Basilar Membrane Tuning

Georg von Békésy's visual observation of the basilar‐membrane travelling wave showed a broad mechanical tuning, quite at variance with the sharp tuning from neural recordings. A few years ago, Johnstone and Boyle applied the Mössbauer technique to the basilar membrane and showed the tuning to be much sharper than previously thought. Our latest results show high‐frequency cutoffs of up to 340 dB per octave. Using a special Mössbauer source with an isomer shift to eliminate the 180° ambiguity of previous work, the phase shift has been shown to asymptote to π/2 lead at low frequencies, rotating approximately linearly with increasing frequency and finally asymptoting sharply to 3π lag at the tuning‐curve cutoff. The low‐frequency slope of about 12 to 24 dB/oct is apparently much less than the neural curves. However, the Mössbauer technique requires a higher sound level than used for neural threshold curve, and if neural curves are constructed with constant firing rates as a parameter, e.g., 50 impulses per second, so that comparable sound pressures are used, then they show a low‐frequency scope of around 36 dB/oct—not too different from the mechanical figure. Hence, taking into account a basilar membrane nonlinearity of the type found by Rhode, the neural and mechanical curves may be reconciled.