(D-149) Characterizing the primary resonator in the cochlea

Introduction:



Hearing loss is one of the most common sensory disorders in the world. Approximately 15% of American adults (37.5 million) of age 18 and over report some trouble hearing. Thorough research of the inner ear biomechanics can help us better understand hearing and its dysfunctions. The basilar membrane is a collagen fiber-imbedded substrate on which the auditory epithelium, called the organ of Corti sits. It plays a key role in cochlear mechano-transduction. The stiffness gradient of the basilar membrane, the primary resonator in the cochlea, underlies the cochlear frequency-location map. Also, the basilar membrane interacts with the cochlear fluids to form the traveling waves. In previous studies, higher-order modes of basilar membrane radial vibrating patterns were reported. We used high-resolution optical coherence tomography (OCT) to measure the vibrating pattern and frequency response of the basilar membrane. To investigate the mechanical behavior of this primary resonator, confident pressure measurement is crucial to determining the system’s best responding frequency (BF).





Materials and Methods:




The cochleae were isolated from young gerbils (15-30 days old). The segment of a cochlea (at the location of 8.5 mm from the base) was separated and placed in a microchamber. Mechanical stimulations were applied to the tissue at different frequencies to provoke passive vibrations in the organ of Corti. These resulting vibrations were recorded using the OCT system. Pressure applied to the tissue was calibrated by using artificial membranes.





Results, Conclusions, and Discussions:



Results



The frequency response of the basilar membrane was consistent with a simple resonator with minimal overshoot. The gain peaked between 0.5 and 1 kHz, which means the BF is roughly between 0.5 and 1 kHz. Higher-order mode vibrations were observed in the basilar membrane. When passively vibrated by mechanical stimulation, the primary mode prevailed throughout the physiological frequency range of the measurement location (< 1.5 kHz). Higher-order mode vibrations appeared near or above the characteristic frequency (CF) at the location ( > 1.5 kHz).



Discussion and Conclusion



In our excised cochleae, outer hair cells’ electromechanical feedback and traveling waves were compromised. Bone removal also changed the boundary conditions of the cochlear fluid space dramatically from its natural states. Compared to the physiological CF of the gerbil cochlea at this location (1.5 kHz for sensitive and 1.1 kHz for insensitive cochlea), the measured BF is even lower. Whether the resonance is affected by microchamber acoustics needs further investigation.



Our observations showed that when the stimulating frequency went higher, the basilar membrane vibrated from the primary to a higher mode. The higher-order mode started to appear near the CF of the measured location. To our knowledge, it is the first direct experimental evidence showing that the basilar membrane exhibits the characteristic behavior of a resonator (mode transition).