A spatial template for the shape of tuning curves in the mammalian cochlea

The shape of the tuning curve of primary auditory neurons of four mammals is characterized using a simple exponential model. The regression analysis formalizes a distinction between the characteristic frequency of a neuron and its "nominal" characteristic frequency in cases of temporary threshold loss in high-frequency neurons. Second, the model offers a stronger quality test for sharpness of tuning than the Q10dB since it takes into account the threshold of the neuron at its characteristic frequency and its "characteristic place" of origin along the cochlear partition. Third, the model reveals that the low-frequency side of the tip segment of the tuning curve is bounded by a constraint or template which is most simply expressed in spatial terms. The template describes the basal-side boundary of an "excitatory region" whose length along the cochlear partition is proportional to the square root of the sound pressure. Tuning curve variability arises because biological dependencies influence the basic template. A "spatial-filter" hypothesis is developed and its generality is discussed, particularly in regard to the case of the acoustic "fovea" of the horseshoe bat. Finally, the possibility is discussed that the template possesses a simple physiological correlate in the form of a spatially localized region marked by a "dc" shift of the mean position of the basilar membrane which sets the sensitivity of the tuning mechanism [E.L. LePage, J. Acoust. Soc. Am. 82, 139-154 (1987)].