0:00:07.867,0:00:11.598
Imagine, for a second,[br]a duck teaching a French class,

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a ping-pong match in orbit [br]around a black hole,

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a dolphin balancing a pineapple.

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You probably haven't actually seen[br]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[br]of something you've never seen?

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That may not seem hard,

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

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It turns out that this is actually[br]a complex problem

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that requires sophisticated coordination[br]inside your brain.

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That's because to create [br]these new, weird images,

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

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like a collage made [br]from fragments of photos.

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The brain has to juggle a sea of thousands[br]of electrical signals

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

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When you look at an object,

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thousands of neurons [br]in your posterior cortex fire.

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These neurons encode various [br]characteristics of the object:

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spiky, fruit, brown, green, and yellow.

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

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

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in this case the one for pineapple.

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In neuroscience, this is called[br]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,[br]assembling a complete mental image.

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Dolphins are encoded by a different[br]neuronal ensemble.

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In fact, every object that you've seen

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is encoded by a neuronal ensemble [br]associated with it,

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the neurons wired together[br]by that synchronized firing.

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

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

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

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So how come you can imagine it anyway?

0:02:04.922,0:02:07.760
One hypothesis, [br]called the Mental Synthesis Theory,

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says that, again, timing is key.

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If the neuronal ensembles [br]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[br]as a single image.

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But something in your brain [br]has to coordinate that firing.

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One plausible candidate [br]is the prefrontal cortex,

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which is involved in [br]all complex cognitive functions.

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Prefrontal cortex neurons are connected[br]to the posterior cortex

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by long, spindly cell extensions[br]called neural fibers.

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

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the prefrontal cortex neurons send[br]electrical signals

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down these neural fibers

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to multiple ensembles[br]in the posterior cortex.

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This activates them in unison.

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If the neuronal ensembles are turned on[br]at the same time,

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

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This conscious purposeful synchronization

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of different neuronal ensembles[br]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[br]neuronal ensembles at the same time.

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The problem is that some neurons

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are much farther away [br]from the prefrontal cortex than others.

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If the signals travel down both fibers[br]at the same rate,

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they'd arrive out of sync.

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You can't change the length [br]of the connections,

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but your brain, [br]especially as it develops in childhood,

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does have a way to change [br]the conduction velocity.

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Neural fibers are wrapped in a fatty[br]substance called myelin.

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Myelin is an insulator

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and speeds up the electrical signals [br]zipping down the nerve fiber.

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Some neural fibers have [br]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 [br]100 times faster or more

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than those with thinner ones.

0:04:06.565,0:04:09.995
Some scientists now think that this[br]difference in myelination

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could be the key [br]to uniform conduction time in the brain,

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and consequently, [br]to our mental synthesis ability.

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A lot of this myelination [br]happens in childhood,

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so from an early age,

0:04:21.814,0:04:26.115
our vibrant imaginations may have a lot[br]to do with building up brains

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whose carefully myelinated connections

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can craft creative symphonies[br]throughout our lives.