MODULE 2

The Ear: How We Hear

To understand how the ear achieves its sensitivity, we must take a look at the anatomy of this sensory mechanism. Basically the ear's function is to turn sound waves into waves in the fluid of the inner ear, and then into neural impulses that are transmitted to the brain. This process is accomplished by a series of mechanical operations.

As you read the text, try to answer the following questions.

The receptor organ for the sense of hearing is the ear. Like all sensory receptors, the ear's overall function is to receive an external stimulus and to convert it into neural impulses which are transmitted to the central nervous system. A simple schematic of this process is shown in Figure 1.

External stimuli (sound waves) . .... . . enter ear where they are converted to . . . . . . neural impulses.

Figure 1. How we hear _

SOUND: THE STIMULUS OF HEARING

The sensation of hearing is stimulated by compression and expansion of air molecules

If you think about the different kinds of sounds that human beings can hear, you can begin to appreciate what a truly flexible receptor the ear is. We can detect sounds that are high or low in frequency. We can hear loud and soft sounds. We can hear blends of sounds: from a rich symphonic crescendo to the random, chaotic mixtures of sound that we call "noise."

Technically speaking, sound is the successive compression and expansion of air molecules. When air molecules are pressed together they are in compression. When air molecules are separated and away from one another, they are in expansion. Sound travels through the air in alternate stages of compression and expansion as shown in Figure 2.

Figure 2 How sound travels through the air

When these phases of compression and expansion take place with a great deal of force, we hear a loud sound. When the compression and expansion take place rapidly, we hear a sound that is high in pitch.

The alternate compression and expansion of air molecules is called sound waves, because when the compression and expansion are plotted on a graph over time, a wave form is the result. Figure 3 shows a series of wave forms generated by a pure (uncomplicated) tone.

Figure 3. Wave form generated by a pure tone

The peaks represent compression and the valleys represent expansion.

Pitch frequency is measured by the number of wave-form cycles generated in a second (Point A to Point B is one cycle). Loudness or amplitude is measured by the deviation of the highest part of the wave from the time line (distance C-D).

In order for one to be able to hear a variety of sounds, then, the ear must be sensitive to changes both in frequency and in amplitude. A bass clarinet can produce tones from 80 cycles per second (cps.)to about 480 480 A piccolo can produce a tone with a frequency of more than 5,000 cps.

The decibel is a relative measure of intensity. That is, the intensity of a sound in decibels tells us how much greater the pressure (or energy) of that sound is than the pressure (or energy) of a standard, reference sound. Often the standard is taken to be the pressure of a 1 000-cps tone at about its absolute threshold for human hearing.

This standard pressure is 0.0002 dynes/cm2. With this standard, a whisper is measured at under 20 decibels; a jet plane produces sound levels of about 160 decibels. The point at which sound becomes painful is between 120 and 140 decibels, depending on the frequency.

FUNCTIONAL ANATOMY OF THE EAR

Figure 4 shows the gross anatomy of the organ of hearing, the ear. Note that it is composed of three sections: the outer ear, the middle ear, and the inner ear.

Figure 4. General structure of the ear

Each of the sections of the ear has a different function in the hearing process. The table below shows which functions belong to each section of the ear.

Auditory Functions of Sections of the Ear
Outer ear -- Collection of sound waves
Middle earAmplification of sound waves
Inner ear Conversion of sound waves into neural impulses

Figure 5. Collection of sound waves:

The outer ear The part of the ear we see, the pinna, collects the sound waves, channeling them to the eardrum through a duct called the auditory canal. The pinna and the auditory canal make up the outer ear. The eardrum separates the outer ear from the middle ear. The eardrum is a thin membrane which is stretched tightly across the end of the auditory canal.

Figure 6. shows Amplification of sound waves: The middle ear is Hammer, Anvil, Stirrup

The pressure of sound waves moves this small membrane back and forth. The vibration of the eardrum moves three small bones, the ossicles, which transmit the vibrations to the inner ear. The eardrum and these small ossiclesÄthe hammer, anvil, and stirrupÄmake up the middle ear. They are connected to one another like a series of levers. Thus, when the hammer is moved by the eardrum, it in turn moves the anvil and the stirrup, mechanically transmitting the sound waves to the inner ear.

The middle ear's main function is to amplify the sound waves. To do so, it acts as a mechanical transformer, converting the sound waves in the air into more forceful vibrations. In fact, these ossicles transform the small pressure on the surface of the eardrum into 22 times as much pressure on the fluid of the inner ear.

Figure 7 Conversion of sound waves to neural impulses

The inner ear The inner ear is the site at which the mechanical vibrations of the ossicles are converted into neural impulses. It is the most complex of the three major parts of the ear. The most important part of the inner ear is the cochlea, where the actual receptor cells are situated.

The cochlea is a fluid-filled structure that is shaped like a spiraling snail's shell. Figure 8 shows the structure of the cochlea. In Section A, the stirrup is shown attached to the oval window of the cochlea. Vibrations are thus transferred to the cochlea at this point.

In Section B you can see the cochlea as if it were unwound. The oval window communicates with the scale vestibuli, a fluid-filled duct. Vibrations, therefore, are transferred to the fluid and carried along the duct. The scale vestibuli is continuous with the scale tympani, a duct which terminates in the round window. The round window is a thin membrane which absorbs the fluid vibrations, thus preventing their reflection back through the duct.

Section C of Figure 8 is a schematic cross section of the cochlea showing the formation of the cochlear duct and the basilar membrane. Section D of the same figure shows an actual cross section of the cochlea. The organ of Corti is located on the basilar membrane. In the organ of Corti are hair cells. When vibrations are carried through the fluid of the cochlea, they displace the basilar membrane and the hair cells upon it. This generates impulses in the nerve endings lying nearby. The impulses are carried by way of the auditory nerve to the temporal lobe of the brain.

Figure 8. The structure of the cochlea (this figure is not too important)

THEORIES OF HEARING

Scientists are fairly certain of the general way in which sound waves are converted into neural impulses. They are less sure, however, of the mechanism which gives us information about the frequency of the sounds we hear. Current belief is that the mechanism for pitch discrimination is located in the cochlea. There are two general theories of pitch perception: place theory and frequency theory.

Place Theories Place theories state that certain points along the basilar membrane are sensi- only to certain frequencies. Early place theory felt that the basilar membrane contained fibers of various thickness (like a piano), so that only specific fibers vibrated at any given frequency. It was later found, though, that the entire cochlea was filled with fluid, so that fibers could not vibrate at frequencies as high as some sound waves we hear. A more recent place theory is the traveling-wave theory. This theory says that since the basilar membrane decreases in thickness as it spirals toward the center of the cochlea, different parts of the membrane itself will vibrate more at specific frequencies.

Figure 9 shows a map of the basilar membrane and the points along the membrane that are (according to the traveling-wave theory) sensitive to specific frequencies.

Figure 9. Sound map of the basilar membrane

Frequency Theories Early frequency theory held that neurons in the cochlea fire at the frequency of the sound waves heard. This simplistic view was objected to on the basis that a single neuron is capable of firing at a maximum of only about 1,000 cycles per second, yet we can hear sounds up to 20,000 cps. To meet this objection, the volley theory was proposed. The volley theory says that specific frequencies are represented by groups of neurons firing in succession. Thus, a 500-cps tone would cause one group of neurons to fire at 500 cps. A 1,000-cps tone would cause two groups of neurons to fire alternately at 500 cps. A 2,000-cps tone would cause four groups of neurons to fire alternately at 500 cps (or, perhaps, 20 groups of neurons to fire alternately at 100 cps).

A COMBINED THEORY OF FREQUENCY DISCRIMINATION

The modern version of place theory, traveling-wave theory, could explain high frequencies onlyÄabove 5,000 cps. On the other hand, frequency theory could be true only for low frequenciesÄbelow 5,000 cps., even including the volley principle. Most recent thinking, therefore, holds that pitch discrimination is accomplished by both functions: a traveling-wave mechanism for high frequencies and a frequency mechanism for low frequencies.

LOUDNESS DISCRIMINATION

Current theory holds that perceived loudness is a function of the number of individual neurons that are firing at a given time. Thus, a very loud tone might activate 250 neurons, while a soft tone would activate only 50 neurons. ..

MODULE 2 PROGRESS CHECK 1

Now test yourself without looking back.

1. The external of the hearing sense is the compression and expansion of

2. When the compression and expansion is very rapid, the sound is said to be high in: a. amplitude.
b. pitch.
c. frequency.
d. loudness.
e. wave form.

3. If one were to compare the wave form of a soft sound with the wave form of a much louder sound. the loud sound will have a greater

4. Name the section of the ear that has each of the following functions.
a. amplification of sound waves
b. Collection of sound waves
c. Conversion of sound waves into neural impulses

5. The ossicles transfer sound vibrations between which parts of the ear?
a. Eardrum and oval window
b. Outer ear and inner ear
c Auditory canal and cochlea
d. Outer ear and middle ear
e. Pinna and anvil

6. Neural impulses in the ear are generated at the:
a. oval window.
b. organ of Cord..
c. eardrum.
d. round window.

7. The traveling-wave theory says that different frequencies are represented by vibration at specific points on the:
a. round window.
b. stirrup.
c. auditory nerve.
d. basilar membrane.

8. A modem interpretation of a frequency theory is the volley theory of hearing. This theory says that frequencies are represented by alternate firing of groups of _______________________________

9. Current thinking holds that more neurons fire when a sound is:|
a. lower in frequency.
b. higher in amplitude.
c. lower in amplitude.

ANSWER KEY


MODULE 2 EXERCISES

Here is a diagram of the ear:

Refer to diagram above. After each of the functions listed below, write the name of the major part of the ear (outer, middle, inner) that carries out the function.
a Converts sound vibrations to neural impulses
b. Amplifies sound vibrations
C. Collects sound waves
d. Transfers vibrations from eardrum to oval window
e. Transfers vibrations from auditory canal to cochlea

______________________________________________3

The shrill sound of a police whistle sets up compressions and expansions of air molecules that are both forceful and rapid. Which of the following would describe the sound of a police whistle?
a. Low frequency
b. High amplitude
C. High frequency
d. Low amplitude

_____________________________________________________1

In terms of its frequency and amplitude, describe the sound of a nearby foghorn.

a. Frequency:

b. Amplitude:

___________________________________________________2

ANSWERS:

1 b, c
2 a low
b. high

3 a inner ear
b. middle ear
c. outer ear
d. middle ear
e. middle ear


Sound vibrations carried by the ossicles set up waves in the fluid- filled cochlea. Those waves displace the__________________________________________________________________________________________3

Match.

1 ) Transfers vibrations to oval window

2) Situated on basilar membrane

3) Carries neural impulses to brain

4) Part of organ of corti that generates impulses in nerve endings of auditory nerve Displaced by fluid vibrations in cochlea
a. Auditory nerve

b. Stirrup

c . Basilar membrane

d. Hair cell

e. Cochlea

f. Organ of Corti


______________________________________________1

The two general types of hearing theories are:

a. place theory.

b. clinical theory.

c. frequency theory.

d. experimental theory.

_______________________________________________________5

One theory of hearing says that frequencies are represented by
vibrations at specific points on the basilar membrane that stimulate
specific neurons. What is the name of this theory?

a. Volley theory
b. Traveling-wave theory
c. (neither)
____________________________________________________4

The valley theory says that:

a. higher frequencies are represented by more rapid firing of individual neurons.
b. higher frequencies are represented by successive
firing of different groups of neurons.
c. (neither)

______________________________________________2

NOW TAKE PROGRESS CHECK 2

1
1 b
2)
3 a
4 d
5 c
2 b
3 basilar membrane
4 b
5 a,c




MODULE 2
PROGRESS CHECK 2


1. Sound is actually the________________________________and ____________________________ of air.

2. When air molecules vibrate very rapidly, frequency is said to be:

a. high.
b. low.
c. loud.
d. complex.

3. When air molecules vibrate forcefully, the sound is said to be high in ______________________________

4. The part of the ear that amplifies sound vibrations is called the___________________________.

5. The structure that is situated on the basilar membrane and converts sound vibrations to neural impulses is called the ____________________________________

6. What is the function of the outer ear?_________________________________________

7. According to the traveling-wave theory, frequency is represented by vibration at specific points on the__________________________

8. According to the volley theory, how are frequencies represented?

ANSWER KEY

UNIT 6 Table of Contents