Page 16 - Winter Issue 2018
P. 16

Age-Related Hearing Loss
Another mechanism that may explain the reduced dynamic may not achieve maximum benefit from selective amplifica-
range is a disruption in the auditory system’s maintenance tion of specific frequency channels.
of a Stable firing rate 0v_er a periofi of time' The maimenfmce The auditory system is organized tonotopically from the co-
of a steady internal environment is known as homeostasis. A . . .

_ _ _ _ _ chlea to the cortex; that is, low-to-high frequencies are rep-
change in the balance of excitatory and inhibitory neurotrans- . . . .

_ _ _ _ _ _ _ _ resented in spatial order. For example, the cochlea is maxi-
mission is one homeostatic mechanism that is associated with many responsive to high frequencies at the basal end (near
aging and hearing loss (Caspary et al., 2013). Communication the middle ear) and maximally responsive to low frequem
between two neurons occurs through neurotransmission; . . . . .

_ _ _ cies at the apical end (top of the cochlear spiral). This spatial
neurons are more likely to fire when they receive excitatory . . . .
_ _ _ _ _ _ _ organization is preserved throughout the auditory system.
input and less likely to fire when they receive inhibitory input. Hearing loss) however) may alter the tonotopic Organization
One possible result of the loss of inhibitory input with aging of central auditory Structures
or hearing loss is an increase in spontaneous neural firing and I
exaggerated responses to auditory stimuli_ For example, the C57 mouse model is used to study hear-
’ 1 ff b th ‘ 1 ‘
Electrophysiological (electroencephalographic [EEG]) studies mg Oés e eds efause ese n,uce Commfm Y exPenen,ce
sensorineural hearing loss relatively early in the adult life
have documented exaggerated responses to sounds presented , _ _ _ , _
_ , _ span. C57 mice show disrupted tonotopic organization in
at conversational listemng levels of about 65-70 dB SPL. The , _ _ _ _ , ,
, the inferior colliculus, the auditory region of the midbrain,
FFR shows exaggerated subcortical responses to the speech _
_ , , _ _ , such that neurons that normally fire best to high-frequency
envelope (slowly varying amplitude variations in speech) in , _ ,
_ _ , sounds begin to respond more to low frequencies (Willott,
older adults with sensorineural hearing loss (Anderson et al., _ , ,
_ _ , , _ 1991). Tonotopic changes may also occur in the auditory
20131)) This exaggeratmn of responses to audltory Snmuh cortex of the brain For exam le when excessive noise dam
may be especially pronounced in the cortex. Magnetoenceph- , _ ' , P ’ _ ,
_ , ages hair cells in specific frequency regions in the cochlea
alographic (MEG) responses (observed on recordings of mag- _ , , , _
_ _ _ _ (e.g., 3-6 kHz), stimulation with signals at these frequencies
netic fields produced by electric currents in the brain) show , , _
_ , does not produce a response in cortical neurons in corre-
overrepresentation of the speech envelope in older adults _ _ ,
, sponding frequency regions but instead produces a response
compared with younger adults (Presacco et al., 2016). Exag- , , _ , _
, in neurons from ad]acent cortical regions (Engineer et al.,
gerated responses to the speech envelope may help to explain 201 1)
why older adults find hearing aid-amplified sound so over- '
whelming when they first start wearing hearing aids. Because of changes in frequency selectivity and tonotopicity,
selective amplification of specific frequencies will not com-
"V\Ihy ls Speech 50 Unclear?" pletely compensate for a decreased ability to discriminate
Older adults often report they can hear the talker, but they between speech sounds based on subtle frequency differenc-
cannot understand what is being said. Speech understand- es. For example, the consonant /g/ has higher frequency en-
ing may be reduced by deficits in the auditory system’s abil- ergy than the consonant /d/. Although the two consonants
ity to represent the timing and frequency cues of speech. The differ in their place of articulation in the vocal tract, the
typical presbyacusic hearing loss compromises audibility in place differences are not visible to the listener from viewing
the high frequencies to a greater extent than in the low fre- the talker’s lips, and, therefore, the listener with hearing loss
quencies (see Figure 1). Therefore, merely amplifying the may have difficulty discriminating between words like “gust”
overall level of sound results in excessive amplification in and “dust” on the basis of frequency differences alone.
the low frequencies where hearing is relatively normal and
in perception of lower frequency background noise. Modern "\I\Ihy Do I Still Have 11»-auble Under-
hearing aids are able to selectively amplify specific frequen- standing Speech with |V|y Hearing Aids?"
cies, within the limitations of the hearing aid microphone Let us assume that your grandmother has been fit with hear.
and CifC1li'El‘Y- HOWBVBI‘. frequency S€1€CtiVi’fY (ability ‘E0 de- ing aids after being diagnosed with a mild-to—moderate hear-
tect differences in frequency) is often decreased in individu- ing loss. You have been looking forward to the next family
als with sensorineural hearing loss compared with individu- gathering, and you are hoping she Participates more in the
als With normal hearing regardless Of stimulus presentation conversation. Your grandmother certainly seems more en-
level (Florentine et al., 1980). Therefore, the hearing aid user gaged, and yet she is still asking others to repeat what was
14 | Acuuseics Thday | Winter 2018

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