements.

另外也幫助我們控制眼球的移動。

Now, that fact is incredibly important because what it means is that people with blindness

剛剛提到的資訊非常重要，因為這是我們可以解釋以下情況的關鍵。因為視覺中樞受損

cause by brain damage to the visual cortex cannot see anything. They don't report seeing

而失去視力的盲胞，沒辦法看見或是辨認出任何影像，

or recognizing anything; however, the other preliminary parts of the brain that get that

然而，因為其他處理視覺影像的部分沒有受損，

message are still healthy, and so, despite being legally blind, these people exhibit

所以儘管照理來說是看不見的，

what is known as blindsight.

他們卻仍然有所謂的盲視。

They can defy all odds and avoid an obstacle course on the ground. They can also be presented

這個能力幫助他們預先感覺到並避開路上的障礙物，甚至當播放人們表現出

faces showing joy, or anxiety and fear, and feel, without knowing why, at all, on Earth they feel

喜悅、焦慮或恐懼的表情給他們看時，在自己渾然不覺的情況下，

that way, a similar emotion.

他們也會做出類似的反應。

So, a lot is going on when we look at things, and if those preliminary structures in the

當我們只是在用眼睛看東西時，腦內的過程其實相當複雜，根據實驗我們可得知某些處理影像的系統

vision system allow certain blind people to be oddly and unconsciously aware of what they're

可以讓部分盲人在不自覺的情況下仍然保有某些部分的意識到眼睛接收到的畫面，

looking at, we may have the ingredients we need for this disconnect.

如果是這樣，我想我們找到破解既視感的關鍵元素了。

Think of it this way: If I experience event A, and then B, and then C, and the inner parts

想像一下：今天我經歷了事件Ａ，接著事件Ｂ，最後為事件Ｃ，而我的大腦已經自動

of my brain commit it to memory and generate an emotional response, but then, a fraction

初步處理了記憶，並且產生了相對情緒，但是在幾毫秒的誤差之後，

of a second later, out-of-sync, my visual cortex finally gets around to telling me,

我的視覺中樞才接收到，並且將訊息傳送到

the conscious aware part of my brain, what I'm seeing. It will say "Ok guys, we're looking

腦中有意識處理視覺影像的部分，大腦才開始廣播：「好的，現在我們看到的是

at A, and then B, and then..." and then the brain says "Whoa, whoa, whoa, wait, wait,

事件Ａ，然後是事件Ｂ，接下來呢...咦？等等，接下來是事件Ｃ對吧？

and then C, right? That's already happened I thought?..." and that may be Deja vu.

這不是已經發生過了嗎？」而這個錯覺，可能就是我們的既視感了。

But what could call these processes dealing with the same information to get temporarily

不過，這種大腦中不同部分無法同步處理資訊的現象，

out-of-sync like that? Well, it's probably a neurological abnormality, possibly an epileptic

怎麼會發生呢？其實這有可能是神經系統異常所造成的，就像當所有神經系統同時作用時，

episode where neurons all fire in sync. We also know that patients who experience chronic

所產生的癲癇現象。我們也發現長期或是經常有既視感的人們當中，

and persistent Deja vu tend to have brain damage in the temporal lobes of their brain-

腦中顳葉部分受損的患者比例較高，

these lobes on either side.

（顳葉在大腦兩側都有）。

Well that sounds kinda bad, right? I mean, when I experience Deja vu should I run to the doctor

聽起來有點恐怖耶？所以當我產生既視感的時候，我該立刻去掛號看醫生了嗎？

right away? Well, not at all. In fact, minor epileptic events are quite common. You've

其實不然，老實說，輕微的癲癇現象其實在日常生活中滿常見的。有種類似的經驗，

probably experienced a similar phenomenon known as a Hypnogogic Jerk. That's a fancy

人們稱之為入睡抽動，我想大部分的人都經歷過。

name for what occurs when you're about to fall asleep and then all of the sudden you

入睡抽動就是當你快進入夢鄉時，突然感覺到自己好像正在往下墜，

feel like you're falling, or that you tripped, and your body jolts itself awake.

或是快要跌倒了，導致你整個身體大幅抽動了一下，驚醒了自己。

What's going on there is that as you fall asleep your muscles relax, and some part of

之所以會發生這現象，是因為當我們進入睡眠時身體的肌肉會放鬆，

your brain that's still awake notices that, and feels like you're no longer supported-

而部分還未休息的大腦員工注意到了這點，誤判你的身體失去支撐力，

you must be falling- and it sends out a huge signal that causes all of your muscles to

表示一定是在往下墜之類的，於是大力送出警訊，促使你全身肌肉動起來，

twitch, waking you up.

結果順便把你嚇醒了。

Oh, and Deja vu is not the only "vu" out there. There's also something known as Presque vu,

另外，既視感並不是唯一的類似神祕現象喔。有另外一個名字叫舌尖現象，英文裡常見的

or "tip of the tongue," and this is what happens when you're familiar with something, you know

"tip of the tongue"，就是想到明明某樣你知道，甚至很熟悉的東西，

you know it, but, for the life of you, at that moment, you cannot recall it. It's like "what's

但是就是在那個努力回想的當下，腦下一片空白。像是：「那個演員的名字叫什麼來著？

that actor's name from that movie? I've seen it 18 times, how can I not remember it?? Wow."

我明明看過那部電影18次有了，怎麼會想不起來？天啊！」

One explanation of how that happens is that other words similar to the target word are

其中一個理論是這樣解釋的：當你的大腦為了找出你要的關鍵字，努力搜尋了所有相關資訊，

being remembered, and to help you out your brain is actively blocking other stuff around

甚至為了讓你不分心並快速找到而暫時擋下所有它認為是額外的資訊，

it, including the word you're actually looking for. Now, this explanation is really nice

結果其實你要的那個關鍵字也不得其門而入了。這個解釋其實很合理，

cause it might actually explain why Presque vu has a shared, or social aspect. Even a

因為它可以套用舌尖現象發生在群體之中的情況下，有時甚至一群人聚在一起時，

bunch of people in a group, all with special brains, if they're given the same "blocking

如果他們聯想到和被擋住的資訊是一樣的，

words" that are related, no one in the group can come up with the name until the subject

那群人中沒有人可以想到答案，反而一直要等到話題換了之後，

is changed and those words are no longer blocked, and then all the sudden someone goes "Oh,

那些大腦擅自攔截的資訊被釋放，才會有一個人突然說「我想到了！

Gary Sinise, that's who it was. Yup, there you go."

他叫蓋瑞·辛尼茲啦！就說我記得的。」

Ok, finally, Jamais vu. This is when something that you know, something that you're familiar

最後一個神祕現象，有些人稱陌生感或未視感，解釋有時明明某樣東西你很熟，

with, all the sudden seems brand new and bizarre. Schizophrenic patients will sometimes see

它卻突然感覺很陌生，甚至很詭異。部分精神分裂症患者有時會看著某位熟人，

people they know and insist that it's not the person the know. Instead, it's an imposter.

並堅稱自己不認識對方，甚至說眼前人是冒牌貨。

But all of us have probably experienced Jamais vu to a certain extent, for instance, when

但其實我們幾乎都曾有過陌生感（未視感），比如說，

you repeat a word over, and over, and over, and over again, the word starts to lose its

當你一直一直一直不斷重複說或聽或寫同一個字時，那個字對你來說會漸漸失去意義，

meaning and you start to think "What? How is this even a word? It's so weird!" Well,

甚至感覺詭異得像是不該存在。你甚至會開始覺得怎麼可能是一個字？也太奇怪了！

scientists believe that happens because continuing to excite the neurons responsible for that

科學家們相信這個現象是因為當我們反覆刺激那個接收該字的感官系統時，

word causes them to become inhibited, and further use is less intense.

有時反而會抑制反應，再刺激也是徒勞。

It is just crazy to think that there is so much out there in the world, in the universe,

想想看，還有這麼多我們還不知道的事情在這世界上甚至在未探索的宇宙中，

that we don't know, that we don't understand, that we haven't yet discovered. But, yet,

但是更奇妙的是，

that very feeling of familiarity for what we do know can't always be trusted. About

有時就連我們以為自己知道的，都有可能只是錯覺。我想不變的大概只有我想說的這句：

the only constant seems to be: As always, thanks for watching.

謝謝你的收看。