WEBVTT

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

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

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They can flex their phantom fingers

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and sometimes even feel 
the chafe of a watchband

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or the throb of an ingrown toenail.

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And astonishingly enough,

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

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So what causes phantom limb sensations?

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The accuracy of these apparitions

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

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And the fact that it’s possible

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

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

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But one thing sets the phantoms
that appear after amputation

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apart from their flesh 
and blood predecessors:

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the vast majority of them are painful.

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To fully understand phantom limbs 
and phantom pain,

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

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Our limbs are full of sensory neurons
responsible for everything

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from the textures we feel 
with our fingertips

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to our understanding 
of where our bodies are in space.

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Neural pathways carry this sensory input
through the spinal cord

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and up to the brain.

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

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most of it remains 
behind after an amputation.

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But the loss of a limb

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

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At the site of an amputation,

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severed nerve endings can thicken 
and become more sensitive,

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transmitting distress signals 
even in response to mild pressure.

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Under normal circumstances,

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

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

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

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and signals can intensify.

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

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There, the somatosensory cortex 
processes them.

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The entire body is mapped in this cortex.

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Sensitive body parts 
with many nerve endings,

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like the lips and hands,

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are represented by the largest areas.

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The cortical homunculus is a model 
of the human body

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

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

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

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

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than in non-violinists.

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The brain also increases 
cortical representation

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when a body part is injured

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in order to heighten sensations 
that alert us to danger.

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This increased representation 
can lead to phantom pain.

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The cortical map is also 
most likely responsible

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

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because they still 
have representation in the brain.

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

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

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Treatment for phantom pain 
usually requires

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a combination of physical therapy,

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medications for pain management,

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prosthetics,

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and time.

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A technique called mirror box therapy

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can be very helpful in developing 
the range of motion

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and reducing pain in the phantom limb.

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

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and the intact limb 
in front of the mirror.

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This tricks the brain 
into seeing the phantom

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rather than just feeling it.

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Scientists are developing 
virtual reality treatments

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

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Prosthetics can also 
create a similar effect—

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many patients report pain

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primarily when they remove 
their prosthetics at night.

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And phantom limbs may in turn

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help patients conceptualize
prosthetics as extensions of their bodies

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and manipulate them intuitively.

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There are still many questions 
about phantom limbs.

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We don’t know why some amputees
escape the pain

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typically associated 
with these apparitions,

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or why some don’t have phantoms at all.

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And further research into phantom limbs

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

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A deeper understanding 
of these apparitions

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

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to build the world as we perceive it.

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They’re an important reminder

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