Download Structure and Function of the Auditory System and more Lecture notes Psychology in PDF only on Docsity!
C H A P T E RC H A P T E R
Structure and Function
of the Auditory System
Structure and Function
of the Auditory System
CHAPTER OBJECTIVES
■ (^) To be able to identify basic anatomical landmarks of the outer ear, middle ear, inner ear, and auditory portions of the central nervous system; ■ (^) To understand the primary functions of the outer ear, middle ear, inner ear, and auditory portions of the central nervous system; ■ To gain insight into how the primary functions of each portion of the auditory system are supported by underlying anatomy and physiology; and ■ To understand some basic perceptual aspects of sound such as hearing threshold, loudness, pitch, and masking.
KEY TERMS AND DEFINITIONS
■ (^) Auditory periphery : The outer ear, middle ear, and inner ear, ending at the nerve fibers exiting the inner ear. ■ (^) Auditory central nervous system : The ascending and descending auditory pathways in the brainstem and cortex. ■ (^) Tonotopic organization : The systematic mapping of sound frequency to the place of maximum stimulation within the auditory system that begins in the cochlea and is pre- served through the auditory cortex. ■ (^) Transducer : A device or system that converts one form of energy to another. The cochlea can be considered a mechanoelectrical transducer because it converts mechanical vibrations to electrical energy to stimulate the afferent nerve fibers leading to the brainstem.
52 SECTION 1 ■^ The Communication Chain
Recall that the primary purpose of the communication chain is to support the transfer of thoughts, ideas, or emotions between two people, the sender and the receiver. Thus far, we have seen that the sender can translate the thought to be communicated into a code of sound sequences representing meaningful words and sentences in the language shared by these two individuals. How is this acoustic code converted into information that can be understood by the mind of the receiver or listener? The mind of the receiver makes use of neural electrical signals generated by the central nervous system. Thus, one of the pri- mary functions of the auditory system is to convert the acoustic energy pro- duced by the talker—human speech sounds—into neural energy that can be deciphered by the brain of the listener. This process is the central topic of this chapter. This chapter is divided into three main sections. The first two deal with the anatomy, physiology, and functional significance of the peripheral and central sections of the auditory system. The peripheral portion of the auditory system is defined here as the structures from the outer ear through the auditory nerve. The auditory portions of the central nervous system begin at the cochlear nucleus and end at the auditory centers of the cortex. The third sec- tion of this chapter details some fundamental aspects of the perception of sound.
PERIPHERAL AUDITORY SYSTEM
Figure 3.1 shows a cross-section of the auditory periphery. This portion of the auditory system is usually further subdivided into the outer ear, middle ear, and inner ear.
Outer Ear
The outer ear consists of two primary components: the pinna and the ear canal. The pinna is the most visible portion of the ear, which extends laterally from the side of the head. It is composed of cartilage and skin. The ear canal is the long, narrow canal leading to the eardrum (or tympanic membrane). The eardrum represents the boundary between the outer ear and the middle ear. The entrance to this canal is called the external auditory meatus. The deep bowl-like portion of the pinna adjacent to the external auditory meatus is known as the concha. The outer ear serves a variety of functions. First, the long (2.5-cm), narrow (5- to 7-mm) canal makes the more delicate middle and inner ear less accessi- ble to foreign objects. The outer third of the canal is composed of skin and
positioned very carefully inside the ear canal to rest alongside the eardrum. Now if we present a series of sinusoidal sound waves, or pure tones, of different frequencies, which all measure 70 dB SPL at the microphone just outside the concha, and if we read the sound pressure levels measured with the other microphone near the eardrum, we will obtain results like those shown by the solid line in Figure 3.2. Notice that, at frequencies less than approximately 1,400 Hz, the microphone that is located near the eardrum measures sound levels of approximately 73 dB SPL. This is only 3 dB higher than the sound level just outside the outer ear. Consequently, the outer ear exerts little effect on the intensity of low-frequency sound. As the frequency of the sound is increased, however, the intensity of the sound measured at the eardrum increases to levels considerably above 70 dB SPL. The maximum sound level at the eardrum is reached at approximately 2,500 Hz and corresponds to a value of approximately 87 dB SPL. Thus, when sound waves having a frequency of
54 SECTION 1 ■^ The Communication Chain
Elevated Source
Frequency (kHz)
1 2 5 10
90
85
80
75
70
65
θ = 0°
α = 0°
α = 60°
dB SPL Measured at Eardrum
FIGURE 3.2 The response of the head, outer ear, and ear canal for various angles of
elevation (a) of the sound source. Zero degrees corresponds to a sound source at eye level and straight ahead (0 degrees azimuth, 0 degrees elevation), whereas 60 degrees represents a source located straight ahead but at a higher elevation. If the listener’s head and outer ear had no influence on the sound level measured at the eardrum, a flat line at 70 dB SPL would result. This figure illustrates the amplification of high- frequency sound by the outer ear and the way that this amplification pattern changes with elevation of the sound source. (Adapted from Shaw EAG. The external ear. In: Keidel WD, Neff WD, eds. Handbook of Sensory Physiology, vol 1. New York: Springer Verlag; 1974:463, with permission.)
2,500 Hz enter the outer ear, their sound pressure levels are increased by 17 dB by the time they strike the eardrum. The function drawn with a solid line in Figure 3.2 illustrates the role that the outer ear serves as a resonator or ampli- fier of high-frequency sounds. The function shown is for the entire outer ear. Experiments similar to the one just described can be conducted to isolate the contribution of various cavities to the resonance of the total outer ear system. Again, the results of such experiments suggest that the two primary structures contributing to the resonance of the outer ear are the concha and the ear canal. The resonance of the outer ear, which is represented by the solid line in Figure 3.2, was obtained with a sound source located directly in front of the subject at eye level. If the sound source is elevated by various amounts, a dif- ferent resonance curve is obtained. Specifically, the notch or dip in the solid function that is located at 10 kHz in Figure 3.2 moves to a higher frequency and the peak of the resonant curve broadens to encompass a wider range of frequencies as the sound source is increased in elevation. This is illustrated in Figure 3.2 by the dotted lines. Each dotted line represents a different angle of elevation. An elevation of 0 degrees corresponds to eye level, whereas a 90- degree elevation would position the sound source directly overhead. The response of the outer ear changes as the elevation of the sound source changes. This results from the angle at which the incident sound wave strikes the various cavities of the outer ear. The result is that a code for sound eleva- tion is provided by the outer ear. This code is the amplitude spectrum of the sound, especially above 3000 Hz, that strikes the eardrum. Thus, the outer ear plays an important role in the perception of the elevation of a sound source. Finally, the outer ear also assists in another aspect of the localization of a sound source. The orientation of the pinnae is such that the pinnae collect sound more efficiently from sound sources located in front of the listener than from sources originating behind the listener. The attenuation of sound waves originat- ing from behind the listener assists in the front/back localization of sound. This is especially true for high-frequency sounds (i.e., sounds with short wavelengths). In summary, the outer ear serves four primary functions. First, it protects the more delicate middle and inner ears from foreign bodies. Second, it boosts or amplifies high-frequency sounds. Third, the outer ear provides the primary cue for the determination of the elevation of a sound’s source. Fourth, the outer ear assists in distinguishing sounds that arise from in front of the listener from those that arise from behind the listener.
Middle Ear
The middle ear consists of a small (2-cm^3 ) air-filled cavity lined with a mucous membrane. It forms the link between the air-filled outer ear and the fluid-filled
CHAPTER 3 ■^ Structure and Function of the Auditory System 55
CHAPTER 3 ■^ Structure and Function of the Auditory System 57
the stapes footplate, then the pressure at the smaller footplate must be greater than that at the larger eardrum. As an analogy, consider water being forced through a hose. If the area of the opening at the far end of the hose is the same as that of the faucet to which it is connected, the water will exit the hose under the same water pressure as it would at the faucet. If a nozzle is now attached to the far end of the hose and it is adjusted to decrease the size of the opening at that end of the hose, the water pressure at that end will be increased in proportion to the degree of constriction produced by the nozzle. The smaller the opening, the greater the water pressure at the nozzle (and the farther the water will be ejected from the nozzle). Applying the same force that exists at the faucet to push the water through a smaller opening created by the nozzle has increased the water pressure at the nozzle. Another analogy explaining the pressure gain associated with areal ratios is one that explains why carpentry nails have a broad head and sharp, narrow point (Vignette 3.1). The middle ear system, primarily through the difference in area between the eardrum and the stapes footplate, compensates for much of the loss of sound energy that would result if the airborne sound waves impinged directly on the fluid-filled inner ear. Approximately 25 to 27 dB of the estimated 30-dB loss has been compensated for by the middle ear. The ability of the middle ear system to amplify or boost the sound pressure depends on signal frequency. Specifically, little pressure amplification occurs for frequencies below 100 Hz or above 2, to 2,500 Hz. Recall, however, that the outer ear amplified sound energy by 20 dB for frequencies from 2,000 to 5,000 Hz. Thus, taken together, the portion of the auditory system peripheral to the stapes footplate increases sound pressure by 20 to 25 dB in a range of approximately 100 to 5,000 Hz. This range of frequencies happens to correspond to the range of frequencies in human speech that are most important for communication. As is noted later in this chapter, a decrease of 10 dB in sound level for moderate-intensity sounds, such as conversational speech, corresponds to a halving of loudness. Without the amplification of sound waves provided by the outer and middle ears, conversational speech pro- duced by the talker would sound like soft speech or whispered speech to the listener and effective communication would be jeopardized. Another less obvious function of the middle ear also involves the outer ear/inner ear link formed by the ossicles. Because of the presence of this mechanical link, the preferred pathway for sound vibrations striking the eardrum will be along the chain formed by the three ossicles. Sound energy, therefore, will be routed directly to the oval window. There is another mem- branous window of the inner ear that also lies along the inner or medial wall of the middle ear cavity. This structure is known as the round window (Fig. 3.1). For the inner ear to be stimulated appropriately by the vibrations of the sound waves, it is important that the oval window and round window not be displaced
58 SECTION 1 ■^ The Communication Chain
Surface Area of nail head is about 10 times greater than that at the tip
- Pressure (^) head = Force (^) hammer / Area (^) head
- Area (^) tip = 0.1 Area (^) head
- Pressure (^) tip = Force (^) hammer / 0.1 Area (^) head
Pressure (^) tip is 10x greater than Pressure (^) head
VIGNETTE 3.1 CONCEPTUAL DEMO
AN ANALOGY FOR P RESSURE
The common carpentry nail provides an illustration of the pressure amplification that occurs when the same force is applied over both a larger and smaller surface area. As shown in the figure, the head of the nail has a greater surface area than the narrow point of the nail. When force is applied to the broader head of the nail with a hammer, that same force is channeled through to the narrow point of the nail. However, because the area of the narrow point at the end of the nail is approximately 10 times smaller than the area at the head of the nail, the pressure applied at the narrow point is 10 times greater. (Recall that pressure is equal to force per unit area or Pressure Force/Area.) The increased pressure at the point of the nail enables it to better pene- trate the material (wood, for example) into which it is being driven. There are, of course, other practical factors affecting the design of the common carpentry nail (for example, the broader head also makes it easier to strike and to remove), but the result- ing pressure amplification is one of the key elements in the design and represents a nice analogy to the pressure amplification achieved by the areal ratio of the tympanic membrane and stapes footplate in the middle ear.
anatomic landmarks of the inner ear depicted in Figure 3.3 are the oval window and the round window. Recall that the footplate of the stapes, the medial-most bone of the three ossicles in the middle ear, is attached to the oval window. The cochlea is cut in cross-section from top (apex) to bottom (base) in Figure 3.4. The winding channel running throughout the bony snail-shaped structure is further subdivided into three compartments. The middle compartment, the scala media , is sandwiched between the other two, the scala tympani and scala vestibuli. The scala media is a cross-section of the membranous labyrinth that runs throughout the osseous labyrinth. All three compartments are filled with fluid, although the fluids are not identical in each compartment. The scala media is filled with a fluid called endolymph, which differs considerably from the fluid in the other two compartments, called perilymph. The scala media houses the sen- sory organ for hearing, the organ of Corti. When the oval window vibrates as a consequence of vibration of the ossicles, a pressure wave is established within the fluid-filled inner ear that causes the scala media, and the structures within it, to move in response to the vibration. This displacement pattern for the scala media is usually simplified by considering the motion of just one of the partitions form- ing the scala media, the basilar membrane (Fig. 3.4). Although the displacement pattern of the basilar membrane is depicted, it should be noted that the entire fluid-filled scala media is undergoing similar displacement.
60 SECTION 1 ■^ The Communication Chain
FIGURE 3.4 Modiolar cross-section of the cochlea illustrating the scalae through
each of the turns. (Adapted from Zemlin WR. Speech and Hearing Science: Anatomy and Physiology. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall; 1988:464, with permission.)
Helicotrema Spiral ligament
Scala vestibuli Scala media (cochlear duct) Scala tympani
Organ of corti
Spiral lamina
Vestibular Auditory nerve branch of auditory nerve
Basilar membrane
Reissner’s membrane
Spiral ganglion
CHAPTER 3 ■^ Structure and Function of the Auditory System 61
Figure 3.5 illustrates the displacement pattern of the basilar membrane at four successive instants in time. When this displacement pattern is visualized directly, the wave established along the basilar membrane is seen moving or traveling from the base to the apex. The displacement pattern increases grad- ually in amplitude as it progresses from the base toward the apex until it reaches a point of maximum displacement. At that point, the amplitude of dis- placement decreases abruptly. The thin solid line connecting the amplitude peaks at various locations for these four instants in time describes the dis- placement envelope. The envelope pattern is symmetrical in that the same pat- tern superimposed on the positive peaks can be flipped upside down and used to describe the negative peaks. As a result, only the positive or upper half of the envelope describing maximum displacement is usually displayed. Figure 3.6 displays the envelopes of the displacement pattern of the basilar membrane that have been observed for different stimulus frequencies. Note that as the frequency of the stimulus increases, the peak of the displacement pattern moves in a basal direction closer to the stapes footplate. At low frequencies,
FIGURE 3.5 Illustration of the traveling wave pattern at four instants in time in
response to a mid-frequency sound. The thin solid lines connect the maximum and minimum displacements at each location along the cochlea and for each instant in time. These lines represent the envelope of the displacement patterns. (Adapted from von Bekesy G. Experiments in Hearing. New York: McGraw-Hill; 1960:462.)
Envelope
Envelope
Time 1
2
3
4
Base
Stapes Distance along the cochlea Helicotrema
Apex
maximum mechanical activity in the cochlea is referred to as tonotopic organiza- tion ( tono frequency; topic place; tonotopic mapping of frequency to place of stimulation). Tonotopic organization is an important property of the auditory system, one that begins in the cochlea and is preserved to the auditory cortex, and forms the basis for viable code for sound frequency. A more detailed picture of the structures within the scala media is provided in Figure 3.7. The sensory organ of hearing, the organ of Corti, is seen to rest on top of the basilar membrane. The organ of Corti contains several thousand sensory receptor cells called hair cells. Each hair cell has several tiny hairs or cilia protruding from the top of the cell. As shown in Figure 3.7, there are two types of hair cells within the organ of Corti: inner hair cells and outer hair cells. Approximately 90% to 95% of the auditory nerve fibers that carry information to the brain make contact with the inner hair cells. The outer hair cells are much greater in number, but do not have as many nerve-fiber connections to the upper portions of the auditory system (only 5% to 10% of nerve fibers con- nect to the outer hair cells). One of the critical functions of the hair cells in the organ of Corti is the conversion ( transducer function) of mechanical energy from the middle-ear vibration to electrical energy that stimulates the connecting nerve fibers. As noted previously, this is a critical link in the communication chain in converting the acoustic signal from the sender to a neural electrical sig- nal that can be deciphered by the brain of the receiver. The conversion takes
CHAPTER 3 ■^ Structure and Function of the Auditory System 63
Reticular lamina Outer hair cells
Tectorial membrane
Inner hair cell
Tunnel
Nerve fibers
Spiral lamina
Habenula perforata
BV Tunnel fibers Basilarmembrane
Pillar cells (rod of corti)
DC DC
DC
HC
FIGURE 3.7 Detailed cross-section of the organ of Corti. BV , basilar vessel; DC ,
Deiter cell; HC , Hensen cell. (Adapted from Pickles JO. An Introduction to the Physiology of Hearing. 2nd ed. London: Academic Press; 1988:29.)
place within the inner ear in the organ of Corti. Intact inner and outer hair cells are needed for this conversion to take place. As we will see later, many agents— such as noise, genetic disorders, or aging—can harm or destroy these hair cells and cause a breakdown in the communication chain. Although exciting research is underway that may alter this statement in the near future, at the moment, such loss of hair cells in the organ of Corti is irreversible. If only a small percentage of outer hair cells (5% to 10%) are actually con- nected to the brain, what purpose do they serve? A complete answer to this ques- tion is beyond the scope of this book. Suffice it to say that the outer hair cells serve to augment the mechanical response of the cochlea when stimulated by vibra- tions. They enable the inner hair cells to be stimulated by much lower sound lev- els than would be possible otherwise. Without them, the inner hair cells would not be able to respond to sound levels lower than about 50 to 60 dB SPL. In addi- tion, in the course of altering the mechanical response of the cochlea for incom- ing sound or mechanical vibration, the outer hair cells generate their own vibra- tions that can be recorded as sounds in the ear canal using a tiny microphone. These sounds, known as otoacoustic emissions (OAEs) can be measured and their presence can be used to verify a normal functioning cochlea (Vignette 3.2). The two primary functions of the auditory portion of the inner ear can be summarized as follows. First, the inner ear performs a frequency analysis on incoming sounds so that different frequencies stimulate different regions of the inner ear (i.e., tonotopic organization). Second, mechanical vibration is amplified and converted into electrical energy by the hair cells. The hair cells are frequently referred to as mechanoelectrical transducers. That is, cells that convert mechanical energy (vibration) into electrical energy that leads to the generation of electrical activity in the auditory nerve.
Auditory Nerve
The electrical potentials generated by auditory nerve fibers are called all-or- none action potentials because they do not vary in amplitude when activated. If the nerve fibers fire, they always fire to the same degree, reaching 100% amplitude. The action potentials, moreover, are very short-lived events, typi- cally requiring less than 1 to 2 ms to rise in amplitude to maximum potential and return to resting state. For this reason, they are frequently referred to as spikes. Information is coded in the auditory portions of the central nervous sys- tem via patterns of neural spikes. For example, as the intensity of sound is increased, the rate at which an auditory nerve fiber fires also increases, although only over a limited range of variation in sound intensity. By varying the intensity and frequency of the sound stimulus, it is possible to determine the frequency to which a given nerve fiber responds best; that is,
64 SECTION 1 ■^ The Communication Chain
the frequency that requires the least amount of intensity to produce an increase in firing rate. This frequency is often referred to as the best frequency or charac- teristic frequency of the nerve fiber. Some nerve fibers have a low characteristic frequency and some have a high characteristic frequency. Fibers with high char- acteristic frequencies come from (inner) hair cells in the base of the cochlea, whereas those with low characteristic frequencies supply the apex. As the nerve fibers exit through the bony core of the cochlea on their way to the brainstem, they maintain an orderly arrangement. The bundle of nerve fibers composing the cochlear branch of the auditory nerve is organized so that fibers with high characteristic frequencies are located around the perimeter, whereas fibers with low characteristic frequencies make up the core of the cochlear nerve. Thus, the auditory nerve is organized, as is the basilar membrane, so that each char- acteristic frequency corresponds to a specific anatomical location or place. This mapping of the frequency of the sound wave to place of maximum activity within an anatomic structure is referred to as tonotopic organization. The neural place that responds to a particular sound stimulus provides the brain with important information regarding the frequency content of that sound. Temporal or time-domain information is also coded by fibers of the audi- tory nerve. Consider, for example, the discharge pattern that occurs within a nerve fiber when that stimulus is a sinusoid that lies within the response area of the nerve fiber. The pattern of spikes that occurs under such conditions is illustrated in Figure 3.8. Note that when the single nerve fiber discharges, it always does so at essentially the same location on the stimulus waveform. In Figure 3.8, this happens to be the positive peak of the waveform. Notice also that it may not fire during every cycle of the stimulus waveform. Nonetheless, if one were to record the interval between successive spikes and examine the number of times each interval occurred, a histogram of the results would look like that shown in Figure 3.8. This histogram, known simply as an interval his- togram, indicates that the most frequent interspike interval corresponds to the period of the waveform. All other peaks in the histogram occur at integer mul- tiples of the period. Thus, the nerve fiber is able to encode the period of the waveform. This holds true for nerve fibers with characteristic frequencies less than approximately 5,000 Hz. As discussed in Chapter 2, if we know the period of a sinusoidal waveform, we know its frequency ( f 1/ T ). Hence, the nerve fibers responding in the manner depicted in Figure 3.8 could code the fre- quency of the acoustic stimulus according to the timing of discharges. For fre- quencies up to 5,000 Hz, the neural firing is synchronized to the sound stimu- lus, as in Figure 3.8A. By combining synchronized firings for several nerve fibers, it is possible to encode the period of sounds up to 5,000 Hz in fre- quency. This type of frequency coding may be useful for a wide variety of sounds, including the coding of the fundamental frequency for complex sounds like vowels or musical notes.
66 SECTION 1 ■^ The Communication Chain
The electrical activity of the auditory nerve can also be recorded from more remote locations. In this case, however, the action potentials are not being mea- sured from single nerve fibers. Recorded electrical activity under these circum- stances represents the composite response from a large number of nerve fibers. For this reason, this composite electrical response is referred to frequently as the whole-nerve action potential. Because the whole-nerve action potential represents
CHAPTER 3 ■^ Structure and Function of the Auditory System 67
Amplitude
Amplitude
Time Time^ Time Interval #1 Interval#
Interval
A Time
Stimulus
Single unit response
1000 Hz Period = 1.0 ms
169 140 120 100 80 60 40 20
1 2 3 4 5 6 7 8 9 10 11 Interval in ms
Number of Spikes
B
FIGURE 3.8 A, Illustration of the synchronization of nerve fiber firings to the stim-
ulus waveform. Note that the nerve fiber always fires at the same point of the wave- form, although it may not fire every cycle. The intervals between successive firings of the nerve fiber can be measured and stored for later analysis. B, A histogram of the intervals measured in (A), referred to as interspike intervals. An interval of 1 ms was the most frequently occurring interval, having occurred approximately 180 times. This cor- responds to the period of the waveform in (A).
We have already reviewed the simple coding of information available in the responses of the auditory nerve fibers. The mapping of frequency to place within the cochlea, for example, was preserved in the responses of the nerve fibers such that fibers having high characteristic frequencies originated from the high-frequency base of the cochlea. The period of the waveform could also be coded for stimulus frequencies less than 5,000 Hz. In addition, the intensity of the stimulus is coded over a limited range (30 to 40 dB) by the discharge rate of the fiber. At the level of the cochlear nucleus, it is already apparent that the ascending auditory pathway begins processing information by converting this fairly simple code into more complex codes. The coding of timing infor- mation, for example, is much more complex in the cochlear nucleus. In addi- tion, some nerve fibers within the cochlear nucleus have a much broader range of intensities (up to 100 dB) over which the discharge rate increases steadily with sound intensity. As one probes nerve fibers at various centers within the auditory CNS, a tremendous diversity of responses is evident.
CHAPTER 3 ■^ Structure and Function of the Auditory System 69
Auditory cortex Medial geniculate body
Inferior colliculus
Cochlear nucleus Superior olive
(Cochlea) (Cochlea)
Superior olive
Cochlear nucleus
Inferior colliculus
Medial geniculate body
FIGURE 3.9 The ascending pathways of the auditory central nervous system. The
blue arrows represent input from the right ear; the white arrows represent input from the left ear. (Adapted from Yost WA, Nielsen DW. Fundamentals of Hearing: An Introduction. 2nd ed. New York: Holt, Rinehart, & Winston; 1985:98, with permission.)
VIGNETTE 3.3 CLINICAL APPLICATIONS
AUDITORY B RAINSTEM RESPONSE M EASUREMENTS
It was mentioned earlier that the whole-nerve action potential represented the summed response of many single nerve fibers firing synchronously in response to an abrupt acoustic signal. It was also mentioned that this potential could be recorded remotely from the ear canal. The left-hand portion of the drawing that accompanies this vignette shows a patient with electrodes pasted to the skin of the forehead, the top of the head (vertex), and the area behind the pinna (the mastoid prominence). The tracing in the right-hand portion of this illustration shows the electrical activity recorded from the patient. The acoustic stimulus is a brief click that produces synchronized responses from nerve fibers in the cochlea and the brainstem portion of the auditory CNS. The tracing represents the average of 2,000 stimulus presentations presented at a moderate inten- sity at a rate of 11 clicks per second. Approximately 3 minutes is required to present all 2,000 stimuli and to obtain the average response shown below. Note that the time scale for the x axis of the tracing spans from 0 to 10 ms. This represents a 10-ms interval beginning with the onset of the click stimulus. The tracing shows several distinct bumps or waves, with the first appearing at approximately 1.5 ms after stimulus onset. This first wave, wave I, is believed to be a remote recording of the whole-nerve action potential from the closest portion of the auditory nerve. Approximately 1 ms later, 2.5 ms after stimulus onset, wave II is observed. This wave is believed to be the response of the more distant portion of the auditory nerve. One millisecond later, the electrical activity has traveled to the next center in the brainstem, the cochlear nucleus, and produces the response recorded as wave III. Wave IV represents the activity of the superior olivary complex. Wave V represents the response of the lateral lemniscus, a structure lying between the superior olives and the inferior colliculus. Waves VI and VII (the two unla- beled bumps after wave V) represent the response of the latter brainstem structure. The response shown in the right-hand tracing is known as an auditory brainstem response (ABR). It has proven very useful in a wide variety of clinical applications, from assessment of the functional integrity of the peripheral and brainstem portions of the ascending auditory CNS to assessment of hearing in infants or difficult-to-test patients.
Amplitude (
μV)
0 2 4 6 8 10ms Time
GRBQ364-3602G-C03[51-78].qxd 3/13/08 8:30PM Page 70 Aptara, Inc.
