Can you still feel a limb that's gone? - Joshua W. Pate

譯者: Lilian Chiu 審譯者: Helen Chang

The vast majority of people who’ve lost a limb can still feel it—

大部分斷肢的人還可以 感覺到斷肢的存在——

not as a memory or vague shape, but in complete lifelike detail.

並不是記憶或是模糊的形狀，

而是有很完整很逼真的細節。

They can flex their phantom fingers

他們能夠彎曲他們的幻指，

and sometimes even feel the chafe of a watchband

有時甚至能感覺到錶帶的磨擦，

or the throb of an ingrown toenail.

或是腳趾甲向內生長帶來的刺痛。

And astonishingly enough,

很驚人的是，

occasionally even people born without a limb can feel a phantom.

偶爾，即使出生時就沒有肢體的人

也能感覺到幻肢。

So what causes phantom limb sensations?

所以，是什麼造成幻肢的感覺？

The accuracy of these apparitions

這些幻覺的精準度

suggests that we have a map of the body in our brains.

意味著我們的大腦中 有一張身體地圖。

And the fact that it’s possible

因為連從來沒有肢體的人

for someone who’s never had a limb to feel one

也能感受到幻肢，

implies we are born with at least the beginnings of this map.

就表示我們在出生時至少 就有一張初版的身體地圖了。

But one thing sets the phantoms that appear after amputation

但截肢之後出現的幻肢

和原本的血肉肢體有一個差別：

apart from their flesh and blood predecessors:

the vast majority of them are painful.

大部分關於幻肢的感覺是痛苦的。

To fully understand phantom limbs and phantom pain,

若要完全了解幻肢和幻痛，

we have to consider the entire pathway from limb to brain.

我們必須要思考從肢體 到大腦的整條路徑。

Our limbs are full of sensory neurons responsible for everything

我們的肢體佈滿很多感覺神經元，

負責的功能很多， 從指尖感受到的質地，

from the textures we feel with our fingertips

to our understanding of where our bodies are in space.

到我們對於身體在空間中 相對位置的理解。

Neural pathways carry this sensory input through the spinal cord

神經傳導路徑會把這些 感覺輸入訊號透過脊髓

and up to the brain.

向上傳到大腦。

Since so much of this path lies outside the limb itself,

因為這路徑有很大部分在四肢之外，

most of it remains behind after an amputation.

大部分路徑在截肢之後還存留著。

But the loss of a limb

但失去了肢體就會改變

alters the way signals travel at every step of the pathway.

訊號在路徑中每一步的傳導方式。

At the site of an amputation,

在截肢處，

severed nerve endings can thicken and become more sensitive,

被切斷的神經末稍可能會 變粗，變得更敏感，

transmitting distress signals even in response to mild pressure.

輕微的壓力，就會讓它 傳送出痛苦的訊號。

Under normal circumstances,

在正常情況下，

these signals would be curtailed in the dorsal horn of the spinal cord.

這些訊號會被脊髓背角給削減掉。

For reasons we don’t fully understand, after an amputation,

但由於某些我們還不全然 了解的理由，在截肢後，

there is a loss of this inhibitory control in the dorsal horn,

背角中的這種抑制 控制功能會消失，

and signals can intensify.

訊號可能會變強。

Once they pass through the spinal cord, sensory signals reach the brain.

一旦感覺訊號通過了脊髓， 就會抵達大腦。

There, the somatosensory cortex processes them.

在大腦中，感覺皮質 會負責處理這些訊號。

The entire body is mapped in this cortex.

這個皮質區有整個身體的地圖。

Sensitive body parts with many nerve endings,

因為有許多神經末稍 所以很敏感的身體部位，

like the lips and hands,

像是嘴脣和手，

are represented by the largest areas.

是由最大塊的區域來代表。

The cortical homunculus is a model of the human body

皮質小人是一種人體模型，

with proportions based on the size of each body part’s representation in the cortex,

身體部位的比例是根據它們

在皮質區中對應的大小。

The amount of cortex devoted to a specific body part can grow or shrink

每個身體部位對應的皮質代表區

可能會長大或縮小，

based on how much sensory input the brain receives from that body part.

依據是看大腦接收的感覺輸入訊號 有多少是來自那個身體部位。

For example, representation of the left hand is larger in violinists

以左手對應的皮質代表區為例，

小提琴家的這個區域 就會比一般人還大。

than in non-violinists.

The brain also increases cortical representation

當一個身體部位受傷時，

大腦也會增加對應的皮質代表區，

when a body part is injured

in order to heighten sensations that alert us to danger.

以強化該部位的感覺， 來警告我們可能的危險。

This increased representation can lead to phantom pain.

代表區的增加，就可能造成幻痛。

The cortical map is also most likely responsible

會感覺到已經不存在的身體部位，

for the feeling of body parts that are no longer there,

也很有可能是皮質地圖造成的，

because they still have representation in the brain.

因為在大腦中還是有 那些部位對應的代表區。

Over time, this representation may shrink and the phantom limb may shrink with it.

隨時間過去，代表區可能會縮小，

幻肢的狀況也可能隨之減輕。

But phantom limb sensations don’t necessarily disappear on their own.

但幻覺的感覺 不見得都會自己消失。

Treatment for phantom pain usually requires

若要治療幻痛，通常會需要

a combination of physical therapy,

結合物理治療、止痛藥物、

medications for pain management,

prosthetics,

假肢，和時間。

and time.

有一種叫做鏡箱治療的技術，

A technique called mirror box therapy

can be very helpful in developing the range of motion

對於發展動作範圍

及減輕幻肢痛楚都很有幫助。

and reducing pain in the phantom limb.

The patient places the phantom limb into a box behind a mirror

病人要把幻肢放入 鏡子後面的箱子內，

and the intact limb in front of the mirror.

好的肢體放在鏡子前面。

This tricks the brain into seeing the phantom

這能欺騙大腦， 讓大腦以為看見了幻肢，

rather than just feeling it.

而不只是感覺到幻肢。

Scientists are developing virtual reality treatments

科學家正在開發虛擬境的治療法，

that make the experience of mirror box therapy even more lifelike.

讓鏡箱治療的體驗能變得更真實。

Prosthetics can also create a similar effect—

假肢也能產生類似的效果——

many patients report pain

許多病人說自己主要是在晚上

primarily when they remove their prosthetics at night.

把假肢拿掉之後才會感到痛楚。

And phantom limbs may in turn

幻肢有可能會反過來

help patients conceptualize prosthetics as extensions of their bodies

協助病人將假肢給概念化，

視為是身體的延伸部分，

and manipulate them intuitively.

並以直覺的方式來操控假肢。

There are still many questions about phantom limbs.

關於幻肢，還有許多沒解開的謎。

We don’t know why some amputees escape the pain

我們不知道為什麼有些被截肢者

沒有通常伴隨著幻肢的痛楚，

typically associated with these apparitions,

or why some don’t have phantoms at all.

也不知道為什麼有些人 根本不會有幻覺。

And further research into phantom limbs

關於幻肢的進一步研究

isn’t just applicable to the people who experience them.

不僅僅是針對感受到幻肢的人。

A deeper understanding of these apparitions

若能對這類幻覺有更深的了解，

will give us insight into the work our brains do every day

我們就能更清楚大腦 每天會做些什麼工作

to build the world as we perceive it.

來建立出我們所感知到的世界。

They’re an important reminder

它們是個重要的提醒，

that the realities we experience are, in fact, subjective.

提醒我們，我們所體驗到的現實，

其實都是主觀的。