SUPPRESSING TRAVELING WAVES REVEALS CRITICAL DYNAMICAL BEHAVIOR IN A MAMMALIAN COCHLEAR SEGMENT

The mammalian cochlea's active process exhibits amplification, sharp frequency selectivity, and compressive nonlinearity, but whether these properties arise intrinsically from hair cells or depend on traveling waves remains uncertain. We developed a preparation that isolates a 500-1500 µm segment of the cochlear duct from 3-4 week-old Mongolian gerbils while maintaining physiological conditions: the apical surface is bathed in artificial endolymph, the basolateral surface in artificial perilymph, temperature is maintained at 36-38°C, and a 100 mV endocochlear potential is applied. Using frequency sweeps and complex zwuis tones with concurrent microphonic recordings and optical coherence tomography, we characterized the isolated segment's mechanical and electrical responses. Despite minimal phase accumulation confirming suppression of traveling waves, the preparation exhibited robust frequency tuning to 1-4 kHz corresponding to the exposed cochlear location. The microphonic response demonstrated amplification and compressive nonlinearity, growing as the one-third power of stimulus intensity at the best frequency. Two-tone stimulation elicited combination tones, further indicating active nonlinear processing. Control experiments without endocochlear potential eliminated these active features, producing only linear passive responses. This platform, which preserves active cochlear tissue under physiological conditions outside the body in the absence of traveling waves, demonstrates that amplification, tuning, and nonlinear compression are intrinsic properties of the organ of Corti and provides a powerful system for investigating cochlear physiology and the mechanisms underlying hearing loss.