Head Shadow, Summation, and Squelch in Bilateral Cochlear-Implant Users With Linked Automatic Gain Controls

Review of Binaural Processing With Asymmetrical Hearing Outcomes in Patients With Bilateral Cochlear Implants

Bilateral cochlear implants (BiCIs) result in several benefits, including improvements in speech understanding in noise and sound source localization. However, the benefit bilateral implants provide among recipients varies considerably across individuals. Here we consider one of the reasons for this variability: difference in hearing function between the two ears, that is, interaural asymmetry. Thus far, investigations of interaural asymmetry have been highly specialized within various research areas. The goal of this review is to integrate these studies in one place, motivating future research in the area of interaural asymmetry. We first consider bottom-up processing, where binaural cues are represented using excitation-inhibition of signals from the left ear and right ear, varying with the location of the sound in space, and represented by the lateral superior olive in the auditory brainstem. We then consider top-down processing via predictive coding, which assumes that perception stems from expectations based on context and prior sensory experience, represented by cascading series of cortical circuits. An internal, perceptual model is maintained and updated in light of incoming sensory input. Together, we hope that this amalgamation of physiological, behavioral, and modeling studies will help bridge gaps in the field of binaural hearing and promote a clearer understanding of the implications of interaural asymmetry for future research on optimal patient interventions.

Cochlear implantation in unilateral hearing loss: impact of short- to medium-term auditory deprivation

Introduction

Single sided deafness (SSD) results in profound cortical reorganization that presents clinically with a significant impact on sound localization and speech comprehension. Cochlear implantation (CI) has been approved for two manufacturers' devices in the United States to restore bilateral function in SSD patients with up to 10 years of auditory deprivation. However, there is great variability in auditory performance and it remains unclear how auditory deprivation affects CI benefits within this 10-year window. This prospective study explores how measured auditory performance relates to real-world experience and device use in a cohort of SSD-CI subjects who have between 0 and 10 years of auditory deprivation.

Methods

Subjects were assessed before implantation and 3-, 6-, and 12-months post-CI activation via Consonant-Nucleus-Consonant (CNC) word recognition and Arizona Biomedical Institute (AzBio) sentence recognition in varying spatial speech and noise presentations that simulate head shadow, squelch, and summation effects (S0N0, SSSDNNH, SNHNSSD; 0 = front, SSD = impacted ear, NH = normal hearing ear). Patient-centered assessments were performed using Tinnitus Handicap Inventory (THI), Spatial Hearing Questionnaire (SHQ), and Health Utility Index Mark 3 (HUI3). Device use data was acquired from manufacturer software. Further subgroup analysis was performed on data stratified by <5 years and 5-10 years duration of deafness.

Results

In the SSD ear, median (IQR) CNC word scores pre-implant and at 3-, 6-, and 12-months post-implant were 0% (0-0%), 24% (8-44%), 28% (4-44%), and 18% (7-33%), respectively. At 6 months post-activation, AzBio scores in S0N0 and SSSDNNH configurations (n = 25) demonstrated statistically significant increases in performance by 5% (p = 0.03) and 20% (p = 0.005), respectively. The median HUI3 score was 0.56 pre-implant, lower than scores for common conditions such as anxiety (0.68) and diabetes (0.77), and comparable to stroke (0.58). Scores improved to 0.83 (0.71-0.91) by 3  months post-activation. These audiologic and subjective benefits were observed even in patients with longer durations of deafness.

Discussion

By merging CI-associated changes in objective and patient-centered measures of auditory function, our findings implicate central mechanisms of auditory compensation and adaptation critical in auditory performance after SSD-CI and quantify the extent to which they affect the real-world experience reported by individuals.

Importance of ipsilateral residual hearing for spatial hearing by bimodal cochlear implant users

Bimodal cochlear implant (CI) listeners have difficulty utilizing spatial cues to segregate competing speech, possibly due to tonotopic mismatch between the acoustic input frequency and electrode place of stimulation. The present study investigated the effects of tonotopic mismatch in the context of residual acoustic hearing in the non-CI ear or residual hearing in both ears. Speech recognition thresholds (SRTs) were measured with two co-located or spatially separated speech maskers in normal-hearing adults listening to acoustic simulations of CIs; low frequency acoustic information was available in the non-CI ear (bimodal listening) or in both ears. Bimodal SRTs were significantly better with tonotopically matched than mismatched electric hearing for both co-located and spatially separated speech maskers. When there was no tonotopic mismatch, residual acoustic hearing in both ears provided a significant benefit when maskers were spatially separated, but not when co-located. The simulation data suggest that hearing preservation in the implanted ear for bimodal CI listeners may significantly benefit utilization of spatial cues to segregate competing speech, especially when the residual acoustic hearing is comparable across two ears. Also, the benefits of bilateral residual acoustic hearing may be best ascertained for spatially separated maskers.

Interaural asymmetry of dynamic range: Abnormal fusion, bilateral interference, and shifts in attention

Speech information in the better ear interferes with the poorer ear in patients with bilateral cochlear implants (BiCIs) who have large asymmetries in speech intelligibility between ears. The goal of the present study was to assess how each ear impacts, and whether one dominates, speech perception using simulated CI processing in older and younger normal-hearing (ONH and YNH) listeners. Dynamic range (DR) was manipulated symmetrically or asymmetrically across spectral bands in a vocoder. We hypothesized that if abnormal integration of speech information occurs with asymmetrical speech understanding, listeners would demonstrate an atypical preference in accuracy when reporting speech presented to the better ear and fusion of speech between the ears (i.e., an increased number of one-word responses when two words were presented). Results from three speech conditions showed that: (1) When the same word was presented to both ears, speech identification accuracy decreased if one or both ears decreased in DR, but listeners usually reported hearing one word. (2) When two words with different vowels were presented to both ears, speech identification accuracy and percentage of two-word responses decreased consistently as DR decreased in one or both ears. (3) When two rhyming words (e.g., bed and led) previously shown to phonologically fuse between ears (e.g., bled) were presented, listeners instead demonstrated interference as DR decreased. The word responded in (2) and (3) came from the right (symmetric) or better (asymmetric) ear, especially in (3) and for ONH listeners in (2). These results suggest that the ear with poorer dynamic range is downweighted by the auditory system, resulting in abnormal fusion and interference, especially for older listeners.

Effects of better-ear glimpsing, binaural unmasking, and spectral resolution on spatial release from masking in cochlear-implant users

Bilateral cochlear-implant (BICI) listeners obtain less spatial release from masking (SRM; speech-recognition improvement for spatially separated vs co-located conditions) than normal-hearing (NH) listeners, especially for symmetrically placed maskers that produce similar long-term target-to-masker ratios at the two ears. Two experiments examined possible causes of this deficit, including limited better-ear glimpsing (using speech information from the more advantageous ear in each time-frequency unit), limited binaural unmasking (using interaural differences to improve signal-in-noise detection), or limited spectral resolution. Listeners had NH (presented with unprocessed or vocoded stimuli) or BICIs. Experiment 1 compared natural symmetric maskers, idealized monaural better-ear masker (IMBM) stimuli that automatically performed better-ear glimpsing, and hybrid stimuli that added worse-ear information, potentially restoring binaural cues. BICI and NH-vocoded SRM was comparable to NH-unprocessed SRM for idealized stimuli but was 14%-22% lower for symmetric stimuli, suggesting limited better-ear glimpsing ability. Hybrid stimuli improved SRM for NH-unprocessed listeners but degraded SRM for BICI and NH-vocoded listeners, suggesting they experienced across-ear interference instead of binaural unmasking. In experiment 2, increasing the number of vocoder channels did not change NH-vocoded SRM. BICI SRM deficits likely reflect a combination of across-ear interference, limited better-ear glimpsing, and poorer binaural unmasking that stems from cochlear-implant-processing limitations other than reduced spectral resolution.

Dichotic listening performance and effort as a function of spectral resolution and interaural symmetry

One potential benefit of bilateral cochlear implants is reduced listening effort in speech-on-speech masking situations. However, the symmetry of the input across ears, possibly related to spectral resolution, could impact binaural benefits. Fifteen young adults with normal hearing performed digit recall with target and interfering digits presented to separate ears and attention directed to the target ear. Recall accuracy and pupil size over time (used as an index of listening effort) were measured for unprocessed, 16-channel vocoded, and 4-channel vocoded digits. Recall accuracy was significantly lower for dichotic (with interfering digits) than for monotic listening. Dichotic recall accuracy was highest when the target was less degraded and the interferer was more degraded. With matched target and interferer spectral resolution, pupil dilation was lower with more degradation. Pupil dilation grew more shallowly over time when the interferer had more degradation. Overall, interferer spectral resolution more strongly affected listening effort than target spectral resolution. These results suggest that interfering speech both lowers performance and increases listening effort, and that the relative spectral resolution of target and interferer affect the listening experience. Ignoring a clearer interferer is more effortful.