Between-ear sound frequency disparity modulates a brainstem biomarker of binaural hearing.

Abstract

The auditory brainstem response (ABR) is an evoked potential that indexes a cascade of neural events elicited by sound. Here we evaluated the influence of sound frequency on a derived component of the ABR known as the binaural interaction component (BIC). Specifically, we evaluated the effect of acoustic interaural (between-ear) frequency mismatch on BIC amplitude. Goals were to (1) increase basic understanding of sound features that influence this long-studied auditory potential and (2) gain insight on the persistence of the BIC with interaural electrode mismatch in human users of bilateral cochlear implants - presently a limitation on the prospective utility of the BIC in audiologic settings. Data were collected in an animal model that is audiometrically similar to humans, the chinchilla (Chinchilla lanigera [6 females]). Frequency disparities and amplitudes of acoustic stimuli were varied over broad ranges, and associated variation of BIC amplitude was quantified. Subsequently, responses were simulated using established models of the brainstem pathway thought to underlie the BIC. Collectively, data demonstrated that at high sound intensities (≥85 dB SPL), the acoustically elicited BIC persisted with interaurally disparate stimulation (click frequencies ≥1.5 octaves apart). However, sharper tuning emerged at moderate sound intensities (65 dB SPL), with largest BIC occurring for stimulus frequencies within approximately 0.8 octaves, equivalent to ±1 mm in cochlear place. Such responses were consistent with simulated responses of the presumed brainstem generator of the BIC, the lateral superior olive. Data suggest that leveraging focused electrical stimulation strategies could improve BIC-based bilateral cochlear implant fitting outcomes.