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THE EYE, THE EAR and the Brain

Sep 1, 2001 12:00 PM, Ted Uzzle


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PART 1: THE EYE AND THE BRAIN

The display of information must meet the needs of the eye. Some of these needs come from the eye itself, and some come from the brain.

THE EYE, AS WE ALL KNOW, IS A LIGHT-sensitive organ of vision that permits us to discriminate among minute variations of shape, color, brightness and distance. Every successful visual display must be adapted to the peculiar characteristics of the eye, and everyone working with video needs to understand the sensory organ being served. The eyes are truly our windows onto the world, but how much do you know of their operation and working? Many of us have a certain ommatophobia, a fear of the eye, or anyway a fear of looking too closely into it.

GROSS ANATOMY OF THE EYE

The eyeball is a sphere approximately 24 mm in diameter. There is a smaller bulge in the front, containing the structures that admit light into the eye. The eye socket is an opening in the skull called the orbit, where fatty tissues, connective tissues and muscles cushion and protect the eyeball.

Movement of the eye is controlled by six muscles for each eyeball. One pair controls up-and-down movement, one pair controls side-to-side movement, and another pair controls diagonal movement. The eyes are in constant involuntary motion, moving 30 to 70 times per second. These astonishing movements are called saccades. The eye is in constant, restless search for parts of the visual field with the greatest light-dark contrast. Amazingly, we are completely unaware of these movements. To prove that to yourself, stand before a mirror and gaze into the reflection of your left eye. Now shift your gaze to the reflection of your right eye. Notice two things: There was no blur as your eyes moved, and you saw no reflection of moving eyes in the mirror.

Protecting the front of the eye is the eyelid. It blinks involuntarily every 2 to 10 seconds, cleaning and wetting the surface of the eye. There are four layers to the eyelid. An outer layer of skin and hair is presented to the world. Underneath, there's a layer of muscles for blinking. Below that is a fibrous layer giving the eyelid its stability and its shape. The innermost layer is the conjunctiva, a mucous membrane that rides across the front of the eye. Beneath the upper eyelid are tear glands and ducts that secrete tears. Teardrops have an inner layer of thick fluid surrounded by water. Surrounding the whole is an oily film that contains an antibacterial enzyme called lysozyme.

The eyeball is protected by the sclera, a thick, tough, flexible outer coating. Over most of its area it is white on the outside but darker on the inside. At the very front of the eye, it becomes transparent so light can enter the eye. The part of the sclera at the front of the eye is called the cornea. The cornea has five layers, each about 2 μm thick and composed of plates, or lamellae. These are collagen fibers running parallel to each other but at an angle to the fibers in the other layers.

Behind the cornea is a chamber containing a clear, colorless liquid called the aqueous humor. This fluid is 99% water; the remainder is a kind of clear serum found in blood. The aqueous humor is also found behind the iris and around the lens. It provides nutrients to the transparent parts of the eye. See Figure 1.

THE OPTICAL SYSTEM

The cornea acts as a lens to refract light into the inner parts of the eye, thus forming an image. There is also a part of the eye called the lens, and this helps in image formation, but actually the cornea does most of the refracting. Also, the flexible cornea and aqueous humor can be distorted by small muscles to change the focal length of the eye's optics. Most of what we think of as focusing the eye is done by the cornea, not the lens.

The technical term for eye focusing is accommodation. The eye at rest (with the optical system at the flattest) is focused for distance viewing. For nearer objects, the cornea, aqueous humor and lens are compressed, rounding them until near objects are focused on the back of the eyeball. There's a limit to the eye's accommodation, determined by the stiffness of the optical system. A young child can see objects as close as 2.5 inches (60 mm), but the optical system becomes less flexible with age. At age 30, adults can see close objects only beyond 4 inches (100 mm); and at age 50, they can see objects no closer than about 10 inches (250 mm). In later years, people may be unable to accommodate their eyes to normal reading or working distances; this condition is called presbyopia. It is a visual analog to presbycusis, the natural loss of hearing with age, caused by the stiffening of the basilar membrane (but for the biology of the ear, see Part 2 of this article). Of course, some people suffer from farsightedness, hyperopia, or nearsightedness, myopia, which are hereditary variations in the sizes of these structures.

At the back of the aqueous humor is the iris, rather like the aperture control of an automatic camera. The iris is a circular, stretchable membrane with a sphincter muscle around its inner circumference. A completely relaxed iris sphincter allows the iris to be wide open, with an open area (pupil) diameter of about 10 mm. In bright light the iris closes to around 2 mm, commanded by a reflex impulse from the brain. That's a reduction in open area of about 96%. The closing action hesitates for about 200 ms after the light is applied and then takes about 2 seconds to reduce to minimum. The iris cannot close fully; if the closure to a 2mm diameter is insufficient, then a neural reflex closes the eyelid.

The iris is coated with a brown pigment. If this pigment is on the outside of the iris, it reflects brown light and the person is said to have brown eyes. If the pigment is on the inside of the iris, the iris then passes brown light and appears, from the outside, to be a complementary color; this person is said to be blue-eyed or green-eyed. Most newborn infants have the brown pigment on the inside of the iris and so are blue-eyed. In the first few months of life the pigment can migrate to the other side, and the infant's eyes then change from blue to brown.

Two thirds of the eyeball is filled with vitreous humor, a gelatinous fluid with the same refractive index as the lens. This means the light will not refract again after leaving the back side of the lens, but will form an image directly at the rear of the eyeball.

THE VASCULAR SYSTEM OF THE EYE

There are no blood vessels in the cornea or the lens of the eye because, of course, blood is not clear and light would not pass through it well. The cornea and the lens are fed oxygen and other nutrients through the aqueous humor and even through the tears. At the junction where the sclera becomes transparent and becomes the cornea, there is the ciliary body. This structure feeds the aqueous humor with needed nutrients, in the area between the iris and the lens. This newly refreshed aqueous humor streams gently out the opening in the iris (the pupil) to the front cavity behind the cornea. Aqueous humor exhausted of nutrients becomes cloudy, and this is drained through the canal of Schlemm, leading ultimately to a vein at the rear of the eye. Failure of the canal of Schlemm causes the aqueous humor to become more and more cloudy, a condition called glaucoma.

At the very rear of the eyeball there are four main arteries and many more smaller veins bringing nutrients to the eye and carrying away waste products. A layer behind the retina, the choroid membrane, is a fabric of interconnected blood vessels and connective tissues, smaller than a postage stamp. This is an extraordinary membrane: At no place else in the body is there such a concentration of vessels and blood flow. Physiologists speculate the choroid membrane not only brings nutrients to the tissues of the eye, but may also control the temperature at the back of the eye and prevent overheating of the retina by brilliant illumination.

When a physician looks in the eye using an ophthalmoscope (invented by the acoustical scientist Hermann von Helmholtz), the retina and choroid membrane are visible. This is the only part of the circulatory system directly visible from outside the body, and for that reason, looking into the eyes is a basic part of the examination of patients with circulatory problems. The ophthalmoscope always includes a magnifier, because the largest of these arteries are but 200 μm in diameter.

THE NEURAL SYSTEM OF THE EYE

At the rear of the eyeball is the retina. This is the transducing device that generates neural impulses from light, making the retina functionally analogous to the organ of Corti in the inner ear.

The optic nerve runs through the sclera and the choroid membrane and connects to the retina slightly off-center (to the nasal side) and below the center of the retina. This point is called the optic disk, or optic papilla. The eye is blind at the optic disk; there is a point in the field of view of each eye where we cannot see. So why do we not see a black dot? The visual system in the brain stretches and connects what's around the blind spot, filling it in, so we aren't aware of it at all. This facility, the stuff of science fiction, is soft-wired; it's an illusion in the brain and not a structure in the eye.

The retina is formed from about 125 million photoreceptors, even though it is but 200 μm in thickness. About 6% of these are cone cells, which give high resolution and color perception but require a fair amount of light to be stimulated. The other 94% of receptors are rod cells, which perceive intensity but not color. Rod cells are much more sensitive in low light than are cone cells: They can perceive a candle at a distance of about one mile (equivalent to one photon in the particle theory of light). Directly opposite the lens, at the center of the retina, is an area called the fovea. In this area, about 500 μm across and taking in 3.5° of the visual field, there is an extraordinary concentration of 5000 cone cells, giving the greatest visual acuity and most accurate color perception. Because there are no rod cells in the fovea, it's not much use in very low light. This explains why astronomers will suggest looking for a faint star by gazing just to the side of where it should be.

Rod and cone cells are connected to bipolar cells, several receptors to each bipolar cell. Bipolar cells, in turn, are connected to ganglion cells, with dozens of bipolar cells per ganglion cell. Somehow this combination of several receptors per gathering cell and very many gathering cells per nerve-transmission cell maintains visual detail while reducing the needed number of nerve pathways to the brain. There are about one million ganglion cells in the retina, and each one communicates individually with the brain. For some unknown reason, the retina is built backwards, with the rods and cones at the rear. This means light must travel through the bipolar cells and the ganglion cells before reaching the light receptors. Near the fovea, the nerve cells are gathered away from the cones concentrated there.

Every nerve cell in the human body is electrically polarized between the cell interior and its exterior. The resting potential is about 50 mV. When a nerve cell or brain cell is stimulated because the cell connected to it fired a pulse at the synapse, or cell connection, it becomes depolarized. It restores its own polarization in turn by firing a pulse at the next cell in line. That's how neural cells communicate. Sensory cells throughout the body work the same way, with the sole exception of the rods and cones in the eye. These fire more or less continuously in the dark — in the absence of stimulation. When a photon of light strikes a visual-pigment molecule in a rod or cone, the pigment is bleached, making it opaque and insensitive to further light. The electrical potential between the interior of the rod or cone and its cell wall decreases, which slows down or stops the cell firing until it is repolarized.

The optic nerve has a diameter approximately the same as a drinking straw. In the case of the ears, the auditory nerve carries the signal to the opposite side of the brain, left ear to right hemisphere, right ear to left hemisphere. What happens to the optic nerve is entirely different. The optic nerves travel 35 to 55 mm back from the eye to a brain structure called the chiasm. Here the nerves split apart in a strange way. The nerve fibers from the nasal half of each retina cross to the other side of the brain, while the nerve fibers from the ear-side of each retina travel straight back to the same hemisphere. This means that nerve impulses from the right side of the entire field of view, from both eyes, go to the left hemisphere, while nerve impulses from the left side of the entire field of view, from both eyes, go to the right hemisphere.

The visual cortex is at the very rear of the brain, in the center. Were the brain a VW beetle, the visual cortex would be the rear bumper. It is about 3 mm in thickness. Here, visual impulses are combined into shape, color, brightness and depth.

THE COLOR SYSTEM OF THE EYE

Cone cells contain a pigment through which light must pass before reaching the receptor. There are three pigments: One passes violet, with a wavelength of 430 nm; one passes blue-green, with a wavelength of 530 nm; and the last pigment passes yellowish-green, with a wavelength of 560 nm. In fact, these optical filters have filter skirts, meaning they pass light of other wavelengths, but with reduced sensitivity. Any monochromatic light (light of a single color) actually activates cone cells of multiple pigments, but at different sensitivities. This also explains why we can see light with wavelengths shorter than 430 nm, and longer than 560 nm.

No cone cells, however, can truly perceive red. The closest we really get is yellowish-green. What we call “red” is really an optical illusion, supplied by the brain by means of extrapolation. Our sensitivity to red is dramatically reduced compared to other colors, and our visual acuity in the red end of the spectrum is extremely bad. Everyone knows (or should know) not to focus a projector using a red test pattern. This is why the red gun in color-video equipment needs the least resolution to be satisfactory (see Figure 2).

The key role of the brain in color perception is demonstrated by color constancy, a phenomenon known well to photographers. A shirt remains the same color when we wear it in bright sunlight, at sunset, in incandescent light, in candlelight and in fluorescent light. But the eye is seeing a quite different color each time. A camera with color film would photograph something quite different each time. The brain, however, tells us it's the same color before our eyes. The brain also tells us the shirt has no color by moonlight or star light, it has only lightness or darkness. In this case, the brain knows only that the cone cells are firing continuously and providing no information.

Folk wisdom has many sayings about believing what you hear and believing what you see. The visual sense is just as prone to illusion as the auditory pathway, and equally filled with mystery and misunderstanding. Maybe belief should rest not on the particular sensory pathway but rather on our understanding of the ways and means through which we view the world.

PART 2: THE EAR AND THE BRAIN

To understand how clients will respond to sound systems, and to help protect your hearing, consider the intricate and surprising auditory pathways.

COMPARE FOR A MOMENT THE EYE and the ear. There is no question that the eye is more sensitive to the human environment. The dark-adapted eye needs only 0.5 attojoules of energy at its retina to perceive light. The ear needs about 100 joules of energy — 20 orders of magnitude more — at the eardrum to perceive a sound.

The dynamic range of the two sensory organs is also dramatically different, but the ear is much more versatile. The range from the threshold of perception to the threshold of damage is about 90 dB in the case of the eye. That's an amazing dynamic range by any estimation. The dynamic range of hearing in an audiologically normal person is five orders of magnitude greater: 140 dB.

Consider also the frequency response of seeing and hearing. This is the range of frequencies over which the sensory organ operates. The eye can sense light ranging in frequency from infrared (460 THz — that's 460 terahertz or 460 trillion hertz) to ultraviolet (750 THz). This is a range of about 0.7 octaves. The ear of a young person of moderate tastes, on the other hand, can hear sounds from around 20 Hz to 20 kHz, 10 octaves.

Both the eye and the ear are connected to the brain, and the sensory mechanisms of both interact in intimate and complex ways with it. We derive more information about the world than the sensory organs alone can provide. Consider this analogy from Albert Bregman:

“Imagine that you are on the edge of a lake and a friend challenges you to play a game. The game is this: Your friend digs two narrow channels up from the side of the lake. Each is a few feet long and a few inches wide and they are spaced a few feet apart. Halfway up each one, your friend stretches a handkerchief and fastens it to the sides of the channel. As waves reach the side of the lake they travel up the channels and cause the two handkerchiefs to go into motion. You are allowed to look at only the handkerchiefs and from their motions to answer a series of questions: How many boats are there on the lake, and where are they? Which is the most powerful one? Which one is closer? Is the wind blowing? Has any large object been dropped suddenly into the lake?” (Bregman, 5-6)

You can see that the sense of hearing is only a tiny fraction of the process of audition, which permits us to form an understanding of the world around us. To understand the amount of brain processing required for audition (as distinct from vision) think of the sophisticated computer power needed by submarines for sonar, compared to the simplicity of technical equipment for interpreting stereoscopic aerial reconnaissance photographs.

GROSS ANATOMY OF THE EAR

The ear itself is divided into three sections: the outer ear, the middle ear and the inner ear (see Figure 1). The outer ear is activated by vibrations in the air and funnels these vibrations into the head. The outer ear protects the much more delicate interior mechanisms. It also amplifies vibrations, like a megaphone in reverse. Finally, the outer ear permits us to identify the directions from which sounds arrive, a function called localization.

The middle ear is located within a hollow space in the skull, at the side of the head. It works by mechanical vibration of bony, muscular and sinewy structures. It serves the function of impedance matching by means of leverage and mechanical advantage. One end of a bony linkage is connected to a receiver with an effective moving area of 55 square mm; the other end of the linkage is connected to an actuator of area 3.2 square mm. Ignoring losses within the linkage, this means an increase of displacement of 17 times for the same energy. The middle ear also equalizes static pressure on both sides of sealed membranes.

The inner ear is an incredibly complex helical structure formed within the skull wall. It operates mechanically (driven by the middle ear), hydrodynamically (by vibrations through fluid-filled chambers), and electrochemically (by the trading of ions between two separately charged fluids). It functions as a transducer, converting vibrations to the spiky firing of nerve cells. It also serves as a wave analyzer, beginning the job of pitch perception, but leaving its remainder to the brain.

Beyond the ear we find the nervous system. The ear is connected to the auditory cortex by means of the eighth cranial nerve.

THE OUTER EAR

The outer ear includes a flap of skin and cartilage at each side of the head. It is the only visible part of the ear. The whole exterior structure is called the pinna, or the auricle. These are as unique and individual as human faces, although we seem somehow wired in our brains to remember and identify faces rather than ears. The parts of the pinna include the helix, a rounded curved ridge around the circumference of the pinna. At the bottom of the helix is the lobe. This is a body-part name most people know; it is where earrings go. Within the dish of the helix is a fold called the anti-helix. A cavity between the top of the helix and the anti-helix is called the triangular fossa. The bowl from which the ear canal departs is the concha. Below the opening of the ear canal, the concha is partially shadowed by the tragus and the anti-tragus; between the two is a slot, the inter-tragal notch. All the asymmetries of the pinna assist directional hearing, both horizontally and vertically. See Figure 2.

The Ear Canal

At the back of the concha begins the ear canal, which runs through the temporal bone of the skull. This is also called the auditory canal, the auditory meatus, and, simply enough, the earhole. Within the ear canal there is deposited earwax, also called, unappetizingly, cerumen. At the rear of the ear canal is the eardrum, or tympanic membrane. This is a circular plate of fibers, both radial and circumferential, attached to the skull at the outer edges, and just conical enough to be slightly concave from the ear canal. The eardrum is quite fragile. If it is ruptured it can heal quickly, but each time with a scar stiffer than the rest of the structure. After enough ruptures, the eardrum becomes stiff enough to affect hearing acuity. The instrument used for inspecting the ear canal is called the otoscope, invented by Thomas Brunton.

THE MIDDLE EAR

The back of the eardrum faces into the middle ear (see Figure 3). This is a hollow chamber in the skull a bit larger than 100 mm3. This chamber is connected to the top of the throat by means of the eustachian tube. This airway permits static air pressure to be equalized on both sides of the eardrum. It can become clogged and also can provide a route of infection to the middle ear.

At the back of the eardrum is the malleus, or hammer. Its longer protrusion, called the manubrium, is connected to the inside of the eardrum, so that when the eardrum moves the hammer is set into a rocking motion. The hammer has another protrusion, called its anterior process, which is connected to the tensor tympanum muscle. The tensor tympanum, as its name would suggest, applies a slight tension to the rear of the eardrum, pulling it into a convex shape. The upper end of the hammer lies next to the incus, or anvil. The two bones are held loosely together by sinews at the malleo-incudal joint, also called the incudomalleolar articulation.

When the eardrum flexes inward, the manubrium moves with it, and the hammer rotates. The head of the hammer pushes the anvil into rotation. There is a long protrusion from the anvil, called the long crus, and at its end is a flexible joint, an articulation between the anvil and the stapes, or stirrup bone. Because the long crus is about 30% longer than the manubrium, there's a leverage action: A short movement of the manubrium makes a longer movement of the long crus.

When the eardrum flexes outward, however, toward a rarefaction wave in air, the incudomalleolar articulation separates slightly, because of flexibility in the tendons that hold them together. This means the hammer and the anvil actually separate during the negative part of each sound-pressure wave in air. They are rigidly attached during compression waves. From this point on, the human hearing mechanism is asymmetrical, responding differently to compression and to rarefaction parts of the wave. This mechanism explains the well-known subjective difference between perception of compression waves (gunshots) and rarefaction waves (the bursting of light bulbs). In reproduced audio this justifies the maintenance of absolute polarity throughout the system.

The stirrup is shaped as a wishbone with a plate across the two tips. This is called the footplate. It covers the oval window, an opening to the innermost part of the ear, and transmits the mechanical vibrations of the ossicles to the fluids of the inner ear. The flexible joint between the anvil and the stirrup has attached to it a muscle called the stapedius, which acts as a fuse to shut off the ear in the case of too-loud sounds. The stapedius actually pulls the stirrup away from the tip of the anvil, separating them. It takes about 170 ms to operate, meaning that it does not act fast enough to provide perfect protection. With the stapedius fully contracted, there is about 20 dB of loss between the anvil and the stirrup, which is called a temporary loudness shift. The stapedius muscle will relax, restoring hearing, within minutes or hours.

THE INNER EAR

The oval window is an opening in the bony wall between the middle ear and the inner ear. It is covered by a flexible membrane, and on its outside rests the footplate of the stirrup. As the stirrup moves in and out, driven by the eardrum through the hammer and the anvil, it communicates a vibration to this membrane, behind which is a gelatinous, serous fluid called the endolymph. The endolymph is similar to the fluid inside all human cells. As the endolymph is compressed, it needs a pressure release, and that is provided by the round window, or fenestra rotunda. The round window is another opening between the middle and inner ear, also covered by a flexible membrane. When the stirrup pushes the endolymph in, the round window bulges back out. Sound has now become a flow of fluids.

The inner ear consists of communicating sacs and ducts called the membranous labyrinth. These are protected from the surfaces of the skull (the bony osseous labyrinth) by being suspended in a form of spinal fluid called perilymph.

The major parts of the inner ear are the vestibule (to which the oval window and the round window open), the semicircular canals, and the cochlea (see Figure 4). The semicircular canals are three curved tubes at right angles to each other, so that each tube curves through one perpendicular plane of three-dimensional space. These are used to sense the orientation of the head. They are filled with endolymph but also include otolith, or ear sand. Otolith consists of crystals of calcium carbonate. As the head moves, the otolith drifts across sensing cells, much like the fluid in a carpenter's bubble level. This mechanism is the basis of the sense of equilibrium and balance.

At the other end of the vestibule is the cochlea, a helically coiled tube spiraling approximately twice around a bony structure called the modiolus. Three chambers run along its length. A very thin shelf of bone, appropriately called the bony shelf, runs along its length, dividing it almost into halves. From the tip of the bony shelf there spring two flexible membranes, Reissner's membrane and the basilar membrane, forming the arms of a Y.

Between the arms of the Y runs the cochlear duct, or scala media. Inside the cochlear duct is found the organ of Corti, part of the peripheral nervous system, and the point where nerves actually sense vibrations of the fluids. Beyond Reissner's membrane, a very thin and flexible sheet, lies the scala vestibuli, which is driven directly by the oval window and the stirrup. It is here that vibration is actually introduced to the cochlea. Beyond the basilar membrane, a stiffer and stronger membrane, is the scala tympani, to which the round window connects. The scala vestibuli and the scala tympani are connected at the apex of the cochlea through the helicotrema, a hole between the two canals. When the footplate of the stirrup presses on the oval window, a compression of the perilymph is transmitted up the scala vestibuli, through the helicotrema, and back down the scala tympani.

THE NEURAL SYSTEM OF THE EAR

The essence of the mystery of the ear is found in the cochlear duct (see Figure 5). This much smaller canal is filled with endolymph, which is much thicker than the perilymph in the other canals. On the inside of the basilar membrane, facing into the cochlear duct, are the hair cells, or cilia. These are nerve cells that move in response to fluid flow. On the outside of the basilar membrane, facing the scala tympani, are the origins of the auditory nerve, called Corti's ganglion. In effect, these are the very fingertips of the auditory nerve. The hair cells are organized into cones, formed rather like the stakes of a teepee, all touching and with a slight twist. They are stimulated by shear forces as vibrations travel through the fluids; they actually tilt back and forth and rub against each other.

In effect, they are capacitor plates with a charge across them, a charge they modulate by their motion. One end of the hair cell touches the perilymph on the other side of the basilar membrane, while the other floats in the endolymph in the cochlear duct. Perilymph has a higher concentration of sodium ions and a lower concentration of potassium ions than does the endolymph. For this reason the resting hair cell has a DC electric potential of about -60 mV. When the bundle of hair cells is deformed in one direction, its electric charge becomes about -40 mV. When deformed in the other direction, it has an electric potential of about -65 mV.

These changes in electric potential are communicated to Corti's ganglion below the basilar membrane, but not as an analog fluctuation. The nerve cells fire, or transmit electrical spikes, at their ends (the synapses), and these spikes arrive eventually at the brain.

The auditory regions in the brain are in the temporal lobes, immediately above the middle and inner ear. However, the left ear is connected to the right temporal lobe while the right ear is connected to the left temporal lobe. The gray matter that processes sounds is in precisely that part of the brain that originates a sense of time, and thus it is often said that while we see objects in space we hear events in time.

There's a misunderstanding floating around in audio that pitch perception is located in the basilar membrane. According to this notion, the organ of Corti would be an in-line filter bank, and the location of moving hair cells allows discrimination between one frequency and another. That's not quite so. Every motion of the eardrum sets every hair in the inner ear into motion; every hair cell fires more or less all the time. Clearly, position on the basilar membrane is connected somehow to pitch perception, but the connection is complex and actually takes place in the brain, not in the inner ear. The temporal lobes of the brain are, in effect, listening to all those hair cells at once, and only then deciding what pitch was heard.

THE MYSTERY OF HEARING

A brief description of the workings of the ear can make them sound simple and straightforward. But in fact, the more we learn of hearing, the deeper a mystery it becomes. For example, it was recently demonstrated (after being disputed for some years) that the ear emits sounds. When someone hears a ringing in the ears (after taking aspirin, for example) that same ringing can be measured and recorded in the ear canal. It is even audible to others. If you hold your ear to someone else's, and if the room is very quiet, you can hear the reported ringing from the other person's ear. This mystery is called cochlear amplification, and no one knows what causes it or how it works.

While the auditory pathways are still not fully understood, we know enough to comprehend that the gift of hearing is instilled with magic and wonder.


Outer

Middle

Inner

Nervous System

Mode of operation

air vibration

mechanical vibration

mechanical, hydrodynamic, electrochemical

electrochemical

Function

protect, amplify, localize

match impedance, equalize pressure

transduction wave analysis

process information


Ted Uzzle is director of instructional development at NSCA and editor emeritus of S&VC.

BIBLIOGRAPHY

The Eye and the Brain

Hubel, David H. Eye, Brain, and Vision. Scientific American Library, 1988.

The Ear and the Brain

Allen, Jont B. and Neeley, Stephen T. “Micromechanical Models of the Cochlea,” Phys. Today, v. 45 n. 7 (July 1992): 40-47.

Bregman, Albert S. Auditory Scene Analysis. MIT Press, 1990.

Glattke, Theodore J. “Elements of Auditory Physiology.” In Fred D. Minifie et al., ed., Normal Aspects of Speech, Hearing, and Language. Prentice-Hall, 1973.

Hudspeth, A. J. “The Hair Cells of the Inner Ear.” Scientific American, v. 248, no. 1. (January 1983): 54-64.

Von Békésy, Georg, translated by E. G. Wever. Experiments in Hearing. American Institute of Physics, reprinted 1989.

Yost, William A. and Nielsen, Donald W. Fundamentals of Hearing. 2nd ed. Holt, Rinehart and Winston, 1985.



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