Page 16 - Winter2018

Age-Related Hearing Loss
Another mechanism that may explain the reduced dynamic range is a disruption in the auditory system’s maintenance of a stable firing rate over a period of time. The maintenance of a steady internal environment is known as homeostasis. A change in the balance of excitatory and inhibitory neurotrans- mission is one homeostatic mechanism that is associated with aging and hearing loss (Caspary et al., 2013). Communication between two neurons occurs through neurotransmission; neurons are more likely to fire when they receive excitatory input and less likely to fire when they receive inhibitory input. One possible result of the loss of inhibitory input with aging or hearing loss is an increase in spontaneous neural firing and exaggerated responses to auditory stimuli.
Electrophysiological (electroencephalographic [EEG]) studies have documented exaggerated responses to sounds presented at conversational listening levels of about 65-70 dB SPL. The FFR shows exaggerated subcortical responses to the speech envelope (slowly varying amplitude variations in speech) in older adults with sensorineural hearing loss (Anderson et al., 2013b). This exaggeration of responses to auditory stimuli may be especially pronounced in the cortex. Magnetoenceph- alographic (MEG) responses (observed on recordings of mag- netic fields produced by electric currents in the brain) show overrepresentation of the speech envelope in older adults compared with younger adults (Presacco et al., 2016). Exag- gerated responses to the speech envelope may help to explain why older adults find hearing aid-amplified sound so over- whelming when they first start wearing hearing aids.
“Why Is Speech So Unclear?”
Older adults often report they can hear the talker, but they cannot understand what is being said. Speech understand- ing may be reduced by deficits in the auditory system’s abil- ity to represent the timing and frequency cues of speech. The typical presbyacusic hearing loss compromises audibility in the high frequencies to a greater extent than in the low fre- quencies (see Figure 1). Therefore, merely amplifying the overall level of sound results in excessive amplification in the low frequencies where hearing is relatively normal and in perception of lower frequency background noise. Modern hearing aids are able to selectively amplify specific frequen- cies, within the limitations of the hearing aid microphone and circuitry. However, frequency selectivity (ability to de- tect differences in frequency) is often decreased in individu- als with sensorineural hearing loss compared with individu- als with normal hearing regardless of stimulus presentation level (Florentine et al., 1980). Therefore, the hearing aid user
may not achieve maximum benefit from selective amplifica- tion of specific frequency channels.
The auditory system is organized tonotopically from the co- chlea to the cortex; that is, low-to-high frequencies are rep- resented in spatial order. For example, the cochlea is maxi- mally responsive to high frequencies at the basal end (near the middle ear) and maximally responsive to low frequen- cies at the apical end (top of the cochlear spiral). This spatial organization is preserved throughout the auditory system. Hearing loss, however, may alter the tonotopic organization of central auditory structures.
For example, the C57 mouse model is used to study hear- ing loss effects because these mice commonly experience sensorineural hearing loss relatively early in the adult life span. C57 mice show disrupted tonotopic organization in the inferior colliculus, the auditory region of the midbrain, such that neurons that normally fire best to high-frequency sounds begin to respond more to low frequencies (Willott, 1991). Tonotopic changes may also occur in the auditory cortex of the brain. For example, when excessive noise dam- ages hair cells in specific frequency regions in the cochlea (e.g., 3-6 kHz), stimulation with signals at these frequencies does not produce a response in cortical neurons in corre- sponding frequency regions but instead produces a response in neurons from adjacent cortical regions (Engineer et al., 2011).
Because of changes in frequency selectivity and tonotopicity, selective amplification of specific frequencies will not com- pletely compensate for a decreased ability to discriminate between speech sounds based on subtle frequency differenc- es. For example, the consonant /g/ has higher frequency en- ergy than the consonant /d/. Although the two consonants differ in their place of articulation in the vocal tract, the place differences are not visible to the listener from viewing the talker’s lips, and, therefore, the listener with hearing loss may have difficulty discriminating between words like “gust” and “dust” on the basis of frequency differences alone.
“Why Do I Still Have Trouble Under- standing Speech with My Hearing Aids?” Let us assume that your grandmother has been fit with hear- ing aids after being diagnosed with a mild-to-moderate hear- ing loss. You have been looking forward to the next family gathering, and you are hoping she participates more in the conversation. Your grandmother certainly seems more en- gaged, and yet she is still asking others to repeat what was
14 | Acoustics Today | Winter 2018