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This paper summarizes the structural and functional damage caused by intense sound exposure in neonatal chicks. Scanning electron microscopy has been used to follow the structural changes to the papilla and their subsequent repair. Pure-tone exposures produced a localized lesion consisting of tectorial membrane destruction, changes in surface organization of the papilla, and hair cell loss. The papilla underwent significant repair following the exposure and new hair cells could be identified on the sensory surface after 4 days of recovery. In addition, various evoked-potential methods provided an objective assessment of auditory function and demonstrated that the peripheral ear was severely impaired by overstimulation. Auditory function returned to near normal levels within 3 days postexposure. The inescapable conclusion from these observations was that hair cell regeneration had little to do with the functional recovery observed during the first 3 days. Tectorial membrane regeneration and the restoration of cochlear micromechanics were combined to form a hypothesis to account for the restoration of auditory function.

The effect of kanamycin on auditory function was investigated in C57BL/6J mice. Results showed that kanamycin treatments (400 mg/kg/day, injected between 10 to 13 days of age) caused severe cochlear damage, particularly to the outer hair cells, and elevation of cochlear nucleus evoked-response thresholds. The treatment also rendered the treated animals susceptible to audiogenic seizures, suggesting a change in the animals' reactivity to intense sounds. However, unlike the effect of noise, exposure to kanamycin did not cause the treated animals to show abnormal augmentation of cochlear nucleus evoked-response amplitude when stimulated at high intensities.

Spectral and temporal response patterns to pure-tone stimuli were collected from single units in the dorsal cochlear nucleus of anesthetized chinchillas. The spectral response profiles were divisible into groups based on the balance of excitation and inhibition. Temporal responses were characterized in chloralose-anesthetized animals by collecting PST-histograms. There appeared to be no simple one-to-one relationship between a unit's spectral and its temporal response pattern. Excitatory spectral responses were generally sharply tuned areas resembling those of auditory nerve fibers. However, unlike the latter, the majority of these had chopper or pauser/buildup temporal responses. Inhibitory spectral responses were of two distinct types: one included lateral inhibitory areas flanking the tuned excitatory areas which occasionally invaded the latter creating a nonmonotonic excitatory response at the unit's characteristic frequency. The other included sharply tuned inhibitory areas. The characteristic frequencies of these units were found to be in close correspondence with those of sharply tuned excitatory units from the same penetration suggesting that these inhibitory units were tonotopically mapped in the same register as tuned excitatory units. The spectral response patterns were studied with three types of anesthesia: ketamine/xylazine, dial/urethane, and chloralose. In each of these groups the patterns were similar. However, the proportions of units showing inhibition was strongly dependent on the choice of anesthetic agent with chloralose yielding the highest proportions (59%) and ketamine/xylazine yielding the lowest (29%).