Auditory Physiology

Auditory Physiology 1

Auditory Physiology

The primary aim of this chapter will be to review the physiological mechanisms that are involved in two basic and extraordinarily important functions of the auditory system: (1) conversion of the vibratory energy that reaches the ear drum into a series of neural impulses on the auditory nerve (this is called transduction), and (2) the spectrumanalysis function of the auditory system; that is, the ability of the auditory system to break a complex sound wave into its individual frequency components.

Before getting into the details, it might be useful to consider some of the fundamental capabilities of the auditory system which, from any point of view, are nothing short of awe inspiring. A brief and by no means exhaustive list appears below.

• The faintest sound that can be detected by the human ear is so weak that it moves the ear drum a distance that is equivalent to one-tenth the diameter of a hydrogenmolecule. If the ear were slightly more sensitive we would hear the random particle oscillations known as Brownian motion.

• The most intense sound that can be heard without causing pain is approximately 140 dB more intense than a barely detectable sound. This means that that the dynamic range of the ear – the ratio of the most intense sound that can be heard without pain to the intensity of a barely audible sound – is an astounding 100 trillion to 1.

• The frequency range of human hearing runs from approximately 20 Hz to 20,000 Hz, a range of about 10 octaves.

• For signal levels approximating conversational speech, the ear can detect frequency differences that are on the order of 0.1%, or approximately 1 Hz for a 1,000 Hz test signal (Wier, Jestaedt, & Green, 1977).

• Later in the chapter we will see that the auditory system utilizes an elegant mechanism that delivers sounds of different frequencies to different physical locations along the cochlea; i.e., a sound of one frequency will produce the greatest neural activity at one physical location while a sound of a slightly different frequency will activate a different location. The difference in frequency that a listener can barely detect corresponds to a difference in physical location along the cochlea of about 10 microns (1 micron = one millionth of a meter, or one thousandth of a millimeter). This distance, in turn, is approximately the width of a single auditory receptor cell(Davis and Silverman, 1970).

• Under ideal conditions listeners can detect intensity differences as small as 0.6 dB (Gulick, Gescheider, and Frisina, 1989).

• Listeners can locate the source of a sound based on differences in the time of arrival between the two ears that are as small as 10 s (i.e., 10 millionths of a second).

Further, the anatomy that supports this processing is a miracle of miniaturization. For example, the middle ear cavity is approximately 3 mm in width and approximately 15 mm in the vertical dimension (Zemlin, 1968), with roughly the volume of a sugar cube. The cochlea, which contains the auditory receptors, is even smaller, at approximately 5 mm in height and approximately 9 mm in diameter at its widest point (Gelfand, 1990).

Overview of the Auditory System

The auditory system can be divided into three major functional subsystems: the conductive mechanism, the sensorineural mechanism, and the central auditory system (see Figure 4-1). In terms of anatomical structures, the conductive mechanism consists of the pinna, the ear canal (also known as the external auditory meatus), the ear drum (also known as the tympanic membrane), and the middle ear, which contains three very small bones called the auditory ossicles. The primary function of the conductive mechanism is to transmit the vibrations that are picked up at the tympanic membrane to the structures of the inner ear, a fluid-filled structure which contains the auditory receptors. However, as we shall see, the middle ear also accomplishes a pressure amplification trick which significantly enhances the sensitivity of the ear.

The sensorineural mechanism consists of the structures of the cochlea and the auditory nerve, also known as the 8th cranial nerve. The auditory nerve conveys neural impulses between the cochlea and the brain stem, which is part of the central auditory system. The inner ear contains specialized sensory receptor cells called hair cells. These cells are responsible for converting the vibratory energy that enters the auditory system into nerve impulses that are transmitted to the central nervous system via the auditory nerve. In addition to the conversion of vibratory energy into neural impulses, the cochlea also carries out a spectrum analysis in which the low frequency components of the signal are directed to one end of the cochlea and the high-frequency components are directed to the other end. As will be seen later in this chapter, the precise role that is played by this frequency analysis is only partially understood.

The electrical signals that are generated by the hair cells in the inner ear are carried by the auditory nerve to central auditory system, which consists of structures in the brain stem and auditory cortex. It is often said that the central auditory system is responsible for higher level functions of auditory analysis, such as the "... recognition, interpretation, and integration of auditory information ..." (Deutsch & Richards, 1979). There is little question that the central auditory system is, in fact, heavily involved in higher level functions such as speech recognition and the ability to recognize familiar voices and familiar melodies. However, the central auditory system also plays a very important role in relatively low-level aspects of auditory analysis, such as sound localization, pitch perception and, quite possibly, spectrum analysis.

The Conductive Mechanism

The Outer Ear

The outermost portion of the conductive mechanism is a cartilaginous structure called the pinna, also known as the auricle (see Figure 4-2). While the approximately funnel shape of the auricle might lead one to believe that the structure may play some role in sound gathering, this appears not to be the case (von Bekesy & Rosenblith, 1958). A prominent visual characteristic of the auricle is the rather convoluted shape consisting of a number of ridges, grooves, and depressions. It appears that this complex topography, along with other factors, plays some role in sound localization (von Bekesy & Rosenblith, 1958; Batteau, 1967; Freedman & Fisher, 1968).

Sound is conducted to the tympanic membrane through the external auditory meatus, also known as the ear canal. The lateral two-thirds of the ear canal is cartilaginous and the medial third is bone. The general shape of the ear canal approximates that of a uniform tube, open at the lateral end and closed medially by the tympanic membrane. The tube averages approximately 2.3 cm in length (Wiener & Ross, 1946). Recall that the resonant frequency pattern of a uniform tube which is closed at one end (by the ear drum in this case) can be determined if its length is known. Using the formula from Chapter 3, the lowest resonant frequency of the ear canal should be approximately 3800 Hz (F1= 35,000/(4 . 2.3) = 35,000/9.2 = 3804 Hz). This figure agrees well with experimental data (Wiener & Ross, 1946; Fleming, 1939), although estimates vary. This resonance is partially responsible for the heightened sensitivity of the auditory system to frequencies in the middle portion of the spectrum (see Chapter 3, Figure 3-24).

The sound wave that enters the ear canal sets the tympanic membrane into vibration. When instantaneous air pressure is relatively high (compression), the membrane will be forced inward, and when instantaneous air pressure is relatively low (rarefaction), the membrane will be forced outward. Consequently, the inward and outward movements of the tympanic membrane mirror those of the sound wave that is driving it; for example, if the tympanic membrane is excited by a 500 Hz sinusoid, the tympanic membrane will move inward and outward sinusoidally at 500 Hz. In general, the instant-to-instant displacements of the tympanic membrane will mirror the instantaneous air pressure waveform that is driving the membrane.

The Middle Ear

The middle ear ortympanic cavity is an air-filled chamber whose volume approximates that of a sugar cube (see Figure 4-3). The middle ear communicates with the nasopharynx via the Eustachian tube. This tube is approximately 35 mm in length in adults and angles downward and forward to connect the anterior wall of the tympanic cavity with the nasopharynx. The tube is normally closed, but opens during yawning and swallowing. When the tube opens, air can travel either into or out of the middle ear to create an equilibrium between the air pressure inside the tympanic cavity and that of the outside air. The Eustachian tube also plays an important role in allowing fluids to drain from the middle ear into the nasopharynx.

In terms of the broad overview presented here, the most important structures in the tympanic cavity are the three ossicles, a series of very small bones referred to collectively as the ossicular chain (see Figure 4-4). The largest of the ossicles is the malleus, which attaches directly to the tympanic membrane. The head of the malleus articulates with the incus, which in turn connects to a very small stirrup-shaped bone called the stapes. The stapes ends in an oval plate called the footplate. The stapes footplate attaches to an opening into the labyrinth called theoval window. The labyrinth is a fluid-filled structure that contains the cochlea and the vestibular system, which is responsible for our sense of balance. The stapes footplate is attached to the oval window via a circular ligament called the annular ligament. Directly below the oval window is a second opening into the labyrinth called the round window. The round window is covered by a very small membrane called the internal tympanic membrane.

A reasonable question to ask about the auditory system is why we have a tympanic membrane and ossicular chain at all. Since a primary effect of these structures is to transmit vibrations to the fluid-filled structures of the inner ear, then why isn't the oval window simply covered with a flexible membrane that is driven directly by the sound wave? Aquatic animals, in fact, make use of a "direct-drive" system with no middle ear. A system of this kind would work in land animals as well, but for reasons that are explained below, a substantial loss of energy would result. The key to understanding the role that is played by the tympanic membrane and ossicular chain is to appreciate the energy loss that occurs when a sound wave is transmitted from the air medium in which the sound is initially generated to the fluid medium that exists inside the inner ear.

We know from everyday experience that we do not hear airborne sound very well when we are underwater. The primary reason for this is that there exists an impedance mismatchbetween the air medium in which the airborne sound is initially generated and the fluid medium into which the vibratory distrubance must be transmitted in order for our underwater listener to hear the sound. Impedance is the total opposition to the flow of energy,[1] and the mismatch results from the fact that air is a low-impedance medium while water (and other similar fluids) is a high-impedance medium. These differences in impedance can be demonstrated simply by running a cupped hand through air and water. There is a general rule that states that energy is reflected back toward the source when a signal reaches the boundary between two media whose impedances do not match. In the case of air and fluid, the impedance mismatch is quite large, and when the signal reaches the air-fluid boundary, only 1/1,000th of the energy is absorbed into the fluid medium, with the remainder being reflected back toward the source. Represented on a decibel scale, the loss of signal intensity is 30 dB. In the formula below, the signal intensity on the airborne side of the air-fluid boundary serves as the reference intensity, and the signal intensity on the fluid side of the boundary serves as the measured intensity.

dB= 10 log10 Im/Ir

= 10 log10 1/1,000

= 10 (-3)

= -30 dB

The negative sign here simply means that the signal will be 30 dB weaker on the fluid side of the boundary. Consequently, if the airborne sound wave were to directly drive a simple membrane covering the oval window, a 30 dB loss in signal intensity would occur at the air-fluid boundary. This is not a minor loss of energy. As we will see in the chapter on auditory perception, a 10 dB decrease in intensity corresponds to a decrease of approximately one-half in our subjective impression of loudness. This means that a 50 dB signal, for example, sounds only one-eighth as loud as an 80 dB signal.

One of the primary functions of the middle ear is to amplify pressure so as to overcome a large portion of this energy loss. This is accomplished in two ways: (1) an increase in pressure that occurs when the vibrations that are picked up on the relatively large surface area of the tympanic membrane are focused on the very small surface area of the stapes footplate, and (2) an increase in force (and therefore pressure as well) that occurs as a result of the mechanical lever action of the ossicular chain. The "area trick," known as the condensation effect, is by far the more important of the two effects. Recall from Chapter 2 that there is an important distinction between force and pressure: force is the amount of push or pull on an object, and is the product of mass and acceleration; pressure, on the other hand, is force per unit area. A major implication of this relationship is that pressure can be amplified without a change in force simply by decreasing the area over which the force is delivered. This is the design principle underlying thumb tacks and knives with sharp cutting edges, and exactly this principle is at work in the middle ear as the energy that is delivered to the relatively large area of the tympanic membrane is focused on the very small area at the stapes footplate. The amount of pressure amplification that results from this concentration of force is proportional to the ratio of the two areas that are involved. The effective area of the tympanic membrane is approximately 0.594 cm2, while the area of the stapes footplate is approximately 0.032 cm2(Durrant & Lovrinic, 1984). Consequently, pressure at the stapes footplate will be approximately 18.6 times greater than pressure at the tympanic membrane (0.594/0.032 = 18.6). This pressure amplification can be represented on a decibel scale. Since we are talking about an increase in pressure, the pressure version of the decibel formula is needed:

dB= 20 log10(0.594/0.032)

= 20 log10(18.6)

= 20 (1.27)

= 25.4 dB

Consequently, of the 30 dB that would be lost at the air-fluid boundary, the condensation effect makes up for roughly 25 dB.

A small amount of additional amplification results from the lever action of the ossicular chain. The basic idea is that the ossicular chain is suspended by ligaments in such a way as to form a lever system, with the fulcrum on the body of the incus. One arm of the lever system consists of the malleus while the other arm consists of the incus (see Figure 4-5). The malleus lever arm is approximately 30% longer than the incus lever arm, producing a lever ratio of 1.3:1. Since the force amplification that occurs in any lever system is proportional to the ratio of the lengths of the two lever arms, force will be amplified by a factor of 1.3. Pressure is the force per unit area, so this increase in force means that pressure will also be amplified by a factor of 1.3. Represented on a decibel scale, this amounts to:

dB= 20 log10(1.3)

= 20 (0.11)

= 2.3 dB

(Notice that the pressure version of the decibel formula is being used here rather than the intensity version. That is because the lever advantage produces an increase in force and, therefore, pressure.) If this 2.3 dB pressure amplification is added to the 25.4 dB that is produced by the condensation effect, we find that the combined action of the middle ear system results in a pressure amplification of 25.4+2.3 = 27.7 dB, nearly all of the 30 dB that would otherwise be lost at the air-fluid boundary.

The Sensorineural Mechanism

The two major auditory structures of the sensorineural mechanism are the cochlea and the auditory nerve. The cochlea is one portion of a larger structure called the labyrinth. As noted earlier, the labyrinth contains both the cochlea (the organ of hearing) and the vestibular system (the organ of balance). The three major divisions of the labyrinth are shown in Figure 4-6. The snail-shaped portion of the labyrinth is the cochlea, which contains the hair cells and many other structures that are important for hearing. The upper portion of the labyrinth contains three structures called the semicircular canals,which are part of the vestibular system. The middle portion of the labyrinth is called the vestibule. The oval window and round window are openings into the vestibule.

The portion of the labyrinth that is shown in panel a of Figure 4-7 is a hollowed-out and fluid-filled bony shell called the bony or osseous labyrinth. Fully contained within the bony labyrinth is a fluid-filled structure called the membranous labyrinth, which can be thought of as something like a convoluted water balloon that fits inside the bony labyrinth (see panel b of Figure 4-7). The fluid that courses through the membranous labyrinth is called endolymph and the fluid outside the membranous labyrinth is called perilymph. Two bulges in the membranous