Without hearing and vision, there would be no spoken languages, and without lan- guages most of the cultural and scientifi c accomplishments of human beings probably would have been impossible. The sense of hearing depends on the ear, a complex sensory instrument that transduces the physical properties of sound waves into neural messages that are sent to the brain. Neural messages from the ears are fi rst interpreted in the temporal lobe’s auditory area and then forwarded to other parts of the brain for additional interpretation (King & Nelken, 2009; Recanzone & Cohen, 2010).
Sound: What Is It?
Hearing, or audition , is the sense that detects the vibratory changes in the air known as sound waves . When an object, such as a tuning fork, vibrates back and forth, it sets in motion successive waves of compression (increased density) and rarefaction (reduced density) of the molecules of the air (see fi gure 5.11 ). When the waves reach the ear, the reception of sound begins. The sound waves in the air cause a chain of small structures in the ear to vibrate in a way that is eventually translated into a neural message to the brain.
Because not all sound waves are alike, the nature of a sound wave determines to a great extent how we sense it. For one thing, sound waves differ in the frequency of cycles of compression and rarefaction of the air (see fi gure 5.11 ). Objects that vibrate slowly create low-frequency sound waves, whereas rapidly vibrating objects produce high-frequency sound waves. The frequency of sound waves is measured in hertz (Hz) units, the number of vibratory cycles per second. The human ear is sensitive to sound waves in the range of 20 to 20,000 Hz. Sound waves also differ in intensity , or how densely compacted the air molecules are in the sound wave.
The frequency of a sound wave largely determines its pitch , or how high or low it sounds to us. For example, striking a glass with a spoon causes a higher-frequency sound wave—which we hear as a higher pitch—than striking a bass drum. The loud- ness of a sound is largely determined by its intensity. Gently tapping a bass drum produces less dense compression and rarefaction, and a quieter sound, than striking it hard. Intensity is measured in decibel (db) units. This scale begins at zero at the abso- lute threshold for detecting a 1,000 Hz tone (and increases by 20 db as the intensity of the stimulus is multiplied by 10). Normal conversation averages about 60 db, whereas
audition (aw-dish ´ -un) Sense of hearing.
sound waves Cyclical changes in air pressure that constitute the stimulus for hearing.
frequency of cycles Rate of vibration of sound waves; determines pitch.
hertz (Hz) Measurement of the frequency of sound waves in cycles per second.
intensity Density of vibrating air molecules, which determines the loudness of sound.
pitch Experience of sound vibrations sensed as high or low.
decibel (db) (des´ i-bel) Measurement of the intensity of perceived sound.
4. The theory of color vision proposes that there are three kinds of cones in the retina that respond primarily to light in either the red, green, or blue range of wavelengths.
a) opponent-process c) psychophysical
b) trichromatic d) sensory adaptation
There are no right or wrong answers to the following questions. They are presented to help you become an active reader and think critically about what you have just read.
1. If cones give us the best visual acuity, what is the advantage of having rods as well?
2. How can the two different theories of color vision both be correct?
Check Your Learning (cont.)
Thinking Critically about Psychology
Correct Answers: 1. d (pp. 125–126),
2. c (p. 126), 3. b (p. 126),
4. b (p. 128)
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sounds of 120 db or more are quite painful (see fi gure 5.12 ). Protect yourself from prolonged expo- sure to sounds over 85 decibels, because they can lead to permanent loss of hearing and ringing in the ears. Even brief exposure to loudness of 150 deci- bels can permanently damage hearing, and most rock concerts exceed these levels.
The timbre of a sound (its characteristic qual- ity) is determined by the complexity of the sound wave—that is, the extent to which it is composed of many waves of different frequency and intensity.
The voices of different people sound different to us largely because of their unique timbres.
The relationship between the physical properties of sound waves and the sensa- tion of sound is not as simple as I have just made it seem. Take loudness, for example.
Two tones of equal intensity may not be heard as equally loud if they are not equal in frequency. Loudness seems greatest for tones of about 3,000 to 4,000 Hz; higher- or lower-frequency sounds of the same intensity seem less loud to us.
The Ear: How Does It Work?
The ear is a sensitive sensory instrument that transduces sound waves into neural impulses to the brain. It is composed of three major sections: the outer ear, the middle ear, and the inner ear (see fi gure 5.13, p. 134 ).
Outer Ear. The external part of the ear, or pinna , which we think of as the “ear,”
is useful as a sound collector and is helpful in locating the origins of sounds. The shape of the pinna is especially important in sound localization, as shown by the fact that temporarily smoothing the pinna with putty impairs sound localization. Connect- ing the outer ear and the middle ear is the hollow external auditory canal . It’s the part that gets waxy and the part through which sound waves reach the eardrum (also known as the tympanic membrane ), the fi rst structure of the middle ear.
Middle Ear. The outermost structure of the middle ear is a thin membrane known as the eardrum , because it resembles the skin on a drum. Sound waves in the air cause the eardrum to vibrate. The vibrating eardrum passes the vibration on to a series of three movable, interconnected bones: the hammer , the anvil , and the stirrup , so
timbre (tam ´ br) Characteristic quality of a sound as determined by the complexity of the sound wave.
pinna (pin ´ nah) External part of the ear.
external auditory canal Tube connecting the pinna to the middle ear.
eardrum Thin membrane that sound waves cause to vibrate; a structure of the middle ear.
hammer, anvil, stirrup Three linked bones of the middle ear, which pass sound waves to the inner ear.
Vibrating tuning fork
Graphic representation of sound wave
Rarefied air molecules
Compressed air molecules
Maximum compression
Maximum rarefaction
One cycle
Figure 5.11
Vibrating objects, such as a tuning fork, create a sound wave of successive compression and rarefaction (expansion) in the air, which can be represented graphically as shown.
0 20 40 60 80 100 120 140 160 180
Pain Threshold
Absolute threshold of human hearing
Whisper Quiet
office Normal conversation
Rocket launch Noisy
automobile
Maximum level of industrial noise considered safe
Loud thunder or rock concert Subway
City bus Decibels
Figure 5.12 The loudness of common sounds as measured in decibel units.
named because of their shapes. These middle-ear structures amplify the vibrations and pass them on to the inner ear.
Inner Ear. The vibrating stirrup shakes another eardrum-like structure called the oval window into motion. This membrane is at the end of a long, curled structure called the cochlea , which is fi lled with fl uid. The vibrating oval window creates waves in the fl uid of the cochlea (see fi gure 5.14 ). The cochlea contains two long tubes that double back on themselves and are connected only at the tip end of the spiral. The pressure of the vibrating waves is relieved by a third eardrum-like membrane at the other end of the cochlea called the round window . Running almost the entire length of the cochlea are several layers of membranes that separate the two tubes. The lower membrane, called the basilar membrane , forms a fl oor on which the ear’s sensory receptors sit. Hairlike receptor cells are contained in the organ of Corti . Vibrations in the cochlear fl uid set the basilar membrane in motion. This movement, in turn, moves the organ of Corti and stimulates the hair cells it contains. These receptors transduce the sound waves in the cochlear fl uid into coded neural impulses sent to the brain (Meyer & others, 2009). The hair cells are sensitive to loud noises and aging.
When they die, hearing declines. To date, there is no medical treatment for restoring them (Groves, 2010).
How does the organ of Corti code neural messages for the brain? The intensity of a sound wave is coded by the number of receptors in the organ of Corti that fi re. Low- intensity sounds stimulate only a few receptors; high-intensity sounds stimulate many receptors.
The frequency of the sound wave is apparently coded in at least two ways. First, sound waves of various frequencies stimulate receptor cells at different places along the organ of Corti. Higher-frequency waves stimulate the organ of Corti close to the oval window; lower-frequency waves stimulate it farther along the cochlea (except for very low frequencies). Second, the frequency of the sound waves is duplicated to some extent in the frequency of the signals sent to the brain by the auditory receptors.
oval window Membrane of the inner ear that vibrates in response to movement of the stirrup, creating waves in the fl uid of the cochlea.
cochlea (cok ´ le¯ -ah) Spiral structure of the inner ear that is fi lled with fl uid and contains the receptors for hearing.
round window Membrane that relieves pressure from the vibrating waves in the cochlear fl uid.
basilar membrane (bas ´ -ı˘ -lar) One of the membranes that separate the two tubes of the cochlea and on which the organ of Corti rests.
organ of Corti (kor ´ te¯ ) Sensory receptor in the cochlea that transduces sound waves into coded neural impulses.
hair cells Receptor neuron cells for hearing located on the organ of Corti.
Outer ear Middle ear Inner ear
Pinna
Hammer Anvil
Semicircular canals
Nerve to brain
Cochlea
Eustachian tube
Eardrum External auditory canal
Stirrup
Round window Oval window
Figure 5.13
Major structures of the ear.
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Only at lower frequencies is each neuron able to signal at the same frequency as the sound wave. At higher frequen- cies, the coding of frequency is achieved by volleys of neu- ral impulses by different groups of neurons, which refl ect the frequency of the sound wave.
Not all sounds travel this complete route from outer ear to cochlea. Some sounds are transmitted through the bones of the head directly to the cochlear fl uid. We hear ourselves speak (and eat) largely through bone conduction hearing.
This is an important consideration in diagnosing hearing problems. People who have suffered damage to the hear- ing apparatus of the middle ear can hear bone-conducted sounds fairly well, but not airborne sounds. People with damage to the auditory nerve—nerve deafness—have dif- fi culty hearing either type of sound.
One more thing about ears deserves mentioning. Ever wonder why people have two of them? For one thing, having a pair of ears gives us a spare in case something goes wrong with one, but the fact that we have two ears also serves an important function in locating the origin of sounds. The ears locate sounds in two ways. First, when a sound wave is coming from straight ahead or from straight behind us, the sound reaches both ears simultaneously. When a sound is coming from the sides or from an angle, it reaches each ear at a slightly different time. The ears are sensitive enough to this difference that they allow us to locate the direction of
sounds, especially high-frequency sounds. The reason you know that the person to the left of you is blowing her nose again is because your left ear hears it before your right
Oval window Tympanic cavity (middle ear)
Cochlea
Basilar membrane and hair cells
Wave traveling down cochlea Round
window Eustachian tube
Hair cells
Basilar membrane
Figure 5.14
Vibrations from sound waves enter the cochlea through the oval window and travel the length of the cochlea, where they are transduced into neural messages by hair cell receptors in the organ of Corti. Two factors tell the listener the location of sounds. First, a sound wave that originates from the side reaches the closer ear slightly sooner than it reaches the farther ear. Second, the head blocks some of the sound wave that reaches the farther ear, reducing the intensity of stimulation to the ear that is farther away from the source of the sound.
The barn owl hunts at night, often inside dark barns and other structures that block the light. It uses its extraordinary sense of hearing to locate its prey in the darkness. The same cues used by humans to localize the source of sounds (differences in sounds reaching the two ears) enable the barn owl to fi nd its scampering prey.
Sound is a physical stimulus made up of successive waves of densely and sparsely com- pressed air. The ear is composed of a series of structures that transmit the sound wave from the outer ear to the inner ear, where it produces vibrations in the fl uid of the cochlea.
The vibrations of the cochlear fl uid are transduced by the receptor cells in the organ of Corti. Coded neural messages are sent to the auditory sensory areas of the brain, where frequency, intensity, and complexity are interpreted as pitch, loudness, and timbre. Differ- ences in the timing and intensity of sound waves reaching the two ears allow us to deter- mine the location of the source of the sound.
There are no right or wrong answers to the following questions. They are presented to help you become an active reader and think critically about what you have just read.
1. In terms of human adaptation and survival, what are the advantages and disadvan- tages of having our ears located on the sides of our heads rather than somewhere else on the body?
2. Juan and Patrick are close friends. Juan hears normally, but Patrick is totally deaf.
How might this difference in the way they experience life infl uence their friendship?
To be sure that you have learned the key points from the preceding section, cover the list of correct answers and try to answer each question. If you give an incorrect answer to any question, return to the page given next to the correct answer to see why your answer was not correct.
1. Objects that vibrate slowly create low-frequency sound waves, which we hear as having .
a) low pitch c) simple timbre
b) high pitch d) complex timbre
2. The sound wave is amplified by the hammer, anvil, and stirrup in the .
a) outer ear c) inner ear
b) middle ear d) pinna
3. The sound wave is transduced into neural impulses in the , which is located in the cochlea in the inner ear.
a) auditory nerve c) organ of Corti
b) cochlear fl uid d) pinna
4. Even with your eyes closed, you know that the person speaking to you is on your left because .
a) the sound reaches your left ear slightly before it reaches your right ear
b) the sound wave that reaches your left ear is slightly more intense than the sound wave that reaches your right ear
c) both of these d) neither of these
Review
Thinking Critically about Psychology Check Your Learning
Correct Answers: 1. a (p. 132),
2. b (pp. 133–134), 3. c (p. 134),
4. c (pp. 135–136)
ear does. Second, cues for the location of high-frequency sounds are also produced by the fact that your head dampens some of the sound reaching the more distant ear, creating a difference in the intensity of the sound waves that reach each of the ears.
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