0:00:06.330,0:00:11.269
The vast majority of people [br]who’ve lost a limb can still feel it—

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

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They can flex their phantom fingers

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and sometimes even feel [br]the chafe of a watchband

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or the throb of an ingrown toenail.

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And astonishingly enough,

0:00:27.917,0:00:33.464
occasionally even people born [br]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 [br]of the body in our brains.

0:00:42.364,0:00:43.747
And the fact that it’s possible

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

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

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But one thing sets the phantoms[br]that appear after amputation

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apart from their flesh [br]and blood predecessors:

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the vast majority of them are painful.

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To fully understand phantom limbs [br]and phantom pain,

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we have to consider the entire pathway [br]from limb to brain.

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Our limbs are full of sensory neurons[br]responsible for everything

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from the textures we feel [br]with our fingertips

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to our understanding [br]of where our bodies are in space.

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Neural pathways carry this sensory input[br]through the spinal cord

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and up to the brain.

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

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most of it remains [br]behind after an amputation.

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But the loss of a limb

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alters the way signals travel [br]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 [br]and become more sensitive,

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transmitting distress signals [br]even in response to mild pressure.

0:01:50.697,0:01:52.386
Under normal circumstances,

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

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

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

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and signals can intensify.

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

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There, the somatosensory cortex [br]processes them.

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The entire body is mapped in this cortex.

0:02:20.611,0:02:23.375
Sensitive body parts [br]with many nerve endings,

0:02:23.375,0:02:25.136
like the lips and hands,

0:02:25.136,0:02:27.710
are represented by the largest areas.

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The cortical homunculus is a model [br]of the human body

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

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

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

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

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than in non-violinists.

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The brain also increases [br]cortical representation

0:02:56.377,0:02:58.489
when a body part is injured

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in order to heighten sensations [br]that alert us to danger.

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This increased representation [br]can lead to phantom pain.

0:03:06.700,0:03:09.617
The cortical map is also [br]most likely responsible

0:03:09.617,0:03:13.223
for the feeling of body parts [br]that are no longer there,

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because they still [br]have representation in the brain.

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

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

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Treatment for phantom pain [br]usually requires

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a combination of physical therapy,

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medications for pain management,

0:03:33.302,0:03:34.377
prosthetics,

0:03:34.377,0:03:36.052
and time.

0:03:36.052,0:03:38.301
A technique called mirror box therapy

0:03:38.301,0:03:41.368
can be very helpful in developing [br]the range of motion

0:03:41.368,0:03:43.967
and reducing pain in the phantom limb.

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The patient places the phantom limb [br]into a box behind a mirror

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and the intact limb [br]in front of the mirror.

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This tricks the brain [br]into seeing the phantom

0:03:53.866,0:03:55.913
rather than just feeling it.

0:03:55.913,0:03:59.045
Scientists are developing [br]virtual reality treatments

0:03:59.045,0:04:03.726
that make the experience [br]of mirror box therapy even more lifelike.

0:04:03.726,0:04:06.700
Prosthetics can also [br]create a similar effect—

0:04:06.700,0:04:08.318
many patients report pain

0:04:08.318,0:04:11.593
primarily when they remove [br]their prosthetics at night.

0:04:11.593,0:04:13.801
And phantom limbs may in turn

0:04:13.801,0:04:18.733
help patients conceptualize[br]prosthetics as extensions of their bodies

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and manipulate them intuitively.

0:04:21.142,0:04:24.530
There are still many questions [br]about phantom limbs.

0:04:24.530,0:04:27.688
We don’t know why some amputees[br]escape the pain

0:04:27.688,0:04:30.008
typically associated [br]with these apparitions,

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or why some don’t have phantoms at all.

0:04:32.490,0:04:35.149
And further research into phantom limbs

0:04:35.149,0:04:38.527
isn’t just applicable to the people [br]who experience them.

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A deeper understanding [br]of these apparitions

0:04:40.797,0:04:45.027
will give us insight into the work[br]our brains do every day

0:04:45.027,0:04:47.625
to build the world as we perceive it.

0:04:47.625,0:04:49.272
They’re an important reminder

0:04:49.272,0:04:53.408
that the realities we experience are,[br]in fact, subjective.