Phase cancellation of auditory nerve fiber responses to combination tones, f2–f1

Abstract

Low-frequency sensitive fibers from the amphibian papilla of anurans (Bufo americanus and Rana pipiens) respond to combination tones of the form f2–f1 in the same way as do mammalian cochlear nerve fibers, even though the structure of the receptor organs is quite different in the two classes. In anurans the shapes of the combination tone and pure tone tuning curves are similar, suggesting that the amphibian papilla fibers are responding to energy generated at the difference frequency. Furthermore, the response of a fiber to a tone pair, whose frequencies are harmonically related and whose fundamental is near the fiber's best excitatory frequency, can be almost totally eliminated by the addition of energy at the fundamental provided it is of appropriate intensity and relative phase. Systematically varying the phase of the fundamental produces a sharp minimum in firing rate with a maximum occurring about 180° later. These results suggest that a basilar membrane is not necessary for propagation of energy at the difference frequency; it can be transmitted by the inner ear fluids and the tectorial membrane. [Supported by NIH Grant NS-09244.]