THE RELATIONSHIP BETWEEN AUDITORY ASYMMETRIES, INTERAU-RAL LOUDNESS MISMATCH, AND FREE-FIELD LOCALIZATION IN BILATERAL COCHLEAR-IMPLANT AND NORMAL-HEARING INDIVIDUALS.

Abstract

Binaural cues [interaural time differences (ITDs) and interaural level differences (ILDs)] are crucial for perceiving changes in spatial locations. In intracranial lateralization tasks, diotic stimuli (i.e., 0-ITD/ILD) are commonly assumed to be perceived as single, fused, and centered auditory images by normal-hearing (NH) individuals. Bilateral cochlear-implant (BICI) users often perceive bilater-ally loudness-balanced stimuli presented simultaneously as off-center, which is an interaural loud-ness mismatch (ILM). The reasons for ILM in BICI users remain unknown. In this dissertation, ILM was measured in BICI users with an intracranial ILD lateralization task. Additionally, it inves-tigated whether interaural neural health asymmetry, estimated from the electrically evoked com-pound action potential (ECAP) amplitude growth function (AGF) slope (i.e., a measure of neural response growth with increasing stimulation levels), explained ILM. In a second study, extensive control measurements in NH individuals were conducted to assess whether they exhibited ILM and whether this could be explained by peripheral interaural asymmetries, including behaviorally meas-ured hearing thresholds and loudness perception asymmetries. Moreover, the possible functional effects of ILM (i.e., increased horizontal localization bias) were investigated in both populations. We hypothesized that BICI users and NH individuals would demonstrate ILM. Additionally, we hypothesized that ILM would be positively correlated with interaural ECAP AGF asymmetry in BICI users and with interaural thresholds and/or loudness-perception asymmetries in NH individu-als. Finally, we hypothesized that ILM would be positively correlated with the magnitude of locali-zation bias in the free field. Our results showed that NH individuals exhibited significant lateraliza-tion biases that were not fully predicted by hearing thresholds, noise detection thresholds, or loud-ness perception asymmetries. Similarly, BICI users demonstrated ILM that was not predicted by the ECAP AGF slope asymmetry. Additionally, larger lateralization biases in NH individuals and ILM in BICI users were not correlated with a larger free-field localization error. The results of these experiments provide valuable data for understanding the effects of binaural asymmetry on binaural perception and spatial hearing in both NH individuals and BICI users.