The second filter is real, but how does it work?
The following speculative suggestions are offered informally concerning the "second filter" in the cochlea. Increased sensitivity is an advantage as important as increased sharpness of tuning. These two features and the "cochlea echo" strongly suggest a resonant system with high Q, which would also explain the prolonged latency near threshold of the action potential of the auditory nerve. The viscous damping of a mechanical resonator in the organ of Corti might be overcome by positive feedback. The seond filter is mechanically fragile and is vulnerable to anoxia. The source of the energy for positive feedback might be the cochlear microphonic of the outer hair cells. A possible transducer is hyaluronic acid located between the cilia and known to have piezoelectric properties.
D000200:Action Potentials; D000328:Adult; D000368:Aged; D001299:Audiometry; D003051:Cochlea / Q000502:physiology*; D005246:Feedback; D006309:Hearing / Q000502:physiology*; D006801:Humans; D000159:Vestibulocochlear Nerve / Q000502:physiology