The role of cortical oscillations during speech processing in older adults with hearing loss

Abstract

Age-related hearing loss is not only a disorder of the inner ear, but also of the brain, which has to complement the degraded speech input while also incurring structural and functional declines. This ‘central’ hearing loss may therefore manifest itself in various aspects of the brain, such as integrity of its structure, but also in its functional properties. For example, aging is associated with a change in absolute and relative oscillatory power in different frequency bands of the human electroencephalogram. These frequency bands have consistently been found to be related to speech comprehension. Specifically, oscillations in the theta frequency band (4-8 Hz) have been found to encode slowly changing acoustic speech cues like prosody, and oscillations in the alpha frequency band (8-12 Hz) have been found to be related to inhibition of irrelevant stimulus information. In the current study, while their electroencephalogram from 128 electrodes was recorded, N = 24 older adults with varying degrees of sensorineural hearing loss listened to an audiobook in four different background noise conditions, embedded in a 2-by-2 design: The number of background talkers as well as the signal-to-noise ratio were varied. We tested to what extent the modulation of theta oscillations as a proxy for older adults’ neuronal capacity to process prosodic cues would predict speech understanding in these challenging listening environments. We also tested the suitability of alpha oscillations as a proxy for the ability to inhibit irrelevant stimulus information. Results are discussed with respect to present theoretical accounts of the aging brain and the function of cortical oscillations during speech processing.