Effects of hair cell lesions on responses of cochlear nerve fibers. II. Single- and two-tone intensity functions in relation to tuning curves.

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Abstract
Single- and 2-tone responses of primary neurons in the cochlear nerves of gerbils (Meriones unguiculatus) were examined. The cochleas of the gerbils were normal or were made abnormal by exposure to impulse noise or treatment with kanamycin. Fibers originating in normal cochleas always exhibited sharp threshold-tuning curves with high-frequency slopes in excess of 300 dB/octave. Two-tone inhibition (TTI), where the response to an excitor tone at the characteristic frequency (CF) of a fiber is reduced by the addition of an inhibitor tone above or below CF, was always demonstrated. The intensity (rate-level) functions produced by single tones placed below and at CF were approximately parallel with similar saturation rates; intensity functions associated with tones placed at increasing frequencies above CF had progressively decreasing slopes and saturation rates. Two-tone responses of fibers innervating abnormal cochleas often showed no inhibition. Coupled with the absence of TTI for inhibitor tones above CF were tuning curves with shallow high-frequency slopes and intensity functions that were parallel for tones above and below CF. The corresponding saturation rates were similar; i.e., a tone placed above CF elicited the same maximum response as a tone placed below CF. High-CF fibers with TTI absent above CF had intensity functions with slopes about 2 times the corresponding fibers of normal cochleas. The absence of TTI for inhibitor tones below CF often corresponded to a hypersensitivity at low frequencies. All fibers exhibiting low-frequency hypersensitivity showed no TTI below CF. Fibers associated with normal complements of hair cells basal to a functionally damaged region along the organ of Corti had TTI below CF, even when the frequency of the inhibitor tone coincided with the region of damage. A number of fibers associated with cochlear lesions had discharge rates that, in response to excitor tones, increased when a 2nd tone was added at frequency-intensity coordinates normally associated with TTI. This phenomenon was termed 2-tone summation. It occurred most often in fibers with steep intensity functions and high saturation rates for 2nd tones placed above CF. Summation for 2nd tones below CF was often associated with hypersensitive thresholds below CF.