Even though it looks like a random, bumpy mess, your brain is incredibly organized.
Your brain has its own version of google maps for navigating the world around it -- but
just how flexible are these maps to changes?
Every sound you can hear or touch you can feel is because neurons in a specific part
of your brain are firing.
Our brains are organized “topographically”so that we can keep track of where things are
happening, … that means we can tell the difference between a touch on the arm or a
touch on the face.
By observing people’s brains in different imaging machines, neuroscientists have largely
mapped out touch, and other, sensations in the brain.
They found touch signals from different parts of the body get sent to different areas of
the brain -- and from that we’ve made somatotopic maps.
The cool thing about these maps is that they maintain spatial relationships, meaning the
receptors in my skin that sense touches to this part of my hand, and this one, send connections
to similarly neighbouring parts of the brain.
You have these maps for each of your senses.
In the case of touch, they’ve drawn out what’s called a homunculus in the brain
--- basically there’s a tiny man living inside your head.
Okay there’s not… but the homunculus is an easy way to see which parts of the primary
sensory cortex, right here, sense different parts of the body.
What you’ll notice is size is not totally proportional..
Bigger parts of the body don’t necessarily get more sensory space in the brain… instead,
regions with more sensitivity get more real estate in the sensory cortex.
And this could be why our lips and fingertips are a lot more sensitive than our arms?
They’re represented as larger in the homunculus, meaning there are more neurons paying close
attention to sensations coming from those regions.
Diagrams like these make it seem like the map is inflexible … but that might not always
be true.
Let’s say you lose a hand -- those sensory neurons don’t just go dark…
Researchers hypothesize that when a limb is lost, new connections may sprout into underused
brain areas, or perhaps weaker connections that were already there can now take over.
When looking at nonhuman primates with amputated limbs, researchers have seen touches to the
lips light up regions of the brain corresponding to where the limb was… which makes sense
if they’re using their mouths to do tasks they would’ve done with their hands before.
There are similar observations in adult humans too, though some argue it might depend on
what the patient uses to compensate for the lost limbs.
But a case study added some surprising new elements to this story.
A young boy with both his hands amputated received a double hand transplant.
Shortly after his transplant, he didn’t have any touch sensation in his new fingertips.
But, when they put him in an MEG machine, they saw that touches to his lips lit up the
parts of his brain that used to be for his hands.
Then tests seven and a half months after the transplant, touches to the lip activated only
the lip area of the sensory cortex, and touches to the fingertips produced unusually strong
responses in the finger area of the cortex.
Does this mean we can regrow and rewire massive parts of our brains within less than a year?
While it’s tempting to interpret things this way, there’s a big debate in the field
right now… and there’s no solid answer yet.
Patients with amputated limbs often report feeling their lost or “phantom” limbs,
sometimes painfully.
This probably means the sensory neurons for the lost limbs aren’t completely gone.
So perhaps in the case of this boy who received the transplant, he didn’t fully regrow new
connections, but other connections were briefly allowed to take over.
While it’ll be helpful and interesting(!) to know what’s really going on here, the
good news is that regardless, follow-ups a year and a half after the transplant, showed
the boy can write, dress, and feed himself more independently than before.
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Thanks for watching, talking body swaps, what about head transplants?
Crystal tackles that heady topic -- here.
One last thing: Have you done the two-point test before?
Test out your sensory homunculus by having a friend poke you with one or two sharp pencils
on the palm of your hand and on your upper arm, and see how accurately you can guess
if they used one or two points on either surface, depending how close and far apart they are.
Thanks for watching!