Correspondence amongst microstructure patterns observed in otoacoustic emissions and Békésy audiometry

Similar patterns of microstructure have been reported in normal ears for Békésy threshold recordings and various forms of otoacoustic emissions (OAE). It has been suggested that they have a common origin associated with the amplifying function of the outer hair cell system and wave interactions occurring within cochlear mechanics. Fine-frequency Békésy audiometry was conducted in ten normal ears and its microstructure was compared with that recorded using two OAq techniques: stimulus frequency (SFOAE) and distortion product (DPOAE). All sweeps encompassed the frequency range from 992 to 2000 Hz in 16-Hz steps. The same probe was used for all Békésy and OAE recordings to eliminate transducer effects. SFOAEs were obtained with stimulus intensities of 0, 3, 6 and 9 dB. DPOAEs were obtained for 2F1-F2 with primary levels (L1/L2) of 40/30, 45/35, 50/40 and 55/45 dB. Reliable microstructure was recorded in all ears. Mean values of microstructure peak spacing ranged from 5.6 to 9.3 per cent amongst methods, consistent with published data. Microstructure was similar within each OAE method for different stimulus intensities for each subject. However, comparisons between Békésy and OAEs, or between OAE methods, did not show the strong correspondence that would be expected if there were a simple common origin to the microstructure. There was weak support for the expected correspondence between Békésy and SFOAE, but no support for any correspondence between Békésy and DPOAE. It is concluded that the various forms of microstructure cannot be explained by a simple common origin.