Abstract
Speech comprehension abilities decline with age and with age-related hearing loss, but it is unclear how this decline expresses in terms of central neural mechanisms. The current study examined neural speech processing in a group of older adults (aged 56–77, n = 16, with varying degrees of sensorineural hearing loss), and compared them to a cohort of young adults (aged 22–31, n = 30, self-reported normal hearing). In a functional MRI experiment, listeners heard and repeated back degraded sentences (4-band vocoded, where the temporal envelope of the acoustic signal is preserved, while the spectral information is substantially degraded). Behaviorally, older adults adapted to degraded speech at the same rate as young listeners, although their overall comprehension of degraded speech was lower. Neurally, both older and young adults relied on the left anterior insula for degraded more than clear speech perception. However, anterior insula engagement in older adults was dependent on hearing acuity. Young adults additionally employed the anterior cingulate cortex (ACC). Interestingly, this age group × degradation interaction was driven by a reduced dynamic range in older adults who displayed elevated levels of ACC activity for both degraded and clear speech, consistent with a persistent upregulation in cognitive control irrespective of task difficulty. For correct speech comprehension, older adults relied on the middle frontal gyrus in addition to a core speech comprehension network recruited by younger adults suggestive of a compensatory mechanism. Taken together, the results indicate that older adults increasingly recruit cognitive control networks, even under optimal listening conditions, at the expense of these systems’ dynamic range.
Highlights
Speech comprehension can become difficult with age and agerelated hearing loss, especially when listening conditions are challenging
In a previous short-term adaptation study in a cohort of young adults (Erb et al, 2013), we have shown that degraded speech processing elicits an increased blood oxygenation leveldependent (BOLD) response in an “executive” network (Eckert et al, 2009) comprising the anterior insula and anterior cingulate cortex (ACC)
Degraded speech processing To reveal regions that are engaged in degraded speech processing, we compared degraded with clear speech trials
Summary
Speech comprehension can become difficult with age and agerelated hearing loss, especially when listening conditions are challenging. Normal-hearing young adults have the capacity to rapidly adapt to degraded speech (Davis et al, 2005; Samuel and Kraljic, 2009; Eisner et al, 2010; Erb et al, 2013). Such short-term perceptual adaptation is not well established in older adults, it bears particular relevance as older adults are frequently affected by hearing loss. In the current functional MRI experiment, we compare these results to a group of older adults with varying degrees of hearing loss to test: (1) whether older listeners are able to (behaviorally) adapt to spectrally severely degraded (“noise-vocoded”) speech at a rate comparable to young listeners and (2) how neural processing of degraded speech differs between young and older adults
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