Title: The Relationship Between Auditory Asymmetries, Interaural Loudness Mismatch, and Free‑Field Localization in Bilateral Cochlear‑Implant and Normal‑Hearing Individuals

Speaker: Obada J. AlQasem

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 bilaterally loudness-balanced stimuli presented simultaneously as off-centered owing to interaural loudness mismatch (ILM). The reasons for ILM in BICI users remain unknown. This dissertation aims to measure ILM in BICI users through an intracranial ILD lateralization task. Additionally, it investigates whether interaural neural health asymmetry, estimated from the electrically evoked compound action potential (ECAP) amplitude growth function (AGF) slope (i.e., a measure of neural response growth with increasing stimulation levels), could explain the ILM. In addition, it aims to conduct extensive control measurements in NH individuals to assess whether they exhibit ILM and whether this can be explained by peripheral interaural asymmetries, including behaviorally measured hearing thresholds and loudness perception asymmetries. Moreover, it aims to investigate the possible functional effects of ILM (i.e., increased horizontal localization bias) 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 individuals. Moreover, we hypothesized that ILM would positively correlate with the magnitude of localization bias in the free field. The results of these experiments provide valuable data needed to understand the effects of binaural asymmetries on binaural perception and spatial hearing in both NH individuals and BICI users.