Bilateral Loudness Balancing and Distorted Spatial Perception in Recipients of Bilateral Cochlear Implants

Abstract

OBJECTIVE Determine if bilateral loudness balancing during mapping of bilateral cochlear implants (CIs) facilitates fused, punctate, and centered auditory images that facilitate lateralization with stimulation on single-electrode pairs. DESIGN Adopting procedures similar to those that are practiced clinically, we used direct stimulation to obtain most-comfortable levels (C levels) in recipients of bilateral Cis. Three pairs of electrodes, located in the base, middle, and apex of the electrode array, were tested. These electrode pairs were loudness-balanced by playing right-left electrode pairs sequentially. In experiment 1, we measured the location, number, and compactness of auditory images in 11 participants in a subjective fusion experiment. In experiment 2, we measured the location and number of the auditory images while imposing a range of interaural level differences (ILDs) in 13 participants in a lateralization experiment. Six of these participants repeated the mapping process and lateralization experiment over three separate days to determine the variability in the procedure. RESULTS In approximately 80% of instances, bilateral loudness balancing was achieved from relatively small adjustments to the C levels (≤3 clinical current units, CU). More important, however, was the observation that in four of eleven participants, simultaneous bilateral stimulation regularly elicited percepts that were not fused into a single auditory object. Across all participants, approximately 23% of percepts were not perceived as fused; this contrasts with the 1-2% incidence of diplacusis observed with normal-hearing individuals. In addition to the unfused images, the perceived location was often offset from the physical ILD. On the whole, only 45% of percepts presented with an ILD of 0 CU were perceived as fused and heard in the center of the head. Taken together, these results suggest that distortions to the spatial map remain common in bilateral CI recipients even after careful bilateral loudness balancing. CONCLUSION The primary conclusion from these experiments is that, even after bilateral loudness balancing, bilateral CI recipients still regularly perceive stimuli that are either unfused, offset from the assumed zero ILD, or both. Thus, while current clinical mapping procedures for bilateral CIs are sufficient to enable many of the benefits of bilateral hearing, they may not elicit percepts that are thought to be optimal for sound-source location. As a result, in the absence of new developments in signal processing for CIs, new mapping procedures may need to be developed for bilateral CI recipients to maximize the benefits of bilateral hearing.

Lateralization of Interaural Level Differences with Multiple Electrode Stimulation in Bilateral Cochlear-Implant Listeners

“…Non-centered 0-CU ILD images seem to result from place of stimulation mismatch (Kan et al 2013), although differential neural survival across the ears may explain such a phenomenon. Non-centered 0-CU ILD auditory images can occur even when the levels are carefully sequentially loudness balanced (Fitzgerald et al 2015). …”

Localization performance correlates with binaural fusion for interaurally mismatched vocoded speech

“…The concept of binaural fusion is often difficult to explain to participants, resulting in participant-dependent differences that may not reflect differences in binaural fusion. Additionally, some CI users may not perceive fused sounds with their processors (Fitzgerald et al, 2015) and thus not have a reference for what a fused percept sounds like, increasing the potential unreliability of the task for that population.…”

Determining the minimum number of electrodes that need to be pitch matched to accurately estimate pitch matches across the array

“…However, although bilateral CI users receive some binaural benefits, they do not receive the same benefits that normal hearing (NH) listeners do (Loizou et al, 2009; Poon et al, 2009; Aronoff et al, 2012). For example, CI users have more difficulty localizing sounds (Aronoff, Freed et al, 2012; Kerber & Seeber, 2012) and more difficulty fusing sounds from the two ears into a unitary coherent percept (Fitzgerald et al, 2015). …”