Effects of age and hearing loss on amplitude modulation processing and perception

On a daily basis, humans are exposed to complex sound waves that exhibit variations over time. These fluctuations, known as envelope amplitude modulations (AMs), convey acoustic information to which the human auditory system demonstrates remarkable sensitivity. Previous research has indicated that both hearing loss and age can have adverse effects on AM perception. These detriments may contribute to the difficulties in auditory perception frequently experienced by older listeners, whether or not they have a hearing impairment. Therefore, a detailed characterization of potential deficits in temporal envelope perception related to age and hearing loss could offer valuable insights into the root causes of these challenges.
This thesis investigated the effects of age and hearing loss on AM perception and explored their potential consequences for speech intelligibility. Four studies focused on AM frequency selectivity, a central aspect of temporal processing and perception in the auditory system often conceptualized as a bank of modulation filters through which envelope fluctuations are processed. In the first study, behavioral AM frequency selectivity in young and older listeners with normal hearing was examined. The results indicated that older listeners exhibited a decline in AM selectivity by a factor of approximately two compared to their young counterparts. The second study investigated AM frequency selectivity in young and older listeners with hearing impairment. Listeners with hearing impairment generally demonstrated better AM frequency selectivity than their similarly aged counterparts with normal hearing. Within the group with hearing impairment, young listeners also exhibited greater AM frequency selectivity compared to older listeners with similar hearing profiles. These findings suggested opposite effects of age and hearing loss on AM frequency selectivity, with age being detrimental and hearing loss being associated with perceptual benefits. It is proposed that the observed improvement with hearing loss may be linked to an increased internal representation of AM signals resulting from a loss of cochlear compression. The third study explored how age- and hearing loss-related effects on AM frequency selectivity may contribute to difficulties in speech intelligibility in noisy environments by influencing the amount of AM masking. The results suggested that an age-related loss of AM selectivity may disrupt speech intelligibility, whereas the benefit in AM perception due to hearing loss may not enhance speech intelligibility. However, due to limited group sample sizes, no direct correlational relationship between measures of AM frequency selectivity and speech intelligibility could be established in this work. Finally, the fourth study aimed to further characterize the impact of an age-related reduction in AM frequency selectivity on speech intelligibility. The study introduced a paradigm manipulating the modulation masking of speech using tone-vocoded stimuli, restricting the tests to envelope-based cues. Although significance was not reached, trends in the data were broadly consistent with a detrimental effect of a loss of AM selectivity on speech intelligibility, warranting further investigations.
Overall, this thesis presents evidence of distinct and opposing suprathreshold effects of age and hearing loss on temporal envelope processing and perception. It proposes that these perceptual deficits might contribute to the considerable difficulties in speech recognition experienced by older listeners. These findings provide a valuable basis for delving deeper into the root causes of challenges in speech intelligibility and for formulating novel compensation strategies.