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.
大多数失去肢体的人们仍能感受到它们的存在，其形式并非记忆或者模糊的形状，而是人能感受到的种种逼真细节。
They can flex their phantom fingers and sometimes even feel the chafe of a watchband or the throb of an ingrown toenail.
他们能弯曲幻肢中的手指，有时甚至可以感受到手表表带的摩擦或者嵌甲的刺痛。
And astonishingly enough, occasionally even people born without a limb can feel a phantom.
更惊人的是，有时那些生来就肢体残疾的人也能感受到这种幻觉。
So what causes phantom limb sensations?
所以是什么造成了幻肢的感知呢？
The accuracy of these apparitions suggests that we have a map of the body in our brains.
这些幻觉感知的精确度暗示着我们的大脑里有一张身体地图。
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.
而生来便肢体残疾的人有可能感受到幻肢的这个事实，说明了我们从出生以来就拥有地图的起点。
But one thing sets the phantoms that appear after amputation apart from their flesh and blood predecessors: the vast majority of them are painful.
但是在截肢后出现的这种幻肢与原有肢体之间的区别在于：大部分幻肢都带有疼痛感。
To fully understand phantom limbs and phantom pain, we have to consider the entire pathway from limb to brain.
为了全面了解幻肢和幻肢痛，我们必须研究从肢体到大脑的整条通道。
Our limbs are full of sensory neurons responsible for everything
我们的肢体上布满了感觉神经元来对外界做出反应，
from the textures we feel with our fingertips to our understanding of where our bodies are in space.
包括我们指尖感受到的质感和对自己身体所处空间位置的理解。
Neural pathways carry this sensory input through the spinal cord and up to the brain.
神经通道把这个感官输入从脊髓运输到大脑。
Since so much of this path lies outside the limb itself, most of it remains behind after an amputation.
由于通道的大部分都独立于肢体之外，它们都会在截肢后继续存在。
But the loss of a limb alters the way signals travel at every step of the pathway.
但是失去肢体会改变信号穿过通道中的每一步过程。
At the site of an amputation, severed nerve endings can thicken and become more sensitive,
在截肢的部位，切断的神经末梢会变厚并且变得更为敏感，
transmitting distress signals even in response to mild pressure.
它即使在温和的刺激下也会传递痛觉信号。
Under normal circumstances, these signals would be curtailed in the dorsal horn of the spinal cord.
在正常情况下，这些信号会在脊髓背角神经元处被截断。
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.
出于一些我们还未知的原因，在截肢后脊髓背角处的抑制作用有一定的减弱，所以痛觉信号就会被放大。
Once they pass through the spinal cord, sensory signals reach the brain.
一旦通过了脊髓这一关，信号就会被传递至大脑。
There, the somatosensory cortex processes them. The entire body is mapped in this cortex.
在那里，躯体感觉皮层会处理这些信号。人的整个身体的知觉都被这个皮层分区处理。
Sensitive body parts with many nerve endings, like the lips and hands, are represented by the largest areas.
布满神经末梢的敏感部位，比如说嘴唇和手，占据了最大面积。

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.
皮质小人是一种人体模型，他的身材比例代表了身体各个部位在皮层中所占比重。
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.
与身体部分所关联的皮层量受大脑所接受的感官输入影响会增多或者减少。
For example, representation of the left hand is larger in violinists than in non-violinists.
打个比方，小提琴家的左手比非小提琴家占据更大面积的皮层。
The brain also increases cortical representation when a body part is injured in order to heighten sensations that alert us to danger.
当身体的一部分受伤时，大脑也会增加这一块皮层的比例，从而放大这种感官并警告我们远离危险。
This increased representation can lead to phantom pain.
增大的皮层量会造成幻肢痛。
The cortical map is also most likely responsible for the feeling of body parts that are no longer there,
皮层地图很有可能让我们不复存在的身体部分有了感知，
because they still have representation in the brain.
因为这些身体部分还存在于大脑的地图中。
Over time, this representation may shrink and the phantom limb may shrink with it.
久而久之，幻肢也许会因为皮层减少而消失。
But phantom limb sensations don't necessarily disappear on their own.
但幻肢痛却不会自己消失。
Treatment for phantom pain usually requires a combination of physical therapy, medications for pain management, prosthetics, and time.
想要治疗幻肢痛通常需要结合物理治疗、止痛药，义肢和时间。
A technique called mirror box therapy can be very helpful in developing the range of motion and reducing pain in the phantom limb.
有一种治疗方法叫做镜箱治疗，它能帮助幻肢运动并减少疼痛感。
The patient places the phantom limb into a box behind a mirror and the intact limb in front of the mirror.
患者把自己的幻肢放入镜子背后的箱子，把完好的手放在镜子前。
This tricks the brain into seeing the phantom rather than just feeling it.
这便让患者的大脑误以为看见了真正的肢体，而不只是感觉到它。
Scientists are developing virtual reality treatments that make the experience of mirror box therapy even more lifelike.
科学家们正在研发虚拟现实治疗，从而使镜箱治疗更加真实。
Prosthetics can also create a similar effect -- many patients report pain primarily when they remove their prosthetics at night.
义肢也能产生同样的效果：许多患者称他们在晚上卸掉义肢时感到了疼痛。
And phantom limbs may in turn help patients conceptualize prosthetics as extensions of their bodies and manipulate them intuitively.
也许幻肢会帮助这些患者，让他们将义肢视作身体的一部分，并更轻松地使用自己的义肢。
There are still many questions about phantom limbs.
围绕着幻肢的疑问还有很多。
We don't know why some amputees escape the pain typically associated with these apparitions, or why some don't have phantoms at all.
我们不知道为什么有些被截肢者并没有感觉到幻肢的痛，而有些人甚至完全感受不到幻肢。
And further research into phantom limbs isn't just applicable to the people who experience them.
我们深入研究幻肢，不仅仅是为了帮助有幻肢感的人。
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.
对幻肢的深入了解还能使我们更深入理解大脑的运行方式，以及大脑怎样建立我们所认知的世界。
They're an important reminder that the realities we experience are, in fact, subjective.
这些认识提醒着我们，我们所经历的现实，一直都是主观上的。