[MUSIC PLAYING]
SPEAKER: The brain is a magical organ.
For us to hear sound, it does things complex and incredible,
but not unknowable.
The brain takes sound waves which journey through the air
and through our skulls and transforms them into sound in our minds.
This sonic odyssey is beautiful, filled with weird landscapes,
unexpected encounters, catastrophe, and joyful
destinations, all like a strange aural fairy tale.
Hearing evolved because it helped animals to survive.
Hearing allowed predators to find prey and eat them,
and hearing allowed prey to spy on predators and to escape.
The floppy piece of skin attached to the sides of our head
is our outer ear, or pinna.
It collects sounds from a wide area of our evironment,
like a reflector disk of a satellite dish.
In humans, the pinna is in a fixed position, but in many animals,
the pinna are highly flexible and can be oriented toward a source of sound.
Curiously, our auditory system has evolved
beyond it's strict utility of survival.
For example, music, through its perception
of sound by our auditory system, evokes sensations and emotions.
Sound is produced when something vibrates,
causing variations in air pressure.
Particles of air collide with other particles, and the wave of movement
bouncing from particle to particle passes outward through the air.
This is called a sound wave.
And now our brave sound waves are approaching the outer ear
and are truly starting their journey.
Once they have entered the ear through the pinna,
they pass down the air-filled auditory canal before hitting the eardrum.
As the sound waves strike the eardrum, the vibrations in the air
are transferred through that membrane into a series of little bones
called the ossicles.
There are three bones in this group, the malleus, incus, and stapes,
also known as the hammer, anvil, and stirrup.
They function to transmit sounds from the air-filled outer ear
into the fluid-filled inner ear by allowing the sound
to enter them through the eardrum, vibrate through them,
and then exit into the inner ear via a second membrane called the oval window.
The inner ear contains the sensory organs for both hearing
as well as for our sense of balance.
The cochlea, an important organ for the auditory process,
is snail-shaped and if rolled up, is only about the size of a pea.
Inside the cochlea is the organ of Corti,
which sits on the basilar membrane and below the tectorial membrane.
And inside the organ of Corti are thousands of auditory hair cells.
These hair cells are, in fact, receptor neurons called stereocilia.
These hair cells convert the motion energy of sound waves
into electrical signals that are communicated to the auditory nerve.
From there, the signal goes into structures deeper in the brain
until at last, it reaches the auditory cortex, where we consciously
experience sound.
To back up, when our sound waves enter the fluid-filled inner ear,
they are thrust into a storm, a storm of vibrations
that they are, in fact, creating.
The basilar membrane below them is violently shaking and tossing them.
The tectorial membrane above them is moving back and forth
and pushing and threatening.
And in the midst of this maelstrom, as they are broken apart,
they spiral through the inner ear and the sound waves finally
begin to weaken.
The inner ear absorbs all their kinetic strength,
and the sound waves begin to realize that this will be their end.
Now, finally, in the chaos of the inner ear, they disappear.
The sound waves have ceased to be, but though the vibrations are now no more,
they are not completely gone.
They have metamorphosed into something new, into neural activity.
And they come to life again in the cortex of the brain.
In the consciousness of our minds, their journey is over.
The brain has taken what was outside and made it inside.
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