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measurements of fine acoustic features under different listening conditions, and for the present purposes, in subjects of different ages. A standard stimulus for auditory studies of temporal processing is a silent gap between two sounds, with the task referred to as gap detection. Here the subject or listener will usually listen to a standard sound without a gap, followed (or preceded) rapidly with two sounds separated by a short temporal gap. From trial-to-trial, the task can be made easier by lengthening the gap, or more difficult by shortening the gap. The subject's task is to identify which sound stimulus contains the temporal gap. Threshold is attained when the subject achieves a specified correct percentage of responses. Depending on the frequencies of the sounds composing the gap, young adult subjects with normal hearing can detect gaps as short as about one msec. It has been found that with age, gap detection thresholds become larger; that is, a longer gap is required by older subjects for correct identification than by young adults with normal hearing (Snell and Frisina, 2000; Snell et al., 2002). Declines in auditory temporal processing are worsened by the extent of hearing loss as measured in a subject's audiogram (Gordon-Salant and Fitzgibbons, 1993; Fitzgibbons and Gordon-Salant, 1996). Very little is known about how processing of rapid changes in sound amplitude-amplitude modulation, or how frequency selectivity changes with age alone in human listeners, although these measures have been used extensively to document temporal and spectral processing deficits associated with hearing loss in young adults.

SPEECH PROCESSING IN NOISE: SPEECH IN NOISE (SPIN), HEARING IN NOISE TEST (HINT)

Psychoacousticians and research audiologists interested in the capabilities of the auditory system in more natural, realistic acoustic situations employ hearing measurement tests that assess the ability of the listener to understand speech in background noise. Two of the most useful, commonly used rigorous procedures for measuring speech comprehension in background noise are the SPIN test and HINT.

These tests involve the presentation of speech signals in different levels of background noise. In the SPIN test, the background noise is a calibrated recording of multi-talker babble—the simultaneous speaking of a group of male and female adult voices, where the voice of any one particular person is not discernible. The SPIN test, administered under headphones, measures each ear independently, whereas the HINT is administered in the free field and utilizes both ears simultaneously. In the HINT, the background noise is a speech-weighted noise devoid of any semantic content. The initial test conditions for the HINT involve having the speech signal and background noise come from the same position, N0, which is directly in front of the subject. The effect of spatial separation of speech and noise is measured in the

HINT. Both tests have been useful measures of the aging auditory system where speech comprehension in background noise was being investigated. For example, Frisina and Frisina (1997), by utilizing the SPIN with human subjects whose audiograms were in the normal hearing range, discovered that aged subjects performed worse in background noise than younger adult subjects. In addition, the effect of sensorineural hearing loss was revealed when age-matched old subjects with different degrees of hearing loss were compared with old subjects within the normal range of hearing. This important study demonstrated that (1) age alone could negatively influence speech recognition in a noise background, and (2) hearing loss itself could result in a decline in speech recognition when measured in a noise background.

SPATIAL PROCESSING: HEARING IN NOISE TEST (HINT)

In addition to testing a subject's ability to understand speech in background noise, the HINT also has spatial processing components to it. Specifically, three main spatial processing conditions typically are utilized: N0, where the speech and noise come from the same position directly in front of the listener; N90, where the speech comes from in front, but the background noise emanates from a location to the right, 90° from center; and N270, where the noise comes from 90° left of center. If responses to speech differ between these locations, it implicates properties or problems due to the brainstem portion of the central auditory system where binaural processing occurs; that is, at the level of the superior olivary complex or auditory midbrain.

EAR AND BRAINSTEM PHYSIOLOGICAL PROCESSING: AUDITORY BRAINSTEM RESPONSE

The most commonly used neurophysiological measure of both peripheral and central (brainstem) auditory processing is the auditory brainstem response (ABR). This noninvasive auditory-evoked potential procedure reflects the integrity of the inner ear and brainstem auditory system extending from the auditory nerve to the level of the lateral lemniscus, just posterior or ventral to the auditory midbrain-inferior colliculus. Scalp electrodes record a series of early neuroelectrical potentials that follow auditory stimulation. The human ABR consists of five waves or peaks, with Waves I and V being the most prominent and useful for clinical purposes or research experiments. This procedure has gained widespread use in neonates, infants, and children who do not respond successfully to behavioral auditory threshold measurement procedures. Its limited use with the old subjects is directed to those whose standard test results suggest the possibility of brainstem tumors or eighth-nerve anomalies. If Wave I of the ABR is sufficiently large for analysis, cochlear sensitivity can be isolated and assessed independently of the brainstem activity for study of the aging auditory system. Generally, ABR latencies, such as Wave V, increase with age for both women and men (Jerger and Hall, 1980; Jerger and Johnson, 1988).

Brainstem auditory temporal processing disorders during aging have been identified in human subjects using forward masking of Wave V (Walton et al., 1999). Here, the intensity of a first (masking) tone is varied to measure the time delay between the masking tone and a probe tone that follows with a variable time delay. Specifically, in humans, age will lengthen the time delay needed for a tone to recover its level due to the presence of the masking tone, thus reducing temporal resolution of the brainstem auditory system.

AUDITORY LATE AND EVENT-RELATED POTENTIALS—CEREBRAL CORTEX

Several late auditory evoked potentials can be measured with scalp recording electrodes in mammals, including humans, including N1, P2, and P3. The latter is sometimes referred to as the P300 endogenous event-related potential and can be elicited by an unusual (target or oddball) stimulus that is presented in the context of a standard stimulus usually being repeated many times. These cortical potentials are of higher amplitudes than ABRs, thus requiring fewer repetitions to be recorded faithfully above the electrical noise floor. There have been a number of studies of the aging auditory system employing the P300 and other auditory cortical potentials, and they generally show an increased latency and decreased amplitude with age (Swartz et al., 1992, 1994).

Animal Model Techniques

AUDITORY SENSITIVITY: ABR AUDIOGRAMS, NOISE THRESHOLDS

These threshold sensitivity measures using pure tones or wideband noise bursts are similar to the ABR neuro-physiological measures as described earlier for humans. Their major advantage for animal applications is that the basic sensitivity of the peripheral and brainstem auditory systems can be measured relatively easily and quickly, especially as compared to classical behavioral paradigms such as operant conditioning techniques. However, unlike human adults, but similar to infants or some young children, the animal needs to be anesthetized for a period of 30 to 60 minutes to assess the full frequency range of hearing in most mammals. So, although this procedure utilizes surface electrodes and is noninvasive, the animal will require a short-acting anesthetic, either inhalation or intraperitoneal injection, to reduce muscle-related electrical activity to the level where the small neural brain signals can be picked up above the electrical background noise (noise floor). Since anesthetics reduce homeostatic body temperature control, mammals undergoing ABR recordings must be kept on a heating pad, oftentimes used in conjunction with a thermometer for servo-control feedback. Age-related changes in peripheral and brain-stem auditory sensitivity have been well-documented utilizing ABRs. For example, different inbred strains of mice lose their hearing at different rates, which has been measured using the mammalian ABR audiogram (Hunter and Willott, 1987; Jacobson et al., 2003). An example of this age-related loss in ABR sensitivity is shown in Figure 76.4, highlighting sex differences for the CBA strain of mouse that loses its hearing slowly, like most human cases of presbycusis (Guimaraes et al., 2004).

Suprathreshold ABR amplitudes also decline with age in strains of mice that have been frequently employed in studies on the neural bases of presbycusis—CBA, C57 (Henry and Lepkowski, 1978; Walton et al., 1995).

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

As in human subjects, the presentation of two simultaneous tones with a specific frequency ratio (DPOAE) or the presentation of a train of clicks (TEOAE) can be employed to obtain an objective neurophysiological assessment of the cochlear outer hair cell system as a function of animal age. Also like ABRs, adult human subjects can have their otoacoustic emissions measured by relaxing in a comfortable chair, whereas mammals need to receive a short-acting anesthetic to achieve the level of reduced muscle activity and twitching (sleep) necessary to obtain good ear canal recordings for the otoacoustic emissions waveforms. Like humans, the amplitudes of otoacoustic emissions decline in aging mammals, in a manner proportionate to the loss of cochlear outer hair cells with age, usually starting at the cochlear base that corresponds to the place that codes higher frequencies (Spongr et al., 1997; Jacobson et al., 2003). An example of DPOAE amplitude that declines as a function of age for female and male CBA mice is provided in Figure 76.5 (Guimaraes et al., 2004).

EFFERENT FEEDBACK SYSTEM: CONTRALATERAL SUPPRESSION OF OTOACOUSTIC EMISSIONS

As described earlier in the section on contralateral suppression (CS) in human subjects, presentation of an acoustic stimulus to the contralateral ear can suppress the amplitude of the ipsilateral otoacoustic emissions via the physiological effects of the auditory efferent feedback system whose nerve cell bodies originate in the superior olivary complex of the brainstem auditory system. Jacobson et al. (2003) performed the analogous experiment in CBA mice that Kim et al. (2002) conducted in human subjects and made a similar discovery: Significant age-related declines in CS magnitudes occur in middle-aged mice, prior to the onset of major declines in the amplitudes of otoacoustic emissions and elevations in ABR audiogram tone thresholds that are characteristic of old age.

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