Excitation, Inhibition, and Disinhibition in the Inner Ear of the Toad (Bufo)

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Abstract
Three distinct types of auditory fibers are found in the VIIIth nerve of the American toad (Bufo americanus). One type is maximally excited by low-frequency tones (100–500 Hz), the second type by mid-frequencies 500–1000 Hz, and the third type by higher frequencies 1100–1700 Hz. Low-frequency units can be inhibited by the addition of a second tone in the mid-frequency range. For total inhibition, the second tone generally must be 10–30 dB more intense than the first. Excitatory and inhibitory tuning curves are disjoint. Total inhibition can be completely disinhibited by the addition of a third tone of approximately equal intensity but higher in frequency than the inhibitory tone. The frequency difference between the best inhibitory and best disinhibitory tones is approximately the best excitatory frequency of the unit. Our results suggest that disinhibition originates from a nonlinear mechanism in the inner ear which is operative even at relatively low intensity levels. Mid- and high-frequency units cannot be inhibited. The peripheral auditory system of the little green toad (Bufo debilis) is similarly organized, except that the high-frequency units respond best to 3000–3600 Hz. The significance of this organization in detection of natural sounds will be discussed. [Supported by NSF Grant GB-18836 and NIH Grant NS-09244.]

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