Cochlear implantation with minimally traumatic surgical techniques and atraumatic electrode arrays has led to an increasing prevalence of adult and pediatric cochlear implant (CI) recipients with the potential for combined Electric and binaural Acoustic Stimulation (EAS). Many studies have demonstrated that adult EAS users consistently exhibit significant benefits for speech understanding in noise and spatial hearing tasks as compared to a CI paired only with a contralateral HA. Research has also demonstrated that sensitivity to interaural time difference (ITD) and interaural level difference (ILD) cues is correlated with EAS benefit for postlingually deafened adult listeners. Despite this active phase of discovery, there is still a striking paucity of research on EAS outcomes in pediatric CI users, the expected trajectory of benefit following EAS fitting, as well as underlying mechanisms driving EAS benefit (or lack thereof) in all populations. There is a disconnect between EAS availability and EAS utilization in all CI recipients and current audiological management of EAS candidates is not data driven. This is problematic given the protracted maturation of the binaural system and the fact that researchers do not understand what additional effects sensorineural hearing loss and combined EAS may have on the developing binaural system. The proposed research activities will describe the time course of binaural development using behavioral and objective responses to interaural differences in timing (phase) and level as well as allow for a natural factor investigation of a clinical intervention, the EAS fitting, on said development. Within the context of a clinical trial, the investigators will compare acute and chronic EAS outcomes for speech recognition and spatial hearing as related to binaural cue sensitivity, cue weightings and underlying neural synchrony necessary for ITD resolution. The investigators have proposed a single-group assignment for both adult and pediatric EAS users and will also include chronological- and hearing-age-matched listeners with normal hearing (NH). A within-subjects, accelerated longitudinal design for both EAS and NH listeners will provide insight into the developmental trajectory of the binaural system for children NH and will provide a benchmark for interpreting effects of sensorineural hearing loss, asymmetry in audibility resulting from cochlear implantation, and EAS use on said trajectory. Our proposed research activities will help close the gap between what is technologically possible with EAS technology and what is clinically implemented by otologists and audiologists investigating development of binaural sensitivity and spatial hearing abilities. The resultant data will comprise the first comprehensive description of behavioral and electrophysiologic measures of binaural hearing in adults and children both with NH and EAS and will uncover information about our EAS clinical populations holding high potential for clinical application in device fittings as well as audiologic and otologic clinical recommendations regarding cochlear implantation.

AIM 1: Emergence of binaural cue sensitivity in EAS users. The investigators will investigate emergence of binaural sensitivity for ITD and ILD cues as well as ITD/ILD cue weighting, which is critical for children with developing binaural function and underlying neural synchronization. Hypotheses: 1a) Pediatric EAS users will demonstrate an emergence of binaural cue sensitivity following a period of chronic EAS use incorporating bilateral LF acoustic amplification, 1b) there will be a relationship between listener age and ITD/ILD weighting for lateralization with the youngest EAS candidates assigning greater weight to ILDs due to immaturities in neural synchronization, and 1c) the time course of pediatric acoustic ITD/ILD development will be correlated with absolute and interaural auditory sensitivity for LF acoustic hearing.

AIM 2: Degree and time course of EAS benefit for speech and spatial discrimination informed by behavioral and objective estimates of binaural cue utilization. The investigators will describe EAS listener performance (CI+HAbilat) and EAS benefit (CI+HAbilat - CI+HAcontra) for (1) tasks of speech recognition in co-located, diffuse, and spatially separated noise, and (2) tasks of spatial discrimination. The investigators will relate these to ITD/ILD sensitivity measured both behaviorally (Aim 1) and electrophysiologically (Aim 2) via cortical auditory evoked potentials (CAEPs) reflecting brainstem-initiated processes inherited by the central auditory system. Hypotheses: 2a) EAS benefit will be observed for speech recognition and spatial discrimination with chronic EAS use, which will be mediated by access to LF ITD cues estimated via both behavioral and objective measures, and 2b) there will be a relationship between behavioral and objective measures of binaural cue utilization and EAS benefit for speech recognition and spatial hearing.