Basilar membrane motion in the pigeon measured with the Mössbauer technique

Vibration measurements were made of the basilar membrane (BM), limbi and columella footplate (CFP) of pigeon using the Mössbauer technique. Recordings were located at 0.23-1.33 mm from the basal end of the BM. The existence of a travelling wave mode, propagating from base to apex, was established for papillae in apparently good physiological condition. For these papillae the characteristic frequency (CF) of the BM isovelocity (0.08 mm X s-1) response was an exponential function of distance with a frequency mapping constant of 0.91 +/- 0.10 mm (equivalent to 0.63 +/- 0.07 mm X oct-1); BM CF at the base was 5.95 +/- 0.65 kHz. Travelling wave motion was not demonstrated for papillae in poor physiological condition; tonotopy of BM CF was still evident, although the correlation with distance was less (1.08 +/- 0.30 mm X oct-1; 4.35 +/- 0.73 kHz at the base). BM motion was linear and the isovelocity responses were less sensitive and less sharp than single unit threshold tuning curves: for papillae in good physiological condition the SPL at BM CF at 0.08 mm X s-1 was 51 +/- 6 dB SPL; Q10 dB was 1.24 +/- 0.38; high- and low-frequency slopes were 20 +/- 6 dB X oct-1 and -14 +/- 4 dB X oct-1, respectively. The response of the BM relative to the CFP for papillae in good physiological condition was reminiscent of a second order resonant system with damping constant of 0.33 +/- 0.06 and group delay at BM CF of 0.89 +/- 0.36 periods.