Electronic Thesis and Dissertation Repository

Thesis Format



Doctor of Philosophy


Health and Rehabilitation Sciences


Macpherson, Ewan A


In normally hearing listeners, binaural hearing relies on symmetrical input from the two ears. Previous studies on human listeners have assessed the effect of induced unilateral conductive hearing loss (UCHL) during adulthood via earplugging. These experiments indicated initial difficulty in sound localization followed by gradual improvement due to adaptation to the perturbed binaural cues. The limited duration of earplug use does not represent the consequences of chronic UCHL secondary to disease. Furthermore, there is insufficient information regarding the adaptation of binaural hearing abilities after treatment of late-onset UCHL.

The current study assessed the binaural hearing abilities of adult listeners with UCHL via behavioral and electrophysiological measurements. The dominant etiology of the hearing loss was otosclerosis. Furthermore, improving binaural hearing abilities following surgical treatment of UCHL caused by otosclerosis was monitored throughout one year post-surgery.

Before including the binaural interaction component (BIC) of the auditory brainstem responses as the electrophysiological measurement, its test-retest reliability was confirmed on normal-hearing listeners. The largest evoked potentials were collected using a midline electrode configuration, and the effect of inter-session interval was investigated.

Participants with UCHL showed elevated interaural time difference discrimination thresholds, but their near-normal sound localization ability suggested possible adaptation to the altered binaural cues. Spatial release from masking was lower than for a control group. Binaural loudness summation was inflated, and this abnormality was salient for those with moderate and moderately-severe UCHL. The brainstem BIC could not be detected in ten of eleven participants.

Measurements were repeated up to three times throughout one year after stapedotomy in patients who registered for corrective surgery. A gradual improvement in binaural hearing tasks after the surgery continued for 6 to 14 months. The objective measurements (BIC) did not show improvement throughout the study for the majority of the participants.

Overall, the results of this study suggest that UCHL-driven deficits in binaural hearing improved throughout a year after the corrective surgery. Some of these changes may last for a long time or remain unresolved, however. Counselling the patients regarding the consequences of UCHL and available intervention options, and creating reasonable expectations about the treatment will be promising.

Summary for Lay Audience

Access to sound with two normally functioning ears, known as binaural hearing, allows a listener to achieve the advantage of three-dimensional hearing, to perceive the location of sounds, and to identify sounds of interest among the background noise. However, asymmetrical auditory input due to hearing loss in one ear interferes with sufficient access to the binaural cues and degrades these abilities.

The current study is the first to assess the effects on binaural hearing of long-lasting hearing loss in one ear during adulthood that is caused by otosclerosis, which is a disease that obstructs sound transmission via abnormal growth of bony tissue in the middle ear. This assessment was conducted via behavioral and physiological measurements (auditory brainstem responses).

The results indicate that chronic asymmetrical hearing loss due to otosclerosis results in difficulty in sound localization, reduced speech intelligibility in noise, and abnormal loudness perception. These changes were more apparent in patients with a high degree of hearing loss. The results of the auditory brainstem response measurements were also consistent with abnormal processing of the binaural sounds that are received asymmetrically.

Following corrective middle-ear surgery, the measurements were repeated, and the results indicated that the recovery process for some binaural hearing abilities may take up to one year, and for some characteristics may remain unresolved. The auditory brainstem responses showed inconsistent improvement after surgery, in contrast with the test-retest reliability of this measurement in normal-hearing listeners.

Overall, the results of this study could be applied as a counselling tool for the patients who experience hearing loss in one ear, to inform them regarding the consequences of unilateral hearing loss, the intervention options to correct their asymmetrical hearing, and to provide a reasonable expectation about surgical treatment.