Silence. Is it emptiness? Is it nothing? When you listen to the si- lence around you in a quiet place, what you really hear are the tiny sounds that are usually drowned out by background noise:
dry leaves blowing across the pavement, the crunch of pebbles beneath your feet, the call of a bird, the skitter of a tiny creature on the path, the wind in the trees. If you close your eyes, you can tell the seasons or the time of day. You can tell if it’s spring or fall from the way the wind rustles through the trees. In fall, the leaves sound crisp; in spring, they are silky soft and the breeze flowing through them is almost inaudible. On a summer night the crickets’ rhythmic chirping seems to come from all around, and rises and falls like waves upon the air. At dawn the birds take over—a cacophony of song you never hear in the heat of midday. In quiet places, we hear the life around us and come into closer contact with our world. We are in touch with nature, in tune with our thoughts, and aware of forces greater than our- selves. As Henry David Thoreau said in Walden (1854), the book he wrote while living alone in the forest: “I wish to hear the silence of the night, for the silence is something positive and to be heard. . . . The silence rings. It is musical and thrills me.”
But when you are sick in bed in a hospital room, you do not hear these things. You certainly do not hear silence. At precisely the moment you are craving peace and quiet, you hear all sorts of noises—loud noises. The sound in intensive-care units can range from forty-five to ninety-eight decibels: the clatter and
whir of machinery, the click of heels on hard-tiled floors, tele- phones ringing, human voices reverberating off walls and ceil- ings and metallic countertops. All through the night, you hear doctors and nurses going about their daily rounds, the chatter of visitors in the hallways, the moaning of other patients in pain.
None of these are comforting sounds. None of them remind you of home and health.
Our perception of sounds begins in the sensory organ de- signed specifically for this purpose: the ear—an incredibly deli- cate and complex instrument that detects movements of air molecules and the differences in frequency and pitch that the movements create. When a pulse of sound reaches the ears, it gets funneled into the auditory canal by thepinnae, the fleshy protuberances that every schoolchild draws as semicircles stick- ing out on either side of the head. In humans this part of the ear is less agile than in dogs or cats or bats, which can quickly turn their pinnae toward a sound to capture it. The next thing the waves of air hit is thetympanum,a tightly stretched membrane that’s located at the end of the external auditory canal and that vibrates like a drum. Hence its name: eardrum. The air distur- bances hit your eardrums and make them vibrate at the same fre- quency as the sound, much the way stereo speakers vibrate with music. But instead of being connected to a speaker, the eardrum is connected to three tiny, loosely hinged bones inside the next chamber, the middle ear. Each bone is delicate and exquisitely shaped. One looks like a hammer and is called by its Latin name, malleus.The next, theincus,looks like an anvil. And the third, thestapes,looks like a stirrup. When the eardrum vibrates, these bones vibrate in tune with its movement and with the move- ment of the air.
In the last chamber, the inner ear, the sound waves reach a carpet of cells. These cells have very fine filaments that wave above the carpet like blades of grass. They are calledhair cells,
and this organ, the real organ of hearing, is called thebasilar membrane of the cochleabecause it is a flat, ribbon-like structure that winds around in a spiral, like a snail shell (kochliasin Greek).
Each filament is a different thickness and thus vibrates best at a different frequency, just as each string on a guitar vibrates at a different frequency according to its thickness. In the final step, the vibrating hairs trigger electrical impulses in the fibers of the auditory nerve cells on which they sit. The rate of firing corresponds to the frequency of the hairs’ vibrations. And so, through this chain of tiny musical instruments, from eardrum through middle ear to inner ear, air movements are translated into electrical signals that travel along nerve pathways through- out the brain.
As the electrical signals are handed off from one nerve cell to the next, they divide intowherestreams (nerve cells that tell you where a sound is coming from) and whatstreams (nerve cells that tell you what the sound is). Bats are particularly good at de- tecting where a sound is coming from, while primates, including humans, are better at detecting what a sound is. We can locate sounds—and in turn use them to locate ourselves in space—
because we hear in stereo. A sound will arrive at one ear just a few thousandths of a second sooner than it arrives at the other ear. The auditory centers in our brains are able to detect this slight discrepancy and employ it to determine where a sound is coming from. A moving sound also gets louder as it approaches us and softer as it draws away from us. Think of an ambulance on a city street—you know how far away it is by the loudness of the siren.
The part of the brain where sounds are interpreted is called theauditory cortex.It lies in a region known as thetemporal lobe.
As its name suggests, this is the area behind the temples, toward the middle of your head, just under the skullbone behind your ears. Just as visual signals from the retina maintain their spatial
organization all the way to the brain, so the keyboard-like layout of the hair cells in the cochlea, extending from low pitch to high, is maintained through these auditory switching stations all the way to the auditory cortex, which contains a pitch keyboard much like that in the cochlea. There tones are laid out from low to high frequency, in an arrangement called atonotopic map.
Marc Raichle, a neurologist at Washington University in St. Louis and also an accomplished pianist, was among the first researchers to demonstrate this layout. In the late 1970s and early 1980s, he was one of a small group of pioneers who used PET scans to study brain function and blood flow. In this type of brain imaging, high-energy radioactive compounds are injected into the bloodstream and decompose within minutes. A scanner that detects radioactivity constructs an instantaneous picture of blood flow in regions where nerve cells are active. When Raichle exposed research participants to a series of pure tones much like a piano scale, he was amazed: areas of blood flow became active in an arrangement that looked just like a piano keyboard. This map-like representation of blood flow corresponded to the re- gions of the brain where nerve cells were firing in an orderly fashion according to pitch. He had achieved a first glimpse of blood flow to the tonotopic map!
Early in the process, before a sound signal reaches the audi- tory cortex but after it leaves the hair cells, the streams of nerve cells divide, some branching toward the speech areas in the brain and others toward areas that are specialized for detection of musical features. Feature extractionis a very important part of how we perceive sound, just as it is essential to how we see.
The speech areas deconstruct sounds into the individual units that constitute spoken language—the phonemes that make up words—while the music areas detect those elements of sound that make up music, including pitch, timbre, contour, and rhythm. Some streams take a shortcut to the part of the brain
that measures timing. Others flow to the various emotion cen- ters in the brain. And thus sounds, particularly music, can trig- ger many different emotional responses.
In the rolling hills just north of Boston, a small oval pond lies hidden in the woods. Since the late nineteenth century, the lo- cals have used this swimming hole to escape the heat of summer.
The water is surprisingly warm. But as you push your inner tube toward the middle of the lake to escape the squishy, weedy bot- tom near the shore, fingers of cold water brush against you.
These come from the spring that bubbles up to feed the pond.
The little pond is drained at the south end by a nameless brook that runs through wetlands filled in summer with purple loosestrife, skunk cabbage, delicate jewelweed, and St. John’s Wort. The brook flows south and empties into a small river called the Spicket. This feeds into the Merrimack, which, after meandering through salty sand flats, joins the Atlantic Ocean northeast of Boston. The Merrimack was, and still is, a main commercial transport route through central New England.
It was because of rivers like the Merrimack and their tribu- taries that in the mid-1800s German, Irish, and French Cana- dian settlers came to this area at the height of the Industrial Rev- olution. Here they built mills to weave cloth made of cotton, wool, and linen. The falls along the streams and rivers provided the power to turn the water wheels that moved the shuttles of the looms. The huge redbrick buildings still dominate the small cities scattered through these hills—Manchester, Lowell, Law- rence, Methuen, Haverhill.
If you follow the no-name brook to the Spicket and then trace the river south, you come to a spot where it drops sud- denly by about forty feet. These falls mark the center of the town of Methuen. An old mill once stood at this site, now a parking lot. On one side of the parking lot stands an imposing
redbrick building. It could be a church or maybe the Town Hall, but in fact it is neither. Built in 1909 for an interior designer, railroad magnate, and real estate developer named Edward Searles, the building was constructed with one sole purpose: to house an organ.
Walk up the steps, then through an unimposing entrance foyer, and you find yourself in the back of a sunny auditorium, two stories high, dominated at the north end by an enormous organ made of American black walnut, reaching to the dome. So amazing is this sight that you stop and stare. The wooden case, which supports the thirty-foot-tall pipes arranged in a graceful arc, is richly carved with figures evoking Michelangelo. Two At- las-like torsos strain to support the tallest pipes to the left and right of the keyboard. A pair of famous composers support the lesser pipes on either side of the stage. Marching toward each side of the stage are stalwart Valkyries with long Teutonic braids and stern faces. And above the center of the stage, a bust of J. S. Bach surveys all in silence.
If you happen to wander into the hall during rehearsal for the July Fourth celebration, you might see the choir practicing un- der the direction of the church’s pastor. He moves quickly back and forth, from conducting the choir to sitting at the organ bench. Hands and feet poised, he gives one last nod to the choir, then drops his hands to the tiered keyboard and lunges forward, simultaneously pressing keys and pedals. Abruptly the hall fills with a great reverberating sound. You are engulfed by the vibra- tions, which seem to rise up through the floor. You stand mo- tionless and allow them to move up your spine and carry you away. Until Edward Searles died, he was the only person privi- leged to hear this sound. He had bought the organ and built the hall in memory of his dead wife, who loved music. When Searles sat alone, sunk in reverie, bathed in the sounds of this enormous organ, he was able to reexperience the emotions he had felt
when his wife had lived, as well as the profound grief he felt after her death.
How is music able to evoke emotions and memories? And what is it about the first reverberating chords of the organ that stirs the listener so deeply? Where is the source of that awe and surprise, that shivery feeling you get when you hear the organ’s first blast?
“Goose bumps. Goose bumps. Goose bumps.” These were Daniel Levitin’s response when he worked with the musician Carlos Santana, the response that made him leave his career as a successful California recording engineer to study neuroscience.
Levitin recalled the exact moment when, preparing to tinker with the sound equipment as Santana played, he felt those goose bumps and asked himself: What was it about the music that had such an effect on him? What was happening in his brain?
Just as with vision, hearing works best when there is contrast.
And contrast entails the element of surprise—the jolt we experi- ence when, say, we emerge from a low cramped space into a large open one, or when a quiet hall fills suddenly with sound. It is the astonishment we feel when we are lulled by the warm New England countryside and then abruptly encounter an enormous organ. Our response is both physical and emotional as we’re struck by this juxtaposition of low and dark with towering light, of quiet stillness with reverberating sound, of bucolic fields with thundering organ. First you feel a shiver in your spine, and maybe a tingling in your fingers. Then you catch your breath.
You inspire and are inspired.
It is the contrast between silence and sound that produces this effect. Had the organ struck its chords in the middle of rush-hour traffic in Manhattan, they would have been barely perceptible. In fact, this is a problem that New York City author- ities have struggled with as sound pollution has increased—how
to make the sirens of emergency vehicles noticeable above the din. Sirens not only need to be louder but also need a changing patternof sound, in order to be heard.
The reason for this is that, at all levels in the nervous sys- tem—a single nerve cell, groups of cells, entire brain regions—
what is most detectable is difference. Nerve cells respond better to a sudden change than they do to repeated stimuli of the same kind or intensity. This is true for all our senses.
The difference between the firing rate of a nerve responding to a sound, and the nerve’s background firing rate, is called the signal-to-noise ratio.In this case, “noise” refers to the low-level chatter of nerve cells’ electrical activity, which goes on all the time. The greater the difference between the specific sound sig- nal and the background activity, the more distinctly the sound is heard. Loud sounds, which cause nerve cells to fire at a higher rate, have the greatest signal-to-noise ratio and are more clearly audible than soft sounds. But soft sounds heard in a very quiet environment can have a signal-to-noise ratio that is equally pro- nounced, and thus they can be heard quite clearly.
Sudden change—a noise, a puff of air—prompts a reaction in all animals called thestartle response—a reflex similar to the knee jerk that is triggered when the doctor taps your knee with a rubber hammer. You are sitting in your chair, hard at work at your computer, and the office prankster sneaks up behind you and claps his hands loudly. You jump—practically leap out of your seat. If you were to place a digital scale on your seat cush- ion, you would find that the reflex increases with the intensity of the sound: the louder the sound, the higher you jump and the harder you land. In fact, this is a standard way to test the startle response in animals: a scale is embedded in the floor of the ani- mal’s cage. In humans, the intensity of this reflex can be mea- sured by gluing small electrodes on top of the eyelids. Each time
you jump, you also blink your eyes, and the intensity of the blink correlates with the degree of startle.
Why would researchers go to such lengths to measure the startle reflex in animals and people? The reason is that this sim- ple reflex is wired by nerve cells in a circuit that runs from the ear through a switching station in the brain stem directly to the brain’s fear center, theamygdala.Measuring this reflex gives an indication of the intensity of the fear response and is routinely used to test the efficacy of anti-anxiety drugs to block it.
Startle is a very primitive and life-saving reflex, because when- ever there is sudden change in the environment, an animal must focus its attention on the scene and must be ready to defend it- self or run. The parts of the brain that form this circuit are the same parts that control our attention and prepare our body to flee during the stress response. If a frightening event has been experienced before, the brain’s stress center, thehypothalamus, instantly gets connected to the circuit and amplifies the startle response. This, too, is life-saving. When you find yourself in a situation that resembles one in which you experienced danger before, your brain’s stress center is already wired to recall the setting instantly and place you on alert.
Sometimes we actually seek that feeling of startle and hint of fear, because these are the ingredients of awe, and awe can be thrilling—just frightening enough to excite but not so much as to terrify. Repeated exposure to startling events, though, can cause us to lose the buzz, for the brain is wired to settle into a state of calm when the continual contrasts become monoto- nous. When we are exposed to many loud noises of the same tone and intensity, the startle response and the nerve-cell firing rates in the brain regions that register sound gradually respond less and less, until they, and we, no longer notice. This is called habituation.