I'm fortunate to have stayed close to my wonderful PhD supervisor, Art Wingfield. A couple of years ago Art and I hosted a Frontiers research topic on how hearing loss affects neural processing. One of our goals was to follow the effects from the periphery (i.e. effects in the cochlea) through higher-level cognitive function.
We've now written a review article that covers these topics (Peelle and Wingfield, 2016). Our theme is one Art has come back to over the years: given the numerous age-related declines in both hearing and cognition, we might expect speech comprehension to be relatively poor in older adults. The fact that it is, in fact, generally quite good speaks to the flexibility of the auditory system and compensatory cognitive and neural mechanisms.
A few highlights:
Hearing impairment affects neural function at every level of the ascending auditory system, from the cochlea to primary auditory cortex. Although frequently demonstrated using noise induced hearing loss, many of the same effects are seen for age-related hearing impairment.
Functional brain imaging in humans routinely shows that when speech is acoustically degraded, listeners engage more regions outside the core speech network, suggesting this activation may play a compensatory role in making up for the reduced acoustic information. (An important caveat is that task effects have to be considered).
Moving forward, an important effort will be understanding how individual differences in both hearing and cognitive abilities affect the brain networks listened use to process spoken language.
We had fun writing this paper, and hope it's a useful resource!