Effects of hair cell lesions on responses of cochlear nerve fibers. I. Lesions, tuning curves, two-tone inhibition, and responses to trapezoidal-wave patterns.

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Abstract
Responses of single afferent fibers to various sound stimuli were recorded in the cochlear nerves of Mongolian gerbils (Meriones unguiculatus) with normal cochleas or cochleas damaged by an ototoxic drug (kanamycin) or exposure to impulse noise. The response properties of fibers innervating abnormal cochleas were compared to those in connection with normal cochleas and were correlated with the presence or absence of hair cells. Normal fibers found with acoustic search stimuli have strikingly homogeneous characteristics in the gerbil preparation. Thresholds at the characteristic frequency (CF) typically varied by less than .+-.5 dB for fibers of similar CF. All fibers innervating normal cochleas showed 2-tone inhibition (TTI); the response to a tone of CF is reduced by the addition of tones above or below CF. At low frequencies, excitation of fiber with low and medium CF occurred during basilar membrane displacement toward scala tympani. Fiber responses obtained from gerbils exposed to impulse noise and associated with losses of outer hair cells (OHC) in excess of about 50% but restricted to the midcochlea had elevated CF thresholds that were not strictly proportional to the amount of OHC loss; TTI above CF even in the presence of high CF-threshold elevation; threshold tuning curves with normal high-frequency slopes, slightly lowered Q10 dB values (less sharply tuned) and low-frequency tails, which were about normal in sensitivity and an increased firing rate (excitation) during basilar membrane displacement toward scala vestibuli at low frequencies. Fibers associated with basal or basal and midcochlear OHC losses due to kanamycin treatment always had elevated CF thresholds that were correlated with the amount of OHC loss; no TTI above CF even in the presence of almost normal CF thresholds; tuning curves with shallow high-frequency slopes of between 70-200 dB/octave, low Q10 dB values and low-frequency tails that were normal, increased or decreased in sensitivity. During stimulation at low frequencies, they were excited during basilar membrane motion or displacement toward scala vestibuli. Cochleograms (hair cell counts as a function of distance along the organ of Corti) in both abnormal groups served only as gross predictors of single-unit characteristics. Functional changes apparently can take place at an ultrastructural level not visible with light microscopy.

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