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。 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。 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? 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.
所以是什么造成了 幻肢的感知呢? 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。 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。 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。 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 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.
我们的肢体上布满了感觉神经元 来对外界做出反应, 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。 神经通道把这个感官输入从脊髓 运输到大脑。 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。 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, 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.
在截肢的部位, 切断的神经末梢会变厚 并且变得更为敏感, 它即使在温和的刺激下 也会传递痛觉信号。 在正常情况下, 这些信号会在脊髓背角神经元处被截断。 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. 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.
一旦通过了脊髓这一关, 信号就会被传递至大脑。 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。 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。 ”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, 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。 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。 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。 增大的皮层量会造成幻肢痛。 皮层地图很有可能让我们 不复存在的身体部分有了感知, 因为这些身体部分 还存在于大脑的地图中。 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. 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.
但幻肢痛却不会自己消失。 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。 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。 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, 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。 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。 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。 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. 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.
围绕着幻肢的疑问还有很多, 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。 我们深入研究幻肢, 不仅仅是为了帮助有幻肢感的人。 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。 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。