A New Hypothesis on the Frequency Discrimination of the Cochlea
Department of Audiology, Trakya University Faculty of Health Sciences, Edirne, Turkey; Department of Physiology, Trakya University School of Medicine, Edirne, Turkey
Department of Otorhinolaryngology, Trakya University School of Medicine, Edirne, Turkey
Department of Physiology, Trakya University School of Medicine, Edirne, Turkey.
Center of In-vitro Fertilization, Marmara University School of Medicine, İstanbul, Turkey
Department of Histology and Embryology, İstanbul Medeniyet University School of Medicine, İstanbul, Turkey
Department of Histology and Embryology, Acıbadem University School of Medicine, İstanbul, Turkey
J Int Adv Otol ; : -
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Key Words: Efferent pathways, cochlear outer hair cells, otoacoustic emissions, spontaneous, basilar membrane, microscopy, electron, scanning
OBJECTIVE: Medial olivocochlear efferent (MOCE) neurons innervate outer hair cells (OHCs) of the cochlea, which in turn leads to basilar membrane motion. We hypothesized that MOCE-induced alterations in basilar membrane motion, independent of traveling waves, is responsible for the cochlear frequency discrimination of sound.
MATERIALS and METHODS: Eleven guinea pigs underwent bilateral otoscopic and audiologic evaluations under general anesthesia. The study comprised two parts. Part I (n=11) included spontaneous otoacoustic emission (SOAE) recordings with or without contralateral pure-tone acoustic stimuli (1 and 8 kHz) at 60 dB sound pressure level (SPL). Part II involved pure-tone (1 or 8 kHz) acoustic trauma in the right ears of two randomly selected subgroups (G1: 1 kHz; n=4 and G8: 8 kHz; n=4). The remaining three animals served as controls. After frequency-specific deafness was confirmed by distortion product otoacoustic emission (DPOAE), SOAEs were recorded in the left ears in the presence of a contralateral pure-tone (1 and 8 kHz) stimulus of 60 dB SPL. Furthermore, the surface of the organ of Corti was examined by scanning electron microscopy (SEM).
RESULTS: The contralateral pure tone led to frequency-specific activation in SOAEs in part I (without trauma) and part II (with trauma) measurements. SEM showed heterogeneous OHC damage along the cochlea in traumatized ears with pure tone.
CONCLUSION: We suggest that MOCEs convey acoustic information from traumatized ears to intact ears. Traumatized ears can show frequency-specific activation in the presence of diffuse damage in OHCs that excludes the passive transmission of the pressure wave from the perilymph to the basilar membrane.