Cochlear microphonic broad tuning curves

Abstract

Abstract. It is known that the cochlear microphonic voltage exhibits much broader tuning than does the basilar membrane motion. The most commonly used explanation for this is that when an electrode is inserted at a particular point inside the scala media, the microphonic potentials of neighbouring hair cells have different phases, leading to cancelation at the electrodes location. In situ recording of functioning outer hair cells (OHCs) for investigating this hypothesis is exceptionally difficult. Therefore, to investigate the discrepancy between the tuning curves of the basilar membrane and those of the cochlear microphonic, and the effect of phase cancellation of adjacent hair cells on the broadness of the cochlear microphonic tuning curves, we use an electromechanical model of the cochlea to devise an experiment. We explore the effect of adjacent hair cells (i.e., longitudinal phase cancellation) on the broadness of the cochlear microphonic tuning curves in different locations. The results of the experiment indicate that active longitudinal coupling (i.e., coupling with active adjacent outer hair cells) only slightly changes the broadness of the CM tuning curves. The results also demonstrate that there is a π phase difference between the potentials produced by the hair bundle and the soma near the place associated with the characteristic frequency based on place-frequency maps (i.e., the best place). We suggest that the transversal phase cancellation (caused by the phase difference between the hair bundle and the soma) plays a far more important role than longitudinal phase cancellation in the broadness of the cochlear microphonic tuning curves. Moreover, by increasing the modelled longitudinal resistance resulting the cochlear microphonic curves exhibiting sharper tuning. The results of the simulations suggest that the passive network of the organ of Corti determines the phase difference between the hair bundle and soma, and hence determines the sharpness of the cochlear microphonic tuning curves.

Finite element modelling of cochlear electrical coupling

“…However, it was shown in Ayat et al (2015) that longitudinal coupling is not required for the cochlear microphonic to have broad tuning curves; rather the broadness is due to phase cancellation between the hair cell and hair bundle voltages. The shorter range of hair cell electrical influence proposed here is consistent with these earlier observations.…”

Spectral Ripples in Round-Window Cochlear Microphonics: Evidence for Multiple Generation Mechanisms

“…We developed our model of CM generation with the goal of explaining the various components seen in the data rather than providing a complete account of the many complexities involved in CM generation. The model ignores the complicated electroanatomy of the spiral cochlea and its possible implications for round-window responses (e.g., Dallos 1984;Chertoff et al 2012;Ayat et al 2015). For instance, the spiral shape of the cochlea may affect the effective electrical attenuation factor, which may not be an entirely monotonic function of longitudinal distance (Chertoff et al 2012).…”