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Hidden Hearing Loss
Studies performed in the last decade have provided increas- ing evidence of peripheral hearing deficits that are not re- vealed in the audiogram or through otoacoustic emissions testing (a measurement of the sounds generated by the hair cells in the inner ear; Dubno et al., 2013). This type of defi- cit is now referred to as “hidden hearing loss” (Schaette and McAlpine, 2011). Aging may lead to one type of hidden hearing loss, a disruption of synapses (connections) between inner hair cells and auditory neurons that carry signals to the brain. This form of hidden hearing loss has been termed cochlear synaptopathy. Evidence of age-related cochlear synaptopathy was found in a mouse model (Sergeyenko et al., 2013). These older mice had normal-hearing thresholds, but neural firing to sounds above the threshold was reduced.
Varying degrees of cochlear synaptopathy may accompany sensorineural hearing loss, which may lead to frustrations experienced by patients and/or audiologists if audiometric thresholds do not predict success with management through hearing aids or cochlear implants. It is hoped that future re- search will be successful in developing proxy measures of syn- aptopathy that can be reliably obtained in a clinical setting.
“Don’t Talk So Fast!”
The term “hidden hearing loss” may also be applied to pro- cessing deficits that affect the individual’s ability to process the temporal or frequency properties of speech stimuli. For example, aging appears to have pronounced effects on the ability of the auditory system to preserve the precise timing characteristics of speech. We use timing cues, such as vowel duration, to distinguish words that differ in voicing, which occurs when the vocal folds of the larynx or voice box vibrate as air passes from the lungs to the oral cavity. For example, the vowel in “wheat” preceding the final voiceless consonant /t/ is shorter than the vowel in “weed” preceding the final voiced consonant /d/. In everyday conversational speech, the final consonant is not sufficiently audible for listeners to make that perceptual judgment without the vowel duration cue. Older adults have reduced ability to identify words on the basis of these and other temporal cues compared with younger adults (Gordon-Salant et al., 2008). In other words, an older adult would require a longer vowel duration to perceive “weed” versus “wheat.” So, the next time you are speaking to your grandmother, try to slow down your rate of speech a bit to increase her ability to use these cues.
The perceptual consequences of disrupted temporal pro- cessing include a reduced ability to understand speech that is spoken rapidly or with an accent. As we age, we may find ourselves relying on open captions when watching many television shows or we may have difficulty understanding the younger relative who speaks rapidly. Decreased temporal processing may also affect the ability to understand speech in challenging listening environments, such as in background noise or in reverberant environments. These are the environ- ments in which hearing aids are the least effective but where older adults report their greatest communication problems. Asaresult,olderadultsbegintoavoidthesetroublesomelis- tening situations and may avoid using their hearing aids.
“Why Are You Shouting?”
When the listener does not understand what was said, the natural tendency is for the speaker to repeat him/herself at a higher level. But the listener may then complain that the speaker is shouting. Individuals with hearing loss may need speech to be spoken at 70 dB sound pressure level (SPL; higher than average conversational speech) to understand the same message that might be understood at 30-40 dB SPL by someone with normal hearing; yet, at 100 dB SPL, speech becomes equally loud for individuals with either hearing loss or normal hearing. Thus, a person with normal hear- ing will have a dynamic range (difference between threshold and maximum tolerable loudness levels) of approximately 100 dB, but the individual with sensorineural hearing loss may have a dynamic range of 50 dB or less. This reduced dynamic range may lead to problems when trying to pro- vide enough amplification to make soft sounds audible while limiting amplification for loud sounds so that they are not uncomfortably loud.
The loss of outer hair cells is one mechanism that may explain the reduced dynamic range observed in people with hearing loss. In the normal-hearing ear, the outer hair cells have abun- dant efferent connections that regulate the amount of ampli- fication applied to sounds (see Figure 2). When outer hair cells are lost, low-level signals are not detected and there is no amplification provided to the signal by the outer hair cells. As the signal level is increased, there is a spread of excitation to neighboring hair cells, which then triggers cochlear amplifi- cation of the signal, resulting in an abrupt perceived increase in loudness. This rapid growth in loudness can occur when the sound level is increased by only 10 or 20 dB.
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