Although estimates vary, some 40–50% of adults over the age of 65 years have some degree of hearing impairment, with this figure rising to 83% of those over the age of 70 years. These data make hearing loss the third most prevalent chronic medical condition among older adults, after arthritis and hypertension. Beyond the obvious difficulty understanding speech, researchers have come to realize that there are also hidden effects of hearing loss that may have significant consequences for brain function.

Successful perception of speech that is degraded by hearing loss can draw on cognitive resources that might otherwise be available for memory or for the comprehension of speech in everyday life. This type of ‘over effort listening’ is associated with increased stress responses, changes in pupil dilation, and poorer behavioral performance on memory tests. In addition, a striking finding from recent large-scale population studies has revealed a strong statistical connection between the appearance and degree of hearing loss and all-cause dementia.

It is thus possible that even a mild-to-moderate hearing loss can inflate the appearance of cognitive decline in the older adult. This sensory–cognitive interaction is a reminder that the auditory system may is a major pathway to the brain and in fact it is “the brain that hears”.