CM tuning can be compatible with sharply tuned receptor potentials

There is convincing evidence that cochlear microphonics (CM) arise primarily from outer hair cells and have a frequency distribution that is much broader than that measured in inner hair cells by Russel and Sellick [6,7]. The broad tuning results from the fact that CM generated at each location decays exponentially along the cochlear duct. This implies that the sources of CM (the outer hair cells) must be more sharply tuned than the CM itself. We modeled approximately the tuning of outer hairs on the assumption that it is proportional to the shear motion between the tectorial membrane and the reticular lamina. At each frequency the spatial distribution of CM is computed by convolving the shear distribution with an exponential decay function. The frequency dependence of CM at a given location can then be found by making a cut through a family of such frequency curves. The resulting CM tuning is much flatter than that of the modeled outer hair-cell receptor potentials and roughly parallels basilar-membrane tuning below the best frequency. Above the best frequency, the theoretical curves show a frequency-dependent plateau similar to that found in physiological CM measurements.