The Effects of Middle-Ear Stiffness on the Auditory Brainstem Neural Encoding of Phase

Abstract

The middle-ear system relies on a balance of mass and stiffness characteristics for transmitting sound from the external environment to the cochlea and auditory neural pathways. Phase is one aspect of sound that must be transmitted and encoded by both ears in contribution to binaural cue sensitivity and spatial hearing. This study investigated the effects of increased middle-ear stiffness on the auditory brainstem neural encoding of phase in human adults between the ages of 18 and 50 years with normal pure-tone threshold sensitivity. The three aims of this study were (i) to investigate the effects of middle-ear stiffness on the auditory brainstem neural encoding of phase; (ii) to investigate the relationships between middle-ear stiffness-induced changes in wideband acoustic immittance measures (i.e., acoustic absorbance and power transmittance) and auditory steady-state response measures; and (iii) to investigate ear-specific effects of middle-ear stiffness on the auditory brainstem neural encoding of phase. The auditory brainstem neural encoding of phase was measured using the auditory steady-state response with and without middle-ear stiffness elicited via contralateral activation of the middle-ear muscle reflex. Middle-ear stiffness was quantified using wideband acoustic immittance assays of acoustic absorbance and power transmittance. Statistical analyses demonstrated decreased auditory steady-state response phase and decreased acoustic absorbance with activation of the middle-ear muscle reflex, consistent with the hypothesis of increased middle-ear stiffness. Contrary to the hypothesis, there were no statistically significant relationships between stiffness-induced changes in wideband acoustic absorbance and auditory steady-state response measures. Finally, stiffness-induced auditory steady-state response phase shifts were statistically comparable for the right and left ears. The findings of this study may have important implications for understanding binaural cue sensitivity and horizontal plane sound localization in several audiologic clinical populations that may demonstrate changes in middle-ear stiffness, including cochlear implant patients who use combined electric and binaural acoustic hearing and patients with otosclerosis.