Tuning and Timing of Organ of Corti Vibrations at the Apex of the Intact Chinchilla Cochlea

The goal of this research was to record sound-evoked vibrations in the organ of Corti at the apex of the intact cochlea of the chinchilla, an animal with a frequency hearing range similar to that of humans. Twelve adult anesthetized chinchillas of either sex were used. Measurements of sound-evoked vibrations of the intact organ of Corti (OoC) were performed using optical coherence tomography (OCT). Acoustic stimuli consisted of single tones of 1-s duration. OoC vibrations were recorded using a Telesto Spectral Domain OCT system (Thorlabs GmbH, Germany) and ThorImage® OCT version 5.4.2 (Thorlabs GmbH, Germany). Further analysis of the output of the ThorImage software was performed by ad hoc programs written using Matlab® R2020b. Recordings were performed at several OoC sites extending from the Hensen's cell region to the vicinity of the basilar membrane (BM). The measurement angle between the optical axis of the OCT system and the BM was approximately 45°. Under that experimental condition, delays among the different OoC locations indicate that BM motion occurs earlier than at other sites. At all OoC sites, sound-evoked vibrations grow nonlinearly with stimulus level at compressive rates. The sharpness of tuning of OoC vibrations increases with stimulus level and death. Iso-velocity curves as a function of frequency are well-tuned around 500 Hz and closely resemble threshold tuning curves of chinchilla auditory-nerve fibers with similar characteristic frequencies. The nonlinear processing of sounds at the apex of the chinchilla cochlea differs significantly from the processing of sounds at the base of the cochlea in the same species.