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(Lonsbury-Martin et al., 1990, 1991; Stover and Norton, 1993; Tadros et al., 2005).

EFFERENT FEEDBACK SYSTEM: CONTRALATERAL SUPPRESSION OF OTOACOUSTIC EMISSIONS

The ascending auditory system (afferent) processes information from the cochlea as this information travels from the inner ear to the brainstem, and then to the higher centers of the brain including the medial geniculate body of the thalamus, and on to the auditory cortex where perception of sounds takes place. Along with this ascending portion of the system, there are nerve cells in the brain that can also send information from the brain back to the inner ear. This descending part of the auditory system, sometimes referred to as the efferent system, can modulate the auditory information processed in the cochlea. For example, in the presence of loud sounds or background noise, the outputs of the cochlea can be reduced when the nerve cells of the efferent system become active.

We can measure the strength of the auditory efferent system by recording the amplitudes of otoacoustic emissions, both in the presence or absence of sounds or noise presented to the opposite (contralateral) ear. When the amplitudes of otoacoustic emissions are reduced in one ear (ipsilateral ear) due to the presence of sound stimulation in the other ear (contralateral ear), the efferent system has been activated. The greater the decrease in the amplitude of these emissions, the greater the strength and health of the descending auditory efferent system. Since the health and abilities of the cochlea for processing important sounds such as speech depend partly on the efficacy of the auditory efferent system, measuring the operation of this system with age is useful and important. For example, for human subjects, Kim et al. (2002) discovered that the auditory efferent system begins to lose its capabilities in middle age, and is almost completely inoperative in most old subjects (see Figure 76.3).

They discovered this by testing human subjects (young adult, middle age, and old) who had audiometric thresholds in the normal hearing range on standard hearing tests as well as DPOAEs in quiet and in the presence of contralateral noise. This is a case where the inner ears of these subjects had normal sensitivity (audiograms in the normal range), but there was clearly an age-related problem in the central auditory system, at least for those portions of the brain associated with the efferent system. Taken together, otoacoustic emissions along with the contralateral suppression (CS) of otoacous-tic emissions provide information on the operating status of the ear-peripheral auditory system, and the brain-auditory feedback system, respectively.

TIMING (TEMPORAL) AND FREQUENCY (SPECTRAL) PROCESSING: GAP DETECTION, AMPLITUDE MODULATION

For human subjects, auditory psychophysics (psychoa-coustics) can provide perceptual information about the system as a whole. Psychoacousticians make careful o O

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