Adult aging and the perception of alternated speech

In this dissertation I explore speech comprehension, focusing specifically on speech comprehension in older adults. Chapter 1 provides a general introduction to topics relevant to speech comprehension and aging. In Chapter 2, I examine the effects of syntactic complexity on young and older adult listeners' speech comprehension. As syntactic complexity of sentences is increased using embedded clauses, older adult listeners require differentially louder presentation volumes to achieve the same accuracy levels as young adult listeners. Older adults with poorer hearing acuity need further increases in presentation volume to reach equivalent accuracy levels.;In Chapter 3, I use alternated speech stimuli to investigate older adults' ability to comprehend speech under a condition where cognitive resources are strained. Older adults' intelligibility score profiles are similarly shaped to young adults when alternated speech is presented at normal speech rate, with young adults having higher intelligibility scores at all tested alternation rates. The age-related gap in intelligibility scores is decreased when older adults are given additional time for processing, by segmenting alternated speech at syntactically relevant boundaries. When speech rate is increased, older adults' intelligibility profiles for alternated speech match those for young adults until high alternation rates and speech rates combine to overwhelm older adults' processing systems.;In Chapter 4, I manipulate the amount of speech content per ear in an alternated speech perception task. Older adult listeners' demonstrate a differentially larger increase in intelligibility than young adult listeners as the amount of speech content is increased to one ear. This finding suggests that older adult listeners are employing a compensatory mechanism for alternated speech comprehension, relying on top-down processing to counteract declines in hearing acuity and processing speed.;In Chapter 5, I present the results of a study examining the effect of hearing acuity and contextual constraint on older adult listeners' accuracy in a running memory task. When older adults' hearing acuity is poorer, accuracy for non-final words is lower than older adults with better hearing, despite the two groups performing equally well on tests of accuracy for final words. This finding supports the effortfulness hypothesis, which proposes that the extra effort that a hearing-impaired listener must expend to achieve perceptual success comes at the cost of processing resources that might otherwise be available for encoding the speech content in memory.;Overall, these studies reflect the dynamic nature of speech comprehension with age. Despite natural physiological and cognitive declines in brain regions necessary for speech comprehension, speech comprehension is well-conserved, as both internal (increased reliance on top-down processing) and external (e.g., additional amplification of target speech) compensation allows older adults to maintain functional, if not optimal, performance.