The vast majority of people who’ve lost a limb can still feel it— not as a memory or vague shape, but in complete lifelike detail. They can flex their phantom fingers and sometimes even feel the chafe of a watchband or the throb of an ingrown toenail. And astonishingly enough, occasionally even people born without a limb can feel a phantom.
大部分斷肢的人還可以 感覺到斷肢的存在—— 並不是記憶或是模糊的形狀, 而是有很完整很逼真的細節。 他們能夠彎曲他們的幻指, 有時甚至能感覺到錶帶的磨擦, 或是腳趾甲向內生長帶來的刺痛。 很驚人的是, 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? The accuracy of these apparitions suggests that we have a map of the body in our brains. And the fact that it’s possible for someone who’s never had a limb to feel one implies we are born with at least the beginnings of this map. But one thing sets the phantoms that appear after amputation apart from their flesh and blood predecessors: the vast majority of them are painful. To fully understand phantom limbs and phantom pain, we have to consider the entire pathway from limb to brain.
所以,是什麼造成幻肢的感覺? 這些幻覺的精準度 意味著我們的大腦中 有一張身體地圖。 因為連從來沒有肢體的人 也能感受到幻肢, 就表示我們在出生時至少 就有一張初版的身體地圖了。 但截肢之後出現的幻肢 和原本的血肉肢體有一個差別: 大部分關於幻肢的感覺是痛苦的。 若要完全了解幻肢和幻痛, 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 from the textures we feel with our fingertips to our understanding of where our bodies are in space. Neural pathways carry this sensory input through the spinal cord and up to the brain. Since so much of this path lies outside the limb itself, most of it remains behind after an amputation. But the loss of a limb alters the way signals travel at every step of the pathway.
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, 到我們對於身體在空間中 相對位置的理解。 神經傳導路徑會把這些 感覺輸入訊號透過脊髓 向上傳到大腦。 因為這路徑有很大部分在四肢之外, 大部分路徑在截肢之後還存留著。 但失去了肢體就會改變 訊號在路徑中每一步的傳導方式。
At the site of an amputation, severed nerve endings can thicken and become more sensitive, transmitting distress signals even in response to mild pressure. Under normal circumstances, these signals would be curtailed in the dorsal horn of the spinal cord. For reasons we don’t fully understand, after an amputation, there is a loss of this inhibitory control in the dorsal horn, and signals can intensify.
在截肢處, 被切斷的神經末稍可能會 變粗,變得更敏感, 輕微的壓力,就會讓它 傳送出痛苦的訊號。 在正常情況下, 這些訊號會被脊髓背角給削減掉。 但由於某些我們還不全然 了解的理由,在截肢後, 背角中的這種抑制 控制功能會消失, 訊號可能會變強。
Once they pass through the spinal cord, sensory signals reach the brain. There, the somatosensory cortex processes them. The entire body is mapped in this cortex. Sensitive body parts with many nerve endings, like the lips and hands, are represented by the largest areas. The cortical homunculus is a model of the human body with proportions based on the size of each body part’s representation in the cortex, The amount of cortex devoted to a specific body part can grow or shrink based on how much sensory input the brain receives from that body part. For example, representation of the left hand is larger in violinists than in non-violinists. The brain also increases cortical representation when a body part is injured in order to heighten sensations that alert us to danger. This increased representation can lead to phantom pain. The cortical map is also most likely responsible for the feeling of body parts that are no longer there, because they still have representation in the brain. Over time, this representation may shrink and the phantom limb may shrink with it.
一旦感覺訊號通過了脊髓, 就會抵達大腦。 在大腦中,感覺皮質 會負責處理這些訊號。 這個皮質區有整個身體的地圖。 因為有許多神經末稍 所以很敏感的身體部位, 像是嘴脣和手, 是由最大塊的區域來代表。 皮質小人是一種人體模型, 身體部位的比例是根據它們 在皮質區中對應的大小。 每個身體部位對應的皮質代表區 可能會長大或縮小, 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。 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。 當一個身體部位受傷時, 大腦也會增加對應的皮質代表區, 以強化該部位的感覺, 來警告我們可能的危險。 代表區的增加,就可能造成幻痛。 會感覺到已經不存在的身體部位, 也很有可能是皮質地圖造成的, 因為在大腦中還是有 那些部位對應的代表區。 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. Treatment for phantom pain usually requires a combination of physical therapy, medications for pain management, prosthetics, and time. A technique called mirror box therapy can be very helpful in developing the range of motion and reducing pain in the phantom limb. The patient places the phantom limb into a box behind a mirror and the intact limb in front of the mirror. This tricks the brain into seeing the phantom rather than just feeling it. Scientists are developing virtual reality treatments that make the experience of mirror box therapy even more lifelike. Prosthetics can also create a similar effect— many patients report pain primarily when they remove their prosthetics at night. And phantom limbs may in turn help patients conceptualize prosthetics as extensions of their bodies and manipulate them intuitively.
但幻覺的感覺 不見得都會自己消失。 若要治療幻痛,通常會需要 結合物理治療、止痛藥物、 假肢,和時間。 有一種叫做鏡箱治療的技術, 對於發展動作範圍 及減輕幻肢痛楚都很有幫助。 病人要把幻肢放入 鏡子後面的箱子內, 好的肢體放在鏡子前面。 這能欺騙大腦, 讓大腦以為看見了幻肢, 而不只是感覺到幻肢。 科學家正在開發虛擬境的治療法, 讓鏡箱治療的體驗能變得更真實。 假肢也能產生類似的效果—— 許多病人說自己主要是在晚上 把假肢拿掉之後才會感到痛楚。 幻肢有可能會反過來 協助病人將假肢給概念化, 視為是身體的延伸部分, 並以直覺的方式來操控假肢。
There are still many questions about phantom limbs. We don’t know why some amputees escape the pain typically associated with these apparitions, or why some don’t have phantoms at all. And further research into phantom limbs isn’t just applicable to the people who experience them. A deeper understanding of these apparitions will give us insight into the work our brains do every day to build the world as we perceive it. They’re an important reminder that the realities we experience are, in fact, subjective.
關於幻肢,還有許多沒解開的謎。 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, 也不知道為什麼有些人 根本不會有幻覺。 關於幻肢的進一步研究 不僅僅是針對感受到幻肢的人。 若能對這類幻覺有更深的了解, 我們就能更清楚大腦 每天會做些什麼工作 來建立出我們所感知到的世界。 它們是個重要的提醒, 提醒我們,我們所體驗到的現實,