Crossed and Uncrossed Olivocochlear Pathways Exacerbate Temporary Shifts in Hearing Sensitivity after Narrow Band Sound Trauma in Normal Ears of Animals with Unilateral Hearing Impairment

Abstract

Olivocochlear (OC) pathways have been shown to reduce the temporary threshold shifts (TTSs) caused by traumatic sounds. More recently they have been shown to exacerbate TTSs under certain conditions. One condition is the normal-hearing ear of animals with a chronic unilateral hearing loss. Testing with pure tone trauma showed that then (a) the normal-hearing ear had a lower-than-normal ‘intrinsic’ susceptibility to intense tones, (b) binaural trauma exacerbated TTSs in the normal-hearing ear through the activity of uncrossed OC (UOC) pathways, and (c) there was no effect on TTSs of the crossed OC (COC) pathway to the normal-hearing ear. The present study is an examination in such animals of effects with noise band trauma. The effects here confirm the previous finding that under such conditions the normal-hearing ear has a lower-than-normal susceptibility to loud sound, and binaural loud sounds exacerbate TTSs in the normal-hearing ear. They extend the previous study by demonstrating that with this traumatic sound, both COC and UOC pathways exacerbate TTSs. These effects contrast against the effects seen in animals with bilaterally normal hearing for the same noise band. Given the commonality of unilateral hearing losses in the normal human population, these data have implications for the functional effects of the OC pathways on loud sound-induced hearing damage.