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A mechanically graded structure supporting frequency-dependent traveling-wave responses in the cochlea. High-frequency responses peak nearer the base and low-frequency responses nearer the apex. Its living response is modified by the cochlear amplifier, so passive mechanical measurements do not capture the entire operating system.

A cochlear hair-cell class contributing mechanical feedback that increases weak-sound sensitivity and sharpens frequency selectivity. Its feedback-dependent effects are part of the cochlear amplifier.