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. 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. 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. 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. 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. 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. 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.