ABSTRACT: This research aims to optimize speech-understanding and spatial-hearing outcomes for cochlear- implant (CI) users, specifically bilateral (BI-CI) and single-sided-deafness (SSD-CI) users. The gap in knowledge is that it is unknown what causes contralateral disruption, how it affects speech understanding and binaural benefits, and how it might be remediated. The long-term goal of our research program is to optimize hearing performance for CI users with two inputs. The overall objective of this application is to determine the mechanisms underlying functional consequences of asymmetry, specifically through two phenomena that are manifestations of contralateral disruption: “bilateral speech interference” (speech presented in one ear impedes speech under- standing in the other, usually the better ear interfering with the poorer) and “interaural loudness mismatch” (bi- lateral stimuli that are equally loud when presented sequentially produce a distorted spatial map when presented simultaneously). Our overarching functional hypothesis is that asymmetric hearing, manifesting in contralateral disruption, is a major factor stunting real-world CI performance. Our overarching mechanistic hypothesis is that the two phenomena stem from asymmetric peripheral distortion, central auditory maladaptive plasticity, or both. We will test our overarching hypotheses through three specific aims: (1) Measure the consequences of contra- lateral disruption (bilateral speech interference and interaural loudness mismatch) from interaural asymmetry for increasingly realistic situations; (2) Determine the extent to which peripheral asymmetry explains contralateral disruption; (3) Determine if contralateral disruption is influenced by central auditory plasticity. These aims will yield the following expected outcomes: (1) identify the situations, prevalence, and magnitude where BI-CI and SSD-CI spatial-hearing outcomes are limited by contralateral disruption; (2) extend objective techniques—pre- viously applied to understand monaural CI performance—to characterize the link between peripheral neural sur- vival and binaural processing; (3) determine if abnormal across-ear processing is susceptible to central plasticity. This contribution is significant because contralateral disruption, viewed through the two asymmetry phenomena of bilateral speech interference and interaural loudness mismatch, is a major roadblock to good binaural hearing. Understanding the causes and negative consequences of contralateral disruption will provide tools to optimize rehabilitation for the 30-50% of BI-CI and nearly 100% of SSD-CI users with asymmetric hearing. The work is innovative because (1) it seeks to understand the roadblock in hearing outcomes based on asymmetry effects, rather than optimizing each ear individually, (2) it compares BI-CI and SSD-CI users to distinguish the mecha- nisms underlying contralateral disruption, and (3) it pairs perceptual and objective measures to differe...