The proposed experiments are a continuation of earlier and ongoing studies of the physiological bases of noise-induced hearing loss carried out in a single-cell model at the level of the cochlear nucleus in both awake and acute primates. The basic experimental strategy continues to make use of a within-neuron design to study the relatively long-lasting effects of brief episodes of what has traditionally been referred to as temporary threshold shift (TTS) on physiological activity. To date, one of the principal findings has revealed that significant and long-lasting alterations can be documented for the discharge responses of the single cell which are not reflected in simultaneously measured, but less sensitive, pure-tone audiometric functions. Further, it has been demonstrated that the standard TTS-like exposure stimuli (100 dB SPL, 3 min) are capable of effecting permanent histological alterations to cochlear duct elements when applied at regular intervals over a period of many moths thus demonstrating the relevance of single-unit analyses towards understanding the biological bases of one of the most common hearing disorders in modern society, i.e., slowly developing hearing loss caused by regular exposure to moderate levels of sound. Following exposure to the standard protocol, central auditory system neurons of all divisions of the cochlear nucleus demonstrate a number of distinct changes that suggest the contribution of both peripheral and central mechanisms towards eliciting the complex effects. The present experiments are designed to distinguish the peripheral from central components of the auditory nervous system's response to moderately intense sound by employing both single eighth nerve fiber and cochlear nucleus cell recording techniques to identify altered inputs to the more centrally located neurons. Such studies will determine the early physiological changes underlying TTS and provide useful insight into the altered sensory encoding processes that probably accompany perceptual changes in the noise=damaged ear.
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