The fascinating science of phantom limbs - Joshua W. Pate

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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 watch band or the throb of an ingrown toenail. What causes these phantom limb sensations? Joshua W. Pate explains how the brain reacts to a missing limb.

Lesson by Joshua W. Pate, directed by Kozmonot Animation Studio.

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.