How Does Experience Shape Binaural Cue Interaction In Bionic Hearing?

Background: It is well established that severely deaf patients who had their hearing restored with bilateral cochlear implants (biCIs) have an impaired ability to sense and exploit binaural cues, so called interaural time differences (ITDs) and interaural leveldifferences (ILDs) to localize sound sources in space or to make sense of complex acoustic environments in which multiple sound sources are active at once. Normally hearing (NH) listeners are highly sensitive to both ILDs and ITDs, and sub-consciouslycombine both cues to arrive at a unified percept of sound source direction. In contrast, biCI patients show a much reduced sensitivity to ITDs, and must therefore base their direction judgments on ILDs alone. This implies that biCI patients must lose a naturalability to use a key binaural cue, but what causes this loss of function, how it emerges and evolves and to what extent it might be reversible with better prosthetic treatment strategies remains unknown.Objectives: This study aims to investigate the "maladaptive plasticity hypothesis", which states that poor binaural hearing in cochlear implant (CI) patients is due largely to inappropriate, uninformative interaural time differences (ITDs) in the pulses deliveredto the patients' ears. We will test this hypothesis in psychoacoustics and electrophysiological experiments on cohorts of neonatally deafened laboratory rats fitted with CIs in young adulthood.Hypotheses: Current clinical CI sound processors will deliver stimuli in which random, uninformative ITD values conflict with useful interaural level difference (ILD) cues. We hypothesize that this conflict leads to a degeneration of ITD sensitivity over time. Wepropose to test this hypothesis by monitoring ITD and ILD sensitivity, and their interaction, over time, through a series of time-intensity-trading experiments. Design and Subjects: Two cohorts of neonatally deafened rats will be trained in astimulus lateralization task. One will receive informative ITDs that are congruent with ILDs, the other will receive stimuli with informative ILDs but random, conflicting ITDs. Behavioral sensitivity will be measured in both cohorts over time, and at the endphysiological ITD and ILD tuning will be measured in the animals' midbrains.Expected Results: We predict that the ITD sensitivity in the random ITD cohort will deteriorate gradually compared to that seen in the informative ITD cohort. Observing the time course of this can inform better future treatment strategies.