Boy Listening

I saw my neighbor on the street yesterday. She said “hello” and I said “hello.” No big deal? Wrong. Hello is a simple word and most of us say it and hear it many times each day. Yet each and every time, those two syllables—or even a simple “beep”—set off a process inside the skull as complicated and remarkable in its sequencing and precision as a Beethoven symphony. 

IN THE EAR

The design of the human ear may not be streamlined, but it is effective, transforming sound waves into electrical signals the brain can understand. It is also particularly well suited to the human voice; our keenest hearing is usually in the range required to hear speech. 

To say hello, I push air out of my throat, making air molecules vibrate and creating a sound wave, a form of energy that can move through air, water, metal or wood, carrying detailed information.

The outer ear is designed to catch that energy in the folds of the earlobe and then funnel it into the ear canal, which acts as a resonance chamber. When the waves hit the eardrum, the ensuing vibrations are carried across the little pocket of the middle ear by a set of tiny bones—the smallest in the body—called the ear ossicles, but more commonly known as the hammer, anvil, and stirrup. Converting the original acoustic energy to mechanical energy, the hammer hits the anvil, the anvil hits the stirrup, and the stirrup, piston-like, hits a membrane-covered opening called the oval window.

On the far side lies the fluid-filled cochlea, the nautilus-like heart of the inner ear. The vibrations transmitted through the oval window send pressure waves through the cochlear fluid; mechanical energy has become hydro energy. Outside, hard, bony walls protect the cochlea. Inside, the basilar membrane runs along its length like a ribbon. Thin as cellophane, the basilar membrane is stiff and narrow at one end, broad and flexible at the other. As sound waves wash through, the basilar membrane acts as a frequency analyzer. Higher-pitched sounds, like hissing, excite the stretch of membrane closest to the oval window; lower pitches, like rumbling, stimulate the farther reaches. Like inhabitants of a long curving residential street, specific sounds always come home to the same location, a particular 1.3 millimeters of membrane and the thirteen hundred neurons that live there, representing a “critical band” of frequencies.

Sitting on top of the basilar membrane is the romantically named organ of Corti. Known as the seat of hearing, it holds thousands of hair cells. Twelve thousand outer cells, organized in three neat rows, amplify weak sounds and sharpen up tuning. Another four thousand inner hair cells, in one row, take on the work of sending signals to the auditory nerve fibers. Like microscopic glow sticks that light up when you snap them, the tiny stereocilia on each hair cell bend under the pressure of the movement of fluid caused by the sound wave and trigger an electrical impulse that travels up the nerve to the brain.

IN THE BRAIN

Now we’re in the central auditory system. The auditory nerve is not just one nerve but a bundle of nerve fibers—a coaxial cable of sorts—connecting the cochlea to the brain stem, where the auditory nerve ends at two collections of neurons called the cochlear nuclei and things start to get complicated. That is as it should be. Brain processing gets more sophisticated as it ascends, the way a good curriculum builds on itself and asks more of students as they move through school. 

The cochlear nuclei sort incoming auditory signals along two different tracks, and contain specialized cells to cover every possible kind of signal. The organization by tone of the basilar membrane, for instance, is repeated in the brain stem, so that some cells respond to high-frequency sounds and others to low-frequency sounds. It is thought that features such as your ability to tell where a sound came from or the fact that you jump at loud noises can be traced to specific cells.

From the cochlear nuclei, sound signals follow two parallel pathways—along the back and the belly of the brain—passing through regions such as the superior olive (really) and the medial geniculate until they reach the auditory cortex in the temporal lobe, just above the ear where the sound started. Here, too, there are specialized cells to extract features of the sound and make sense of it. With practice, the auditory cortex gets better and better at listening.

As a result, babies learn to talk, a trained technician can tune a piano by ear, and you can appreciate that Beethoven symphony. 

Listening with Headphones

Coming next: Vision

Parts of this post and the illustration of the auditory system originally appeared in I Can Hear You Whisper: An Intimate Journey through the Science of Sound and Language (Dutton, 2014).

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