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SPATIAL HEARING AND TEMPORAL PROCESSING IN OLD AND HEARING-IMPAIRED INDIVIDUALS
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
Small timing differences occur when sounds reach one ear before the other, creating interaural phase differences (IPDs). The phase-locked activity in the auditory nerve can, at low frequencies, preserve IPDs. IPDs are used for localising and separating sounds from different directions. Chapters 3, 5, and 6 report three studies of the independent effects of age and sensorineural hearing loss on the temporal processing of sound that aids spatial hearing. Chapters 2 and 4 describe two supporting methodological studies. Chapter 2 compared the duration of training required for stable IPD-discrimination thresholds for two stimulus presentation procedures. The procedure requiring the least training was adopted for subsequent studies. Age and hearing loss are related and both may affect sensitivity to IPDs. Chapter 3 demonstrated that hearing loss, regardless of listener age, is related to poorer sensitivity to IPDs in the temporal fine structure (TFS), but not in the temporal envelope. Chapter 3 also showed that age, independent of hearing loss, is related to poorer envelope-IPD sensitivity at low modulation rates, and somewhat poorer TFS-IPD sensitivity. In Chapter 5, listener age and IPD sensitivity were both compared to subcortical neural phase locking measured through the frequency-following response (FFR). Phase coherence in the envelope-FFR at 145 Hz modulation and in the TFS-FFR deteriorated with age, suggesting less precise phase locking in old age. However, age-related changes to IPD sensitivity were not strongly related to age-related changes in FFR phase coherence. IPD sensitivity declines may be predominantly caused by deterioration of binaural processing independent of subcortical phase locking. Chapter 4 showed that electrodes at the mastoids recorded TFS-FFR generated earlier in the auditory pathway than electrodes from the nape of the neck to forehead, which recorded FFR generated later in the brainstem. However, these electrode montages did not reveal different age- or hearing-loss-related FFR deficits in Chapter 5. Chapter 6 determined whether hearing loss affected the ability to use TFS IPDs to achieve better speech perception. On average, old hearing-impaired listeners gained a small, but significant, benefit from a lateral separation of the speech sources. Replacing the TFS with binaurally in-phase sine waves (removing the TFS IPDs) significantly reduced the benefit of lateral separation. How much a listener benefitted from intact TFS IPDs in speech perception was strongly related to the extent of their hearing loss at low frequencies and their monaural processing of TFS, but not to their ability to discriminate IPDs. In general, this thesis shows that low-frequency hearing loss is associated with poor sensitivity to TFS IPDs and the ability to benefit from them when sounds are laterally separated. The thesis also shows that old age can reduce sensitivity to IPDs and weaken subcortical temporal coding. Although only partly related, these effects are likely to cause problems for old individuals in challenging listening environments.