Page 15 - Winter Issue 2018
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Hidden Hearing Loss The perceptual consequences of disrupted temporal pro-
Studies performed in the last decade have provided increas- cessing include a reduced ability to understand speech that
ing evidence of peripheral hearing deficits that are not re- is spoken rapidly or with an accent. As we age, we may find
vealed in the audiogram or through otoacoustic emissions ourselves relying on open captions when watching many
testing (a measurement of the sounds generated by the hair television shows or we may have difficulty understanding
cells in the inner ear; Dubno et al., 2013). This type of defi- the younger relative who speaks rapidly. Decreased temporal
cit is now referred to as “hidden hearing loss” (Schaette and processing may also affect the ability to understand speech in
McAlpine, 2011). Aging may lead to one type of hidden challenging listening environments, such as in background
hearing loss, a disruption of synapses (connections) between noise or in reverberant environments. These are the environ-
inner hair cells and auditory neurons that carry signals to ments in which hearing aids are the least effective but where
the brain. This form of hidden hearing loss has been termed older adults report their greatest communication problems.
cochlear synaptopathy. Evidence of age—related cochlear As a result, older adults begin to avoid these troublesome lis-
synaptopathy was found in a mouse model (Sergeyenko et tening situations and may avoid using their hearing aids.
al., 2013). These older mice had normal—hearing thresholds,
but neural firing to sounds above the threshold was reduced. "\I\Ihy Are You Shouting?"
V . d f hl When the listener does not understand what was said, the
arying egrees o coc ear synaptopathy may accompany . _
sensorineural hearing loss, which may lead to frustrations llatllla_l tendency ls fol the? Speaker to repeat hlm/llelself
experienced by patients and/or audiologists if audiometric at a lllgllel level‘ Bllt the llslellel max then cllmplaln that
thresholds do not predict success with management through the Speaker ls Sllolltlllg lmllvlduals wltll llearlllg loss may
hearing aids or cochlear implants It is hoped that future re_ need speech to be spoken at 70 dB sound pressure level (SPL;
Search will be Successful in developing proxy measures of SYn_ higher than average conversational speech) to understand
aptopathy that can be reliably obtained in a clinic 31 Setting the same message that might be understood at 30-40 dB SPL
by someone with normal hearing; yet, at 100 dB SPL, speech
..D°n,t Talk so Fast!” becomes equally loud for individuals with either hearing
The term “hidden hearing loss” may also be applied to pro- loss °_l normal lleallllg Thus’ a_ Pelsoll wltll normal heal"
cessing deficits that affect the individual’s ability to process mg wlll llave a “lyllamlc lallge (dlffelellce between tlllesllold
the temporal or frequency properties of speech stimuli. For and maxlmum tlllefallle lolllllless levels) of aPPl°x_lmately
example’ aging appears to have Pronounced effects on the 100 dB, but the individual with sensorineural hearing loss
ability of the auditory system to preserve the precise timing may llllve a “lyllamlc lallge of 50 dB or less‘ Tllls reduced
characteristics of speech. We use timing cues, such as vowel “l_Yllllmlc range m_llY leflll to problems when tlylllg to Pl_°'
duration, to distinguish words that differ in voicing, which vld“ ‘Enough alllpllllclltloll to make Soft Sounds alldlllle wlllle
occurs when the vocal folds of the larynx or voice box vibrate llmltlllg ampllllclltloll fol loud Sounds so that they ale not
as air passes from the lungs to the oral cavity. For example, uncomfortably loud‘
the vowel in “wheat” preceding the final voiceless consonant The loss of outer hair cells is one mechanism that may explain
/t/ is shorter than the vowel in “weed” preceding the final the reduced dynamic range observed in people with hearing
voiced consonant /d/. In everyday conversational speech, loss. In the normal—hearing ear, the outer hair cells have abun-
the final consonant is not sufficiently audible for listeners to dant efferent connections that regulate the amount of ampli-
make that perceptual judgment without the vowel duration fication applied to sounds (see Figure 2). W'hen outer hair
cue. Older adults have reduced ability to identify words on cells are lost, low-level signals are not detected and there is no
the basis of these and other temporal cues compared with amplification provided to the signal by the outer hair cells. As
younger adults (Gordon-Salant et al., 2008). In other words, the signal level is increased, there is a spread of excitation to
an older adult would require a longer vowel duration to neighboring hair cells, which then triggers cochlear amplifi-
perceive “weed” versus “wheat.” So, the next time you are cation of the signal, resulting in an abrupt perceived increase
speaking to your grandmother, try to slow down your rate of in loudness. This rapid growth in loudness can occur when
speech a bit to increase her ability to use these cues. the sound level is increased by only 10 or 20 dB.
Winter 2018 | Acnustics Thday | ‘I5

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