Sound frequency processing in the inner ear: A new theory

Abstract

Frequency discrimination, a cornerstone of auditory perception and its technological applications, remains incompletely understood. Current theories fall short of fully explaining the human ear's ability to distinguish sounds of varying frequencies. The limitations observed in hearing assessments and hearing aid prostheses represent a fundamental challenge in the field of hearing sciences. This paper introduces a novel theoretical framework, building upon our prior research, to elucidate the mechanism underlying frequency discrimination. This proposed mechanism has the potential to change the hearing theories and the fundamental operating principles of hearing aid technologies. This framework provides a neuro-mechanical foundation for cochlear filter bank and Spectro-temporal models by bridging the gap between cochlear biomechanics and neural processing. We hypothesize that an outer hair cells (OHC) to inner hair cells (IHC) reflex arc contributes active neural sharpening for frequency discrimination, while the OHC-Prestin-IHC complex generates the highprecision temporal code necessary for the central auditory system to perform its complex analysis. The incoming acoustic signal can be transmitted to IHC by OHC through a process quantization in the organ of Corti. This signal is encoded at ribbon synapses of IHC where the acoustic signal is transformed into a digital one. The resulting signal code is transmitted to the auditory cortex as a stimulus envelope encoded by the Type 1 spiral ganglion neurons or the afferent auditory nerve which can be thought of as a pulse-code modulation transmitter. While the traveling wave theory, as proposed by von Bekesy, has traditionally been invoked to explain pitch perception and sound processing within the cochlea, our framework diverges by suggesting that the derived acoustic code is perceived across all segments of the cochlea, rather than being confined to a tonotopically mapped region on the basilar membrane. Frequency discrimination, within this model, can be conceptualized as a reflex activation mediated by the electromotile responses of OHC (driven by the motor protein Prestin) in conjunction with the medial olivocochlear efferent (MOCE) system. This MOCE reflex activation can be triggered by the acoustic code generated by IHCs and the electromotile responses of OHCs across the entire cochlear duct. OHCs, along with the motor protein Prestin, which facilitates electromotile responses independently of basilar membrane motion, may play a significant role in this frequency discrimination. In this context, the basilar membrane provides only a general vibration pattern for OHC activity.