THE EYE, THE EAR and the BrainTed Uzzle explains the gross anatomy and operations of the eye and the ear, important basics for video and sound professionals to understand. 9/01/2001 8:00 AM Eastern
THE EYE, THE EAR and the Brain
Sep 1, 2001 12:00 PM, Ted Uzzle
PART 1: THE EYE AND THE BRAIN
The display of information must meet the needs of the eye. Someof these needs come from the eye itself, and some come from thebrain.
THE EYE, AS WE ALL KNOW, IS A LIGHT-sensitive organ of vision thatpermits us to discriminate among minute variations of shape, color,brightness and distance. Every successful visual display must beadapted to the peculiar characteristics of the eye, and everyoneworking with video needs to understand the sensory organ being served.The eyes are truly our windows onto the world, but how much do you knowof 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 asmaller bulge in the front, containing the structures that admit lightinto the eye. The eye socket is an opening in the skull called theorbit, where fatty tissues, connective tissues and muscles cushion andprotect 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-sidemovement, and another pair controls diagonal movement. The eyes are inconstant involuntary motion, moving 30 to 70 times per second. Theseastonishing movements are called saccades. The eye is inconstant, restless search for parts of the visual field with thegreatest light-dark contrast. Amazingly, we are completely unaware ofthese movements. To prove that to yourself, stand before a mirror andgaze into the reflection of your left eye. Now shift your gaze to thereflection of your right eye. Notice two things: There was no blur asyour eyes moved, and you saw no reflection of moving eyes in themirror.
Protecting the front of the eye is the eyelid. It blinksinvoluntarily every 2 to 10 seconds, cleaning and wetting the surfaceof the eye. There are four layers to the eyelid. An outer layer of skinand hair is presented to the world. Underneath, there's a layer ofmuscles for blinking. Below that is a fibrous layer giving the eyelidits stability and its shape. The innermost layer is the conjunctiva, amucous membrane that rides across the front of the eye. Beneath theupper eyelid are tear glands and ducts that secrete tears. Teardropshave an inner layer of thick fluid surrounded by water. Surrounding thewhole is an oily film that contains an antibacterial enzyme calledlysozyme.
The eyeball is protected by the sclera, a thick, tough, flexibleouter coating. Over most of its area it is white on the outside butdarker on the inside. At the very front of the eye, it becomestransparent so light can enter the eye. The part of the sclera at thefront of the eye is called the cornea. The cornea has five layers, eachabout 2 μm thick and composed of plates, or lamellae. Theseare collagen fibers running parallel to each other but at an angle tothe fibers in the other layers.
Behind thecornea is a chamber containing a clear, colorless liquid called theaqueous humor. This fluid is 99% water; the remainder is a kindof clear serum found in blood. The aqueous humor is also found behindthe iris and around the lens. It provides nutrients to the transparentparts of the eye. See Figure 1.
THE OPTICAL SYSTEM
The cornea acts as a lens to refract light into the inner parts ofthe eye, thus forming an image. There is also a part of the eye calledthe lens, and this helps in image formation, but actually the corneadoes most of the refracting. Also, the flexible cornea and aqueoushumor can be distorted by small muscles to change the focal length ofthe eye's optics. Most of what we think of as focusing the eye is doneby the cornea, not the lens.
The technical term for eye focusing is accommodation. The eyeat rest (with the optical system at the flattest) is focused fordistance viewing. For nearer objects, the cornea, aqueous humor andlens are compressed, rounding them until near objects are focused onthe back of the eyeball. There's a limit to the eye's accommodation,determined by the stiffness of the optical system. A young child cansee objects as close as 2.5 inches (60 mm), but the optical systembecomes less flexible with age. At age 30, adults can see close objectsonly beyond 4 inches (100 mm); and at age 50, they can see objects nocloser than about 10 inches (250 mm). In later years, people may beunable to accommodate their eyes to normal reading or workingdistances; this condition is called presbyopia. It is a visualanalog to presbycusis, the natural loss of hearing with age,caused by the stiffening of the basilar membrane (but for the biologyof the ear, see Part 2 of this article). Of course, some people sufferfrom farsightedness, hyperopia, or nearsightedness, myopia, which arehereditary variations in the sizes of these structures.
At the back of the aqueous humor is the iris, rather like theaperture control of an automatic camera. The iris is a circular,stretchable membrane with a sphincter muscle around its innercircumference. A completely relaxed iris sphincter allows the iris tobe wide open, with an open area (pupil) diameter of about 10 mm. Inbright light the iris closes to around 2 mm, commanded by a refleximpulse from the brain. That's a reduction in open area of about 96%.The closing action hesitates for about 200 ms after the light isapplied and then takes about 2 seconds to reduce to minimum. The iriscannot 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 theoutside of the iris, it reflects brown light and the person is said tohave brown eyes. If the pigment is on the inside of the iris, the iristhen passes brown light and appears, from the outside, to be acomplementary color; this person is said to be blue-eyed or green-eyed.Most newborn infants have the brown pigment on the inside of the irisand so are blue-eyed. In the first few months of life the pigment canmigrate to the other side, and the infant's eyes then change from blueto brown.
Two thirds of the eyeball is filled with vitreous humor, agelatinous fluid with the same refractive index as the lens. This meansthe light will not refract again after leaving the back side of thelens, 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 eyebecause, of course, blood is not clear and light would not pass throughit well. The cornea and the lens are fed oxygen and other nutrientsthrough the aqueous humor and even through the tears. At the junctionwhere the sclera becomes transparent and becomes the cornea, there isthe ciliary body. This structure feeds the aqueous humor with needednutrients, in the area between the iris and the lens. This newlyrefreshed aqueous humor streams gently out the opening in the iris (thepupil) to the front cavity behind the cornea. Aqueous humor exhaustedof nutrients becomes cloudy, and this is drained through the canal ofSchlemm, leading ultimately to a vein at the rear of the eye. Failureof the canal of Schlemm causes the aqueous humor to become more andmore cloudy, a condition called glaucoma.
At the very rear of the eyeball there are four main arteries andmany more smaller veins bringing nutrients to the eye and carrying awaywaste products. A layer behind the retina, the choroid membrane, is afabric of interconnected blood vessels and connective tissues, smallerthan a postage stamp. This is an extraordinary membrane: At no placeelse in the body is there such a concentration of vessels and bloodflow. Physiologists speculate the choroid membrane not only bringsnutrients to the tissues of the eye, but may also control thetemperature at the back of the eye and prevent overheating of theretina by brilliant illumination.
When a physician looks in the eye using an ophthalmoscope (inventedby the acoustical scientist Hermann von Helmholtz), the retina andchoroid membrane are visible. This is the only part of the circulatorysystem directly visible from outside the body, and for that reason,looking into the eyes is a basic part of the examination of patientswith circulatory problems. The ophthalmoscope always includes amagnifier, because the largest of these arteries are but 200 μm indiameter.
THE NEURAL SYSTEM OF THE EYE
At the rear of the eyeball is the retina. This is the transducingdevice that generates neural impulses from light, making the retinafunctionally analogous to the organ of Corti in the inner ear.
The optic nerve runs through the sclera and the choroid membrane andconnects to the retina slightly off-center (to the nasal side) andbelow the center of the retina. This point is called the optic disk, oroptic papilla. The eye is blind at the optic disk; there is a point inthe field of view of each eye where we cannot see. So why do we not seea black dot? The visual system in the brain stretches and connectswhat's around the blind spot, filling it in, so we aren't aware of itat 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, eventhough it is but 200 μm in thickness. About 6% of these are conecells, which give high resolution and color perception but require afair amount of light to be stimulated. The other 94% of receptors arerod cells, which perceive intensity but not color. Rod cells are muchmore sensitive in low light than are cone cells: They can perceive acandle at a distance of about one mile (equivalent to one photon in theparticle theory of light). Directly opposite the lens, at the center ofthe retina, is an area called the fovea. In this area, about 500μm across and taking in 3.5° of the visual field, there is anextraordinary concentration of 5000 cone cells, giving the greatestvisual acuity and most accurate color perception. Because there are norod cells in the fovea, it's not much use in very low light. Thisexplains why astronomers will suggest looking for a faint star bygazing just to the side of where it should be.
Rod and cone cells are connected to bipolar cells, several receptorsto each bipolar cell. Bipolar cells, in turn, are connected to ganglioncells, with dozens of bipolar cells per ganglion cell. Somehow thiscombination of several receptors per gathering cell and very manygathering cells per nerve-transmission cell maintains visual detailwhile reducing the needed number of nerve pathways to the brain. Thereare about one million ganglion cells in the retina, and each onecommunicates individually with the brain. For some unknown reason, theretina is built backwards, with the rods and cones at the rear. Thismeans light must travel through the bipolar cells and the ganglioncells before reaching the light receptors. Near the fovea, the nervecells are gathered away from the cones concentrated there.
Every nerve cell in the human body is electrically polarized betweenthe cell interior and its exterior. The resting potential is about 50mV. When a nerve cell or brain cell is stimulated because the cellconnected to it fired a pulse at the synapse, or cell connection, itbecomes depolarized. It restores its own polarization in turn by firinga pulse at the next cell in line. That's how neural cells communicate.Sensory cells throughout the body work the same way, with the soleexception of the rods and cones in the eye. These fire more or lesscontinuously in the dark — in the absence of stimulation. When aphoton of light strikes a visual-pigment molecule in a rod or cone, thepigment is bleached, making it opaque and insensitive to further light.The electrical potential between the interior of the rod or cone andits cell wall decreases, which slows down or stops the cell firinguntil it is repolarized.
The optic nerve has a diameter approximately the same as a drinkingstraw. In the case of the ears, the auditory nerve carries the signalto the opposite side of the brain, left ear to right hemisphere, rightear to left hemisphere. What happens to the optic nerve is entirelydifferent. The optic nerves travel 35 to 55 mm back from the eye to abrain structure called the chiasm. Here the nerves split apart in astrange way. The nerve fibers from the nasal half of each retina crossto the other side of the brain, while the nerve fibers from theear-side of each retina travel straight back to the same hemisphere.This means that nerve impulses from the right side of the entire fieldof view, from both eyes, go to the left hemisphere, while nerveimpulses from the left side of the entire field of view, from botheyes, 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 intoshape, color, brightness and depth.
THE COLOR SYSTEM OF THE EYE
Cone cells contain a pigment through which light must pass beforereaching the receptor. There are three pigments: One passes violet,with a wavelength of 430 nm; one passes blue-green, with a wavelengthof 530 nm; and the last pigment passes yellowish-green, with awavelength of 560 nm. In fact, these optical filters have filterskirts, meaning they pass light of other wavelengths, but with reducedsensitivity. Any monochromatic light (light of a single color) actuallyactivates cone cells of multiple pigments, but at differentsensitivities. This also explains why we can see light with wavelengthsshorter than 430 nm, and longer than 560 nm.
No conecells, however, can truly perceive red. The closest we really get isyellowish-green. What we call “red” is really an opticalillusion, supplied by the brain by means of extrapolation. Oursensitivity 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 redtest pattern. This is why the red gun in color-video equipment needsthe least resolution to be satisfactory (see Figure 2).
The key role of the brain in color perception is demonstrated bycolor constancy, a phenomenon known well to photographers. A shirtremains the same color when we wear it in bright sunlight, at sunset,in incandescent light, in candlelight and in fluorescent light. But theeye is seeing a quite different color each time. A camera with colorfilm would photograph something quite different each time. The brain,however, tells us it's the same color before our eyes. The brain alsotells us the shirt has no color by moonlight or star light, it has onlylightness or darkness. In this case, the brain knows only that the conecells are firing continuously and providing no information.
Folk wisdom has many sayings about believing what you hear andbelieving what you see. The visual sense is just as prone to illusionas the auditory pathway, and equally filled with mystery andmisunderstanding. Maybe belief should rest not on the particularsensory pathway but rather on our understanding of the ways and meansthrough which we view the world.
PART 2: THE EAR AND THE BRAIN
To understand how clients will respond to sound systems, and tohelp protect your hearing, consider the intricate and surprisingauditory pathways.
COMPARE FOR A MOMENT THE EYE and the ear. There is no question thatthe eye is more sensitive to the human environment. The dark-adaptedeye needs only 0.5 attojoules of energy at its retina to perceivelight. The ear needs about 100 joules of energy — 20 orders ofmagnitude more — at the eardrum to perceive a sound.
The dynamic range of the two sensory organs is also dramaticallydifferent, but the ear is much more versatile. The range from thethreshold of perception to the threshold of damage is about 90 dB inthe case of the eye. That's an amazing dynamic range by any estimation.The dynamic range of hearing in an audiologically normal person is fiveorders of magnitude greater: 140 dB.
Consider also the frequency response of seeing and hearing. This isthe range of frequencies over which the sensory organ operates. The eyecan 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 ofmoderate tastes, on the other hand, can hear sounds from around 20 Hzto 20 kHz, 10 octaves.
Both the eye and the ear are connected to the brain, and the sensorymechanisms of both interact in intimate and complex ways with it. Wederive more information about the world than the sensory organs alonecan provide. Consider this analogy from Albert Bregman:
“Imagine that you are on the edge of a lake and a friendchallenges you to play a game. The game is this: Your friend digs twonarrow channels up from the side of the lake. Each is a few feet longand a few inches wide and they are spaced a few feet apart. Halfway upeach one, your friend stretches a handkerchief and fastens it to thesides of the channel. As waves reach the side of the lake they travelup the channels and cause the two handkerchiefs to go into motion. Youare allowed to look at only the handkerchiefs and from their motions toanswer a series of questions: How many boats are there on the lake, andwhere are they? Which is the most powerful one? Which one is closer? Isthe wind blowing? Has any large object been dropped suddenly into thelake?” (Bregman, 5-6)
You can see that the sense of hearing is only a tiny fractionof the process of audition, which permits us to form an understandingof the world around us. To understand the amount of brain processingrequired for audition (as distinct from vision) think of thesophisticated computer power needed by submarines for sonar, comparedto the simplicity of technical equipment for interpreting stereoscopicaerial reconnaissance photographs.
GROSSANATOMY OF THE EAR
The ear itself is divided into three sections: the outer ear, themiddle ear and the inner ear (see Figure 1). The outer ear isactivated by vibrations in the air and funnels these vibrations intothe head. The outer ear protects the much more delicate interiormechanisms. It also amplifies vibrations, like a megaphone in reverse.Finally, the outer ear permits us to identify the directions from whichsounds arrive, a function called localization.
The middle ear is located within a hollow space in the skull, at theside of the head. It works by mechanical vibration of bony, muscularand sinewy structures. It serves the function of impedance matching bymeans of leverage and mechanical advantage. One end of a bony linkageis connected to a receiver with an effective moving area of 55 squaremm; the other end of the linkage is connected to an actuator of area3.2 square mm. Ignoring losses within the linkage, this means anincrease of displacement of 17 times for the same energy. The middleear also equalizes static pressure on both sides of sealedmembranes.
The inner ear is an incredibly complex helical structure formedwithin the skull wall. It operates mechanically (driven by the middleear), hydrodynamically (by vibrations through fluid-filled chambers),and electrochemically (by the trading of ions between two separatelycharged fluids). It functions as a transducer, converting vibrations tothe spiky firing of nerve cells. It also serves as a wave analyzer,beginning the job of pitch perception, but leaving its remainder to thebrain.
Beyond the ear we find the nervous system. The ear is connected tothe auditory cortex by means of the eighth cranial nerve.
The outer ear includes a flap of skin and cartilage at each side ofthe head. It is the only visible part of the ear. The whole exteriorstructure is called the pinna, or the auricle. These are asunique and individual as human faces, although we seem somehow wired inour brains to remember and identify faces rather than ears. The partsof the pinna include the helix, a rounded curved ridge around thecircumference 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 cavitybetween the top of the helix and the anti-helix is called thetriangular fossa. The bowl from which the ear canal departs isthe concha. Below the opening of the ear canal, the concha is partiallyshadowed by the tragus and the anti-tragus; between the two is a slot,the inter-tragal notch. All the asymmetries of the pinna assistdirectional hearing, both horizontally and vertically. See Figure2.
The Ear Canal
At the back of the concha begins the ear canal, which runs throughthe temporal bone of the skull. This is also called the auditory canal,the auditory meatus, and, simply enough, the earhole. Within the earcanal there is deposited earwax, also called, unappetizingly, cerumen.At the rear of the ear canal is the eardrum, or tympanic membrane. Thisis a circular plate of fibers, both radial and circumferential,attached to the skull at the outer edges, and just conical enough to beslightly concave from the ear canal. The eardrum is quite fragile. Ifit is ruptured it can heal quickly, but each time with a scar stifferthan the rest of the structure. After enough ruptures, the eardrumbecomes stiff enough to affect hearing acuity. The instrument used forinspecting the ear canal is called the otoscope, invented byThomas Brunton.
The back of the eardrum faces into the middle ear (see Figure3). This is a hollow chamber in the skull a bit larger than 100mm3. This chamber is connected to the top of the throat by means of theeustachian tube. This airway permits static air pressure to beequalized on both sides of the eardrum. It can become clogged and alsocan 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 insideof the eardrum, so that when the eardrum moves the hammer is set into arocking motion. The hammer has another protrusion, called its anteriorprocess, which is connected to the tensor tympanum muscle. The tensortympanum, as its name would suggest, applies a slight tension to therear of the eardrum, pulling it into a convex shape. The upper end ofthe hammer lies next to the incus, or anvil. The twobones 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 thehammer rotates. The head of the hammer pushes the anvil into rotation.There is a long protrusion from the anvil, called the long crus, and atits end is a flexible joint, an articulation between the anvil and thestapes, or stirrup bone. Because the long crus is about 30%longer than the manubrium, there's a leverage action: A short movementof the manubrium makes a longer movement of the long crus.
When the eardrum flexes outward, however, toward a rarefaction wavein air, the incudomalleolar articulation separates slightly, because offlexibility in the tendons that hold them together. This means thehammer and the anvil actually separate during the negative part of eachsound-pressure wave in air. They are rigidly attached duringcompression waves. From this point on, the human hearing mechanism isasymmetrical, responding differently to compression and to rarefactionparts of the wave. This mechanism explains the well-known subjectivedifference between perception of compression waves (gunshots) andrarefaction waves (the bursting of light bulbs). In reproduced audiothis justifies the maintenance of absolute polarity throughout thesystem.
The stirrup is shaped as a wishbone with a plate across the twotips. This is called the footplate. It covers the oval window, anopening to the innermost part of the ear, and transmits the mechanicalvibrations of the ossicles to the fluids of the inner ear. The flexiblejoint between the anvil and the stirrup has attached to it a musclecalled the stapedius, which acts as a fuse to shut off the ear in thecase of too-loud sounds. The stapedius actually pulls the stirrup awayfrom the tip of the anvil, separating them. It takes about 170 ms tooperate, meaning that it does not act fast enough to provide perfectprotection. With the stapedius fully contracted, there is about 20 dBof loss between the anvil and the stirrup, which is called atemporary 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 middleear and the inner ear. It is covered by a flexible membrane, and on itsoutside rests the footplate of the stirrup. As the stirrup moves in andout, driven by the eardrum through the hammer and the anvil, itcommunicates a vibration to this membrane, behind which is agelatinous, serous fluid called the endolymph. The endolymph is similarto the fluid inside all human cells. As the endolymph is compressed, itneeds a pressure release, and that is provided by the round window, orfenestra rotunda. The round window is another opening betweenthe middle and inner ear, also covered by a flexible membrane. When thestirrup 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 themembranous labyrinth. These are protected from the surfaces of theskull (the bony osseous labyrinth) by being suspended in a form ofspinal fluid called perilymph.
The majorparts of the inner ear are the vestibule (to which the oval window andthe round window open), the semicircular canals, and the cochlea (seeFigure 4). The semicircular canals are three curved tubes atright angles to each other, so that each tube curves through oneperpendicular plane of three-dimensional space. These are used to sensethe orientation of the head. They are filled with endolymph but alsoinclude otolith, or ear sand. Otolith consists of crystals ofcalcium carbonate. As the head moves, the otolith drifts across sensingcells, much like the fluid in a carpenter's bubble level. Thismechanism is the basis of the sense of equilibrium and balance.
At the other end of the vestibule is the cochlea, a helically coiledtube spiraling approximately twice around a bony structure called themodiolus. Three chambers run along its length. A very thin shelfof bone, appropriately called the bony shelf, runs along itslength, dividing it almost into halves. From the tip of the bony shelfthere spring two flexible membranes, Reissner's membrane and thebasilar membrane, forming the arms of a Y.
Between the arms of the Y runs the cochlear duct, or scalamedia. Inside the cochlear duct is found the organ of Corti, partof the peripheral nervous system, and the point where nerves actuallysense vibrations of the fluids. Beyond Reissner's membrane, a very thinand flexible sheet, lies the scala vestibuli, which is drivendirectly by the oval window and the stirrup. It is here that vibrationis actually introduced to the cochlea. Beyond the basilar membrane, astiffer and stronger membrane, is the scala tympani, to whichthe round window connects. The scala vestibuli and the scala tympaniare connected at the apex of the cochlea through thehelicotrema, a hole between the two canals. When the footplateof the stirrup presses on the oval window, a compression of theperilymph is transmitted up the scala vestibuli, through thehelicotrema, and back down the scala tympani.
THE NEURALSYSTEM 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 withendolymph, which is much thicker than the perilymph in the othercanals. On the inside of the basilar membrane, facing into the cochlearduct, are the hair cells, or cilia. These are nerve cells that move inresponse to fluid flow. On the outside of the basilar membrane, facingthe scala tympani, are the origins of the auditory nerve, calledCorti's ganglion. In effect, these are the very fingertips of theauditory nerve. The hair cells are organized into cones, formed ratherlike the stakes of a teepee, all touching and with a slight twist. Theyare 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, acharge they modulate by their motion. One end of the hair cell touchesthe perilymph on the other side of the basilar membrane, while theother floats in the endolymph in the cochlear duct. Perilymph has ahigher concentration of sodium ions and a lower concentration ofpotassium ions than does the endolymph. For this reason the restinghair cell has a DC electric potential of about -60 mV. When the bundleof hair cells is deformed in one direction, its electric charge becomesabout -40 mV. When deformed in the other direction, it has an electricpotential of about -65 mV.
These changes in electric potential are communicated to Corti'sganglion below the basilar membrane, but not as an analog fluctuation.The nerve cells fire, or transmit electrical spikes, at their ends (thesynapses), 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 isconnected to the right temporal lobe while the right ear is connectedto the left temporal lobe. The gray matter that processes sounds is inprecisely that part of the brain that originates a sense of time, andthus it is often said that while we see objects in space wehear events in time.
There's a misunderstanding floating around in audio that pitchperception is located in the basilar membrane. According to thisnotion, the organ of Corti would be an in-line filter bank, and thelocation of moving hair cells allows discrimination between onefrequency and another. That's not quite so. Every motion of the eardrumsets every hair in the inner ear into motion; every hair cell firesmore or less all the time. Clearly, position on the basilar membrane isconnected somehow to pitch perception, but the connection is complexand actually takes place in the brain, not in the inner ear. Thetemporal lobes of the brain are, in effect, listening to all those haircells 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 soundsimple and straightforward. But in fact, the more we learn of hearing,the deeper a mystery it becomes. For example, it was recentlydemonstrated (after being disputed for some years) that the ear emitssounds. When someone hears a ringing in the ears (after taking aspirin,for example) that same ringing can be measured and recorded in the earcanal. It is even audible to others. If you hold your ear to someoneelse's, and if the room is very quiet, you can hear the reportedringing from the other person's ear. This mystery is called cochlearamplification, and no one knows what causes it or how it works.
While the auditory pathways are still not fully understood, we knowenough to comprehend that the gift of hearing is instilled with magicand wonder.
|Mode of operation|
mechanical, hydrodynamic, electrochemical
protect, amplify, localize
match impedance, equalize pressure
transduction wave analysis
Ted Uzzle is director of instructional development at NSCA andeditor emeritus of S&VC.
The Eye and the Brain
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The Ear and the Brain
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