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Methods for Studying Hearing Impairment and Auditory Problems of the Aged

Robert D. Frisina and D. Robert Frisina

Age-related hearing loss—presbycusis—is the foremost communication disorder of our elderly, and one of their top three chronic medical conditions. Currently, there are no cures for the sensorineural hearing loss and auditory processing problems that affect the majority of persons over age 60. We hope that increased basic research with animal models coupled with human clinical studies will lead to breakthrough translational studies aimed at prevention and eventual biomedical cure. The present chapter reviews a variety of effective procedures for measuring hearing loss as a function ofage, including both classical and experimental paradigms. A theme of this exposition is that by utilizing key testing procedures, we can gain an understanding about how age and age-related ototoxic insults and conditions can affect either the cochlea (portion of the inner ear used for hearing) or the central auditory system (portions of the brain used for hearing).

Human Investigations

AUDITORY SENSITIVITY: AUDIOGRAMS, NOISE THRESHOLDS, SPEECH DISCRIMINATION

A classic sign and symptom of age-related hearing loss in human listeners and patients is a gradual decline of hearing sensitivity in the higher frequencies, oftentimes beginning in the middle-age years. The classic manner in which hearing sensitivity is measured results in a pure-tone audiogram, and is the basic measurement of hearing sensitivity performed by all audiologists. In producing the audiogram, thresholds to single tones are measured by asking the subject or listener to respond when a tone of a certain frequency is detected. In a standard audiogram, measurements are made at discrete frequencies from 250 Hz up to 4 kHz. For research purposes, or early diagnosis of presbycusis, ultra-high frequency audiograms can be measured. In such cases the test frequencies can go as high as 20 kHz and require special audiometer calibration and headphones. Audiometric frequencies at 6 and 8 kHz are particularly useful for early diagnosis and research (Tadros et al., 2005), and frequencies up to 14 kHz can be useful for experimental studies of age-related hearing loss in humans.

Overall sensitivity of the ear can be measured using wideband signals such as white noise, speech-weighted noise, and speech sounds themselves, to determine threshold sensitivity. The most commonly used speech measure is called the speech reception threshold (SRT). Audiologists use two-syllable words with equal stress, called spondees, to determine one's speech reception threshold. The SRT correlates highly with speech-frequency pure tone audiogram averages. Speech discrimination tests, as contrasted with SRT, are suprathreshold measures that test the capacity of the cochlea to resolve speech sounds in quiet conditions. A commonly used standardized test is the Northwestern University (NU-6), consisting of 50 words in each of several lists. Each word contains three phonemes in a consonant-vowel-consonant format. The phoneme distribution in each word list approximates the frequency of occurrence of each phoneme in everyday speech. This test is routinely administered in quiet at an intensity level of 30 dB greater than the SRT level, a condition that is characteristic of conversational speech in quiet environments. Old adults with mild or moderate degrees of sensorineural hearing loss are expected to do well on this measure; thus it is a valuable screening tool used to detect unique aberrations that might occur in a given case. Performance in noisy environments involves more than the cochlea and cannot be predicted from this measure in quiet; therefore, additional tests are designed to measure more closely the ability to communicate in everyday noise environments.

OUTER HAIR CELL SYSTEM: OTOACOUSTIC EMISSIONS—TRANSIENT, DISTORTION PRODUCT

There are two types of hair cells in the auditory portion of the mammalian inner ear. There is one row of inner hair cells that spiral from the cochlear base to the apex, and generally three rows of outer hair cells in the cochlear

Handbook of Models for Human Aging

Copyright © 2006 by Academic Press All rights of reproduction in any form reserved.

Figure 76.1 In the 1-6 kHz frequency range, DPOAE amplitudes decline with age, particularly from middle age to old age. For all three age groups, the greatest amplitudes are in the 4-5 kHz region, which is the region showing the greatest decline in DPOAE amplitudes from middle to old age. The values on the right side of the graph are the mean values for the three age groups, across all of the frequencies tested. Error bars represent standard errors of the mean (S.E.M.). From Kim et al. (2002), with permission.

Figure 76.1 In the 1-6 kHz frequency range, DPOAE amplitudes decline with age, particularly from middle age to old age. For all three age groups, the greatest amplitudes are in the 4-5 kHz region, which is the region showing the greatest decline in DPOAE amplitudes from middle to old age. The values on the right side of the graph are the mean values for the three age groups, across all of the frequencies tested. Error bars represent standard errors of the mean (S.E.M.). From Kim et al. (2002), with permission.

spiral. Inner hair cells provide the main channels through which we hear; that is, they carry sound information from the cochlea to those portions of the brain used for hearing. About 95% of the auditory nerve fibers in the eighth cranial nerve form synapses with inner hair cells. In contrast, outer hair cells provide nonlinear electro-hydro-mechanical inputs to the inner hair cells that make the latter much more sensitive to quiet sounds and neuroethologically-relevant complex vocalizations such as speech, and allow them to participate more fully in neural feedback loops from the brain to the ear.

Otoacoustic emissions are faint sounds originating from the outer hair cells of the cochlea that can be recorded from the external ear canal (Kemp, 1978). They are sometimes produced spontaneously, but the most common recording procedure is when sounds are put into the ear, and then on a msec time scale, otoacoustic emissions are produced by the outer hair cells in response to the input sounds. When acoustic clicks are used to elicit the otoacoustic emissions, these physiological responses are referred to as transient-evoked otoacoustic emissions— TEOAEs. When two tones are presented at the same time, the nonlinear properties of the outer hair cell system produce distortion product frequencies, such as cubic or quadratic distortion products, that are not present in the original acoustic stimulus. The most commonly used distortion-product otoacoustic emission (DPOAE) is the 2F1-F2 component, where F1 and F2 are two frequencies at an optimal frequency ratio. Otoacoustic emissions are quite advantageous for measuring the m u

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