[SoundStage!]Getting Technical
Back-Issue Article

January 2004

How We Hear

For audiophiles, more so than most other people, the sense of hearing is critical. Without it, how would you be sure that your system is better than your friend's?

Human hearing is quite amazing, and the ranges within which we can perceive loudness and frequency are incredibly broad. Our ability to enjoy and create music -- and most importantly, to be able to understand the complex nature of speech -- are all functions of our auditory system.

The ear

Why is our outer ear shaped in the bizarre form it is? For two reasons: to collect sound similar to the way a megaphone does, and to create small reflections that aid in our localization of sound. These reflections that occur in the folds of our ears differ depending on the direction of sound.

Sound is funneled from the outer ear into the ear canal, which is better known as the thing you don't stick things into -- but if you do, it's only about 3cm long, so don't stick anything in longer than that. The ear canal is shaped to amplify higher sound frequencies between 1500 to 7000Hz, which is one of the reasons why this is our most sensitive frequency range, though we can generally perceive frequencies from 20 to 20,000Hz.

Sound waves travel down the ear canal to slam up against the thin tissue of the eardrum, which vibrates. Since the ear's three tiny middle bones are connected to the eardrum, they also begin to vibrate.

The chamber containing these bones is connected to the back of the mouth by the Eustachian tube, whose function is to keep the air pressure inside the ear equal to that outside. When you're flying and your ears pop, you're opening your Eustachian tube and adjusting the middle ear to the new pressures.

Connected to middle bones is the oval window of the inner ear. Located here is the cochlea, a snaillike structure full of fluid and microscopic hairs. The vibrations of the middle bones are transmitted into the fluid via the oval window of the cochlea, where the mechanical energy of the sound vibrations is transformed into electrical nerve impulses.

The specific structure that achieves this lies within the cochlea and is called the organ of Corti. It contains columns of hair cells that, when bent by the movement of the fluid, cause a nerve impulse to be fired. Actually, to be more precise (or more pedantic), the membrane upon which the hair cells sit moves. This bumps the tops of the hair cells and that is how they bend. Which group of hair cells bends determines our perception of frequency, while the number of bending hair cells determines our perception of loudness, which ranges from 20 to 140dB. This is a very simplistic description, since in the case of both our perception of frequency and loudness, neural processing occurs that makes it possible for us to hear such wide ranges.

From the hair cells many nerve fibers extend, all of which end up in the same place -- the brain -- because hearing, in reality, is all in your head.

The brain

Next, I'll discuss how the information collected by our ears is processed by the brain. This brings back fond memories of anatomy class; I can almost smell the formaldehyde.

Right before the nerve fibers enter the brain, they join up, forming the auditory nerve. This passes through our skull -- thick or thin -- into the brain's cochlear nuclei.

Before I proceed, it's important to note that similar hair cells sit next to one another. Likewise, their similar nerve fibers stick together as they pass into the brain. This organized arrangement of cell and fiber is similar for all of our senses, which makes sense. You wouldn't want stuff all over the place the way it is in my office. I can't find anything when I want to. Unfortunately, one of the down-sides of this arrangement is that if you get a nasty bump on the head, or suffer a stroke, the damage wipes out a lot of one specific thing, not a little bit of everything.

As I said, we have now traveled into the brain. From here on, we are only concerned with the journey our nerve impulses take from the ear to the auditory cortex, which is the sound-receiving area of the brain. As with our other senses, all the information entering one side of the brain travels to the opposite side of the brain, except it's not really "all" the information. In reality, only 60% of auditory nerve fibers cross to the opposite side, while the rest stick to their original side. And as inexplicable as this is, it gets even stranger. Those fibers that do cross work faster and better, guaranteeing that your left ear is more strongly represented on the right side of your head, and vice-versa.

There are other implications concerning this nerve-fiber crossover. Everyone has heard of left-brain/right-brain, and knows that there are differences between the sides. But the widely held notion that one side dominates and therefore predicts your personality is nonsense. With hearing, the auditory cortex of the right side concerns itself with the perception of music and other nonverbal sounds, while the left side concerns itself with the extremely important perception of speech.

This physical separation of tasks results in bizarre behaviors for those with brain damage. If significant damage occurs to the left side, people can no longer remember or understand spoken words, yet can still read. Meanwhile, if the right side is damaged, musicians completely lose their ability to play music or even sing "Happy Birthday."

Returning to our tour of auditory nerve impulses through the brain, before the information from our ears reaches our auditory cortex, there are a number of stops along the way. Furthermore, auditory processing has three separate paths: the startle reflex and localization of sound (which are both subconscious processes), and conscious perception of sound. To make this simple, I'll describe the route taken by one auditory nerve. Each nerve takes the same route, just on opposite sides, and the structures mentioned are on both sides of our brain. The first stop of the auditory nerve is in our hindbrain, the ancient part of our brain that lays underneath the big, convoluted cortex we all picture when we think of a brain. Within the hindbrain sits the cochlear nucleus that responds to different frequencies and also initiates the startle reflex. From the cochlear nucleus, the next stop in the hindbrain is the olivary nucleus. Here, the nerves begin to cross over to the opposite side. The olivary nucleus responds to the time differences between perceived sounds. Therefore, its role is the localization of sound, which is a subject worth going into.

To be able to react appropriately to sounds -- that is, to jump when you hear a car horn -- the brain needs to able to locate where those sounds are coming from. To successfully do this, the brain must know the direction the sound is coming from and its distance. As mentioned earlier, our outer ear is able to provide some information about direction, but the remaining information is provided by a time-of-arrival mechanism. The mechanism requires both ears, because our brain judges the time difference between similar incoming sounds and from this calculates the direction. The brain requires both sources of information because this time-of-arrival mechanism cannot tell if a sound is coming from in front of, above, or behind the head. The processing of direction is done by the olivary nucleus, and it does a good job, as evidenced by the following example. Experiments have demonstrated that blindfolded people are able to stop walking before hitting a wall. But, if they're given ear plugs, they'll walk smack into the same wall.

Distance is perceived by changes in loudness. The louder the sound, the nearer it is, and vice versa. Information about distance, plus the direction information from the olivary nucleus, travels to the inferior colliculus of our midbrain. This structure helps form a map of the sounds in our surroundings. The information then travels to the medial geniculate nucleus that sits in the thalamus. Here auditory information is passed on to our emotional center before it is sent to the auditory cortex. (You can thank a properly functioning medial geniculate nucleus for causing you to smile when Led Zeppelin comes on the radio.) Auditory information is also passed directly on to its final destination: the auditory cortex.

The auditory cortex sits on either side of your head just above your ears and is part of the temporal lobe. The auditory cortex is organized just like hair cells, with neurons that respond to similar frequencies situated near each other. There is also an area dedicated to the localization of sounds, since even though the olivary nucleus and the superior colliculus do most of the work, the auditory cortex makes us aware of where sounds are.

The ability to recognize language is located in its own special place -- Wernicke's area -- within the auditory cortex. If anything happens to this area, you will lose your ability to understand speech but will still be able to talk, although your words will be mixed up and come across as babbling. It's interesting to note that if damage occurs to another special region -- Broca's area of your frontal lobe -- you can still understand speech perfectly but are unable to utter a single word.

The auditory cortex is the final destination of the sound waves that have reached your ears. Here, the information about the sound meets with other information, such as what you see, your memories, and any emotional connotations associated with the sound. Most importantly, the auditory cortex makes us conscious of what our ears are sensing; it allows us "to know" what we are hearing. When you turn on your sound system, your auditory cortex will tell you if the music has good imaging, atmosphere, depth, smoothness, and fullness -- or if it's time for a component upgrade.

...Alison Aulph
alisona@soundstage.com

 

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