Measurements of the stiffness map challenge a basic tenet of cochlear theories

The cochlear frequency map is believed to depend on the progressive decrease in partition stiffness from base to apex. Measurements on cochleae from human cadavers by von Békésy (1960) suggested that the elasticity of the partition increases by a factor of 100 from the stapes to the helicotrema. However, conventional models require a factor of nearly 10,000 to support the frequency range of normal hearing if entirely determined by partition stiffness. To test this assumption, we measured point stiffness along the width and length of the partition in the gerbil cochlea. Two major findings result from this study: (1) contrary to von Békésy's results, both cellular and extracellular elements of the sensory epithelium exhibit stiffness gradients; and (2) the stiffness changes by only a factor of 100 over the whole cochlea. Our results imply that present ideas regarding partition vibration need to be significantly revised.