Page 10 - Volume 12, Issue 2 - Spring 2012

stimulation has a compressive nonlinearity (Rhode, 1971), which results in much sharper mechanical tuning at sound levels close to threshold and broader tuning at higher sound levels. The pure-tone excitation pattern of the traveling wave along the basilar membrane has a larger relative amplitude at lower sound levels at locations just basal to its tonotopic place than its relative amplitude at higher sound levels (i.e., this is the region of compressively nonlinear growth in amplitude). The outer hair cells in the organ of Corti act as a saturating feedback amplifier to achieving this sharper tuning at sound levels near threshold. From the perspective of identifying ears with hearing loss, damage to outer or inner hair cells or reductions in the electrical potential across the hair cells can lead to sensorineural hearing loss.
Intermodulation distortion and two-tone suppression effects are observed when two or more pure tones are pre- sented simultaneously. These processes are intimately related to the compressive nonlinearity of basilar-membrane mechanics. In the 19th century, Helmholtz described an auditory theory for the combination tones heard out by lis- teners when two pure tones are presented simultaneously; detailed psychophysical data and models are available (Goldstein, 1967). A correlate to such distortion tones was found in single-fiber recordings from the auditory nerve (Goldstein and Kiang, 1968), and in recordings of basilar- membrane displacement (Nuttall et al., 1990; Robles et al., 1990). In single-fiber recordings from the auditory nerve in monkey (Nomoto et al., 1964) and cat (Kiang et al., 1965, Sachs and Kiang, 1967), the presence of one pure tone influ- enced the neural firing rate associated with another pure tone at a higher or lower frequency. This two-tone suppression effect was also found in mechanical recordings on the basilar membrane (Rhode, 1977), which supports the modern view that sharp mechanical tuning is responsible for sharp neural tuning. Such suppression effects (and other effects at more central levels in the auditory system) improve a listener’s abil- ity to perceive speech in noise. For a pure tone (called the probe tone) presented at a low to moderate level, a suppres- sion tuning curve is constructed by measuring the sound pressure level (SPL) needed of a second suppressor tone to reduce the response to the probe tone by a reference amount at a set of suppressor frequencies above and below the probe frequency. A suppression tuning curve is a plot of this critical SPL versus suppressor frequency.
The acoustic reflex forms a feedback pathway connecting middle-ear, cochlear and neural levels of auditory function. This reflex triggers an action of the stapedius muscle to stiffen the ossicular chain of the middle ear (at least at low frequen- cies) in response to the presentation of a relatively high-level sound. Neural signals generated in response to sound ascend via the VIIIth nerve and are processed centrally; neural effer- ents descend via the VIIth (facial) nerve to activate the stapedius muscle, which influences middle-ear transmission.
One task of an aural acoustical test in the clinical setting is to reveal potential sources of dysfunction within the mid- dle ear, cochlea and neural processes that affect the acousti- cal response measured in the ear canal. The next section summarizes past approaches to middle-ear testing.
Middle-ear tests: Acoustic impedance and admittance tympanometry
While acoustical impedance measurements of the ear were initially reported in the late 1920’s and 1930’s, Metz (1946) was the first to obtain clinically significant results in measuring acoustical impedance in substantial groups of ears with normal function and ears with middle-ear disease. The impedance was measured using an acoustic bridge technique at ambient pressure in the ear canal. The bridge had a sound source symmetrically placed in the middle of a cylindrical tube, one end of which was coupled into the ear canal in a leak-free manner. Its opposite end was terminated in a set of acoustic couplers of adjustable, but known dimensions. The sound source generated an outgoing sinusoidal sound wave of equal amplitude but opposite phase in the two tubes. The observer listened using a stethoscope via a tube coupled to each side of the tube near its middle with a Y-tube connec- tion, and adjusted the coupler dimensions until a minimum audible sound was detected. This null represented the fre- quency-specific condition in which the impedances on both sides were equal. The acoustic bridge had to be re-adjusted at other test frequencies, and the acoustic effect of the volume of air in the part of the ear canal between the tube end and the eardrum was unaccounted for. Metz also used the acoustic bridge to measure the acoustic reflex response in terms of a shift in the acoustic impedance of the ear. The acoustic reflex threshold is the lowest level activator that elic- its a detectable shift in the acoustic impedance.
An innovation that led to improved clinical utility was the measurement of acoustic impedance over a range of ear- canal air pressures in a test called tympanometry (Terkildsen and Thomsen, 1959). This pressurization dif- ferentially affected the mobility of the eardrum as assessed using a pure tone. Tympanometers to measure aural acoustic admittance (i.e., the inverse of impedance) at a sin- gle frequency close to 226 Hz use a pump to control air pressure and a probe with a leak-free insertion into the ear canal. Tympanometers were in widespread clinical use by the 1970s and are the dominant clinical testing device of middle-ear function used today.
Acoustic admittance is useful in a measurement in which the acoustic pressure in the ear canal is the input variable and the volume velocity swept out by the eardrum is the output variable. In an ear with normal function, the resulting admit- tance magnitude tympanogram is plotted as a function of excess air pressure within the ear canal. This pressure is con- trolled by the pump over a range of approximately 2-3% devi- ation from atmospheric pressure (e.g., -300 to +200 daPa). The admittance magnitude at the probe has a single-peaked shape that can be used to measure admittance magnitude at the eardrum under certain assumptions. These assumptions, which are of limited validity, are: the eardrum is immobile at the most positive or most negative “tail” pressure, the cross- sectional area does not vary with air pressure, and acoustic wave effects in the ear canal are negligible (Shanks and Lilly, 1981). If the eardrum is immobile at the tail pressures, then the probe admittance equals the admittance of the enclosed volume of air, which acts at low frequencies as a compliance.
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