Single auditory nerve fiber activity and behavioral thresholds in pathological cochleas

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Abstract
Four chinchillas were exposed to 4 kHz CF octave band of noise at 86 dB for five days. After a recovery period of approximately six months, behavioral audiograms were obtained and auditory nerve fiber activity was recorded. The animals were sacrificed and the cochleas embedded in Araldite resin to obtain surface preparations and 1μ sections of the Organ of Corti. Permanent threshold shifts ranged from 5 to 20 dB between 4 and 8 kHz. VIII nerve recordings showed single fiber thresholds were elevated up to 70 dB between 4 and 14 kHz, while higher and lower CF fibers were essentially normal. Spontaneous activity in some of the animals was equal to or greater than normal. Cochleagrams all showed narrow lesions of inner and/or outer hair cells over approximately a 1‐mm distance. A comparison of the three realms of data revealed the following: (1) The greatest loss in sensitivity from the noise exposure was seen in the single nerve fiber thresholds while the smallest change was seen in the behavioral threshold shifts; (2) Based on the frequency‐place maps of the chinchilla cochlea, the range of fibers with elevated thresholds exceeded the extent of the outer hair cell lesions. A number of anatomical changes were seen that effectively increased the extent of the damage found in the cochleagram. These changes include distortions in the surface topography of the Organ of Corti affecting the orientation of IHC; missing pillar cells in the presence of normal OHC and the orientation of IHC; missing pillar cells in the presence of normal OHC and IHC populations and protrusion of the IHC cuticular plate into the subtectorial space. [Work supported by NSF and NIOSH.]

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