[music playing]

[birds chirping]

(calling) Eddie?

[birds chirping]

(calling) Eddie?

(calling) April! April!

(calling) Eddie!

So it's pretty obvious why vision matters. People have a visceral reaction to why vision matters. But when it comes to communicating, you wouldn't really understand what I was trying to convey if you couldn't hear me.

(calling) Eddie?

So probably the most important thing about audition when it comes to everyday life is just the ability to communicate and to speak to one another.

A sound wave is the basic unit of acoustic information.

(calling) April!

These vibrations of air molecules are transmitted in various shapes and sizes. And as with light waves, we have evolved biological mechanisms to detect and interpret these waves.

A sound wave is just jostling air molecules. You can think of it rather like people trying to get through a stadium tunnel, and they're bumping into each other.

[crowd murmuring]

And it's that physical wave of jostling air molecules that hits our eardrums.

So how do we detect these acoustic vibrations around us and extract what's important from the raw sensory data?

(calling) April!

To answer that question, we must go inside the ear.

Sound waves hit the eardrum, which initiate a mechanical motion in the middle ear.

This mechanical motion is conducted by three tiny bone structures known for resembling the shapes of a hammer, anvil, and stirrup.

Which then vibrates the fluid-filled cochlea.

Inside the cochlea, we find a specialized set of cells that translates fluid vibrations into nerve impulses.

On that membrane are these tiny little hair cells. They're called hair cells because they literally have roads of hair, stereocilia, that get mechanically displaced back and forth.

[splashing thumps]

And when they move back and forth, you get ions that flow into a neuron that change the voltage inside that neuron. So charged particles go into the cell and flow out of the cell as these channels open and close with the movement of the stereocilia. When that cell reaches a certain voltage because it's exceeded some threshold, it will emit a nerve impulse, a firing of a neuron that is transmitted up higher into the brain.

Every single neuron is either, at one instant, firing or not. And if you only had two or three of them, it would be a very crude kind of percept. You actually have about 30,000 hair cells in the ear.

One of the other things that's kind of fun about the auditory system is these vibrations of the stereocilia are actually mechanically linked to the vibrations in the air.

(calling) Eddie?

So at one end of the cochlea, these vibrations are slow and you get vibrations of opening and closing of the hair cells' channels very slowly. At the other end, they're very rapid. And that actually matters in the auditory system.

After the raw sensory data is captured by the ear, it moves along the auditory nerve through the thalamus and into the cortex. Evidence suggests that like the visual system, there are two distinct streams of processing that occur in the auditory cortex. From primary auditory cortex, information about what the stimulus is gets passed along the ventral stream to the temporal lobe. Information about where a stimulus is gets processed by the dorsal stream and end up in the parietal lobe.

Our sense of hearing is a finely evolved system, able to discern the very slightest changes in pitch, volume, and timbre. We unconsciously use the slight differences in time between a sound wave hitting each ear to place a sound in the environment, perching our heads from side to side to help localize it.

(calling) April!

(calling) Eddie!

(calling) April!

[music playing faintly]

But our auditory system is also quite fragile.

April. April!

[music playing faintly]

Prolonged exposure, or even one intense episode, can lead to the irreversible loss of hair cells, and thus the inability to detect certain pitches as well and to hear lower amplitude waves.

Hearing loss can be genetically influenced. It can also be environmentally influenced.

The most sensitive part of this whole system, in terms of everyday life for most of us, is in the cochlea, in the actual hair cells themselves. Unlike many other parts of the body, hair cells in the auditory cochlea don't regenerate. And they can be damaged if they move too much.

[tapping drumsticks]

And one of the ways they move too much is from having too loud a sound. So these things are actually vibrating mechanically, and if too loud a sound comes, it can cause them to rupture, which causes a preprogrammed reaction of cell death. And once they die, they're gone. So noise exposure is a huge part of hearing loss.

[music playing faintly]

If we can avoid damaging hair cells, hearing is the source of great enjoyment, of appreciating music and very loud music at times. It is a fragile system. It is resilient. And it is a feat of millions of years of evolution, always ready for the next vibration.

[music playing]