Evidence for mechanical origin of peripheral inhibition in the anuran inner ear

Abstract

The auditory fibers in the eighth nerve of the American toad (Bufo americanus) that have their best excitatory frequencies in the range 100–600 Hz can be totally inhibited by the addition of a second tone of appropriate frequency and intensity. Each of these fibers can also be excited by a pair of tones if their frequency difference is approximately equal to the unit's best excitatory frequency, even though neither tone by itself has any excitatory effect on the unit. The intensities of the two “difference tones” in many cases need only be 10–20 dB greater than the intensity of the best excitatory tone to evoke comparable spike rates. The excitation of these low-frequency fibers by difference tones seems to reside in nonlinear mechanical events in the inner ear. Midfrequency-sensitive units, having their best frequencies in the range 500–1000 Hz, and the high-frequency-sensitive units, with their best frequencies in the range 1100–1700 Hz, cannot be inhibited by tones nor can they be excited by pairs of difference tones. Similar results in other anuran species provide evidence that inhibition in the peripheral auditory system of frogs and toads is of mechanical origin. [Supported by NSF Grant GB-18836 and NIH Grant NS-09244.]