WEBVTT

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Imagine, for a second,
a duck teaching a french class,

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a ping-pong match in orbit 
around a black hole,

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a dolphin balancing a pineapple.

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You probably haven't actually seen
any of these things,

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but you could imagine them instantly.

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How does your brain produce an image
of something you've never seen?

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That may not seem hard,

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but that's only because 
we're so used to doing it.

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It turns out that this is actually
a complex problem

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that requires sophisticated coordination
inside your brain.

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That's because to create 
these new, weird images,

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your brain takes familiar pieces 
and assembles them in new ways,

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like a collage made 
from fragments of photos.

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The brain has to juggle a sea of thousands
of electrical signals

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getting them all to their destination
at precisely the right time.

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When you look at an object,

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thousands of neurons 
in your posterior cortex fire.

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These neurons encode various 
characteristics of the object -

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spiky, fruit, brown, green, and yellow.

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This synchronous firing strengthens the
connections between that set of neurons,

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linking them together into what's known
as a neuronal ensemble,

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in this case the one for pineapple.

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In neuroscience, this is called
the Hebbian principle,

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neurons that fire together wire together.

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If you try to imagine a pineapple later,

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the whole ensemble will light up,
assembling a complete mental image.

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Dolphins are encoded by a different
neuronal ensemble.

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In fact, every object that you've seen

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is encoded by a neuronal ensemble 
associated with it,

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the neurons wired together
by that synchronized firing.

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But this principle doesn't explain
the infinite number of objects

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that we can conjure up in our imaginations
without ever seeing them.

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The neuronal ensemble for a dolphin
balancing a pineapple doesn't exist.

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So how come you can imagine it anyway?

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One hypothesis, 
called the Mental Synthesis Theory,

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says that, again, timing is key.

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If the neuronal ensembles 
for the dolphin and pineapple

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are activated at the same time,

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we can perceive the two separate objects
as a single image.

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But something in your brain 
has to coordinate that firing.

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One plausible candidate 
is the prefrontal cortex,

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which is involved in 
all complex cognitive functions.

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Prefrontal cortex neurons are connected
to the posterior cortex

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by long, spindly cell extensions
called neural fibers.

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The mental synthesis theory proposes
that like a puppeteer pulling the strings,

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the prefrontal cortex neurons send
electrical signals

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down these neural fibers

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to multiple ensembles
in the posterior cortex.

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This activates them in unison.

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If the neuronal ensembles are turned on
at the same time,

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you experience the composite image
just as if you'd actually seen it.

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This conscious purposeful synchronization

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of different neuronal ensembles
by the prefrontal cortex

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is called mental synthesis.

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In order for mental sythesis to work,

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signals would have to arrive at both
neuronal ensembles at the same time.

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The problem is that some neurons

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are much farther away 
from the prefrontal cortex than others.

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If the signals travel down both fibers
at the same rate,

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they'd arrive out of sync.

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You can't change the length 
of the connections,

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but your brain, 
especially as it develops in childhood,

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does have a way to change 
the conduction velocity.

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Neural fibers are wrapped in a fatty
substance called myelin.

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Myelin is an insulator

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and speeds up the electrical signals 
zipping down the nerve fiber.

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Some neural fibers have 
as many as 100 layers of myelin.

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Others only have a few.

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And fibers with thicker layers of myelin

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can conduct signals 
100 times faster or more

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than those with thinner ones.

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Some scientists now think that this
difference in myelination

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could be the key 
to uniform conduction time in the brain,

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and consequently, 
to our mental synthesis ability.

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A lot of this myelination 
happens in childhood,

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so from an early age,

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our vibrant imaginations may have a lot
to do with building up brains

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whose carefully myelinated connections

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can craft creative symphonies
throughout our lives.