What Do Recent Discoveries in Cochlear Mechanics Tell Us About Otoacoustic Emissions?

Recent optical measurements of vibrations of the organ of Corti contrast with those of the basilar membrane in their larger amplitude and much wider bandwidth of active gain and nonlinearity [6-9, 15, 16, 19, 20]. A report by Ren and He (19) demonstrates a considerably richer spectrum of intermodulation distortion in the motion of the reticular lamina than in the basilar membrane of the Mongolian gerbil cochlea. Data from this species from my lab also show a broad distortion spectrum much more similar to the reticular lamina than the basilar membrane in the Ren and He data. This result, along with other evidence of broad-band nonlinearity in ear canal acoustics strongly suggests the need to reevaluate otoacoustic emission (OAE) phenomena. In particular, these experimental data do not support reverse slow basilar membrane traveling waves [23] as the dominant mode of conducting OAE signals to the ear canal, but they are instead conducted primarily by fluid pressure in scala vestibuli, at least for this species in the basal region of the cochlea. Several important unresolved issues in OAEs may be resolved by simultaneous measurements of vibrations in the organ of Corti and basilar membrane, together with comparable measurements in ear canal acoustics.