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

Abstract

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.

Keywords: asymmetrical hearing; auditory deprivation; binaural hearing; cochlear implants; spatial hearing.

Figure 1.

(A) Bottom-up binaural processing via the LSO. Stimulation arrives at the auditory nerve via electrical pulses and travels through action potentials to the LSO. (B) LSO output for accurately encoded binaural cues. The x-axis corresponds to ILD in dB or ITD in µs. The y-axis corresponds to the spike rate measured by single unit recording. Spike rates are modulated by binaural cue, showing distinct responses for different ILD or ITD magnitudes. (C) LSO output for poorly encoded binaural cues. The x-axis corresponds to ILD in dB or ITD in µs. The y-axis corresponds to the spike rate measured by single unit recording. Spike rates are weakly modulated by binaural cue, showing indistinct responses for different ILD or ITD magnitudes. ILD=interaural level difference; ITD=interaural time difference; LSO=lateral superior olive.

Figure 2.

Schematic representations of distinct and indistinct stimuli. Units representing sound features corresponding to the stimulus are predicted, and units representing features different from the stimulus are not predicted. Distinctness corresponds to greater prediction confidence. Lines between blocks are meant to represent feedforward and feedback connections, providing excitatory or inhibitory input. Connections between blocks within a single layer (e.g., sideband inhibition) and connections between one block and itself are excluded to avoid visually cluttering the figure.