Unilateral hearing losses alter loud sound-induced temporary threshold shifts and efferent effects in the normal-hearing ear

In animals with bilaterally normal hearing, olivocochlear pathways can prot ect the cochlea from the temporary shifts in hearing sensitivity (temporary threshold shifts; TTSs) caused by short-duration intense loud sounds. The crossed olivocochlear pathway provides protection during binaural loud soun d, and uncrossed pathways protect when monaural or binaural loud sounds occ ur in noise backgrounds. Here I demonstrate that when there is a chronic un ilateral hearing loss, effects of loud sounds, and efferent effects on loud sound, in the normal-hearing ear differ markedly from normal. Three catego ries of test animals with unilateral hearing loss were tested for effects a t the normal-hearing ear. In all categories a monaural loud tone to the nor mal-hearing ear produced lower-than-normal TTSs, apparently because of a to nic resetting of that ear's susceptibility to loud sound. Second, in the tw o test categories in which the hearing-loss ear was only partly damaged, bi naural loud sound exacerbated TTSs in the normal-hearing ear because it cau sed threshold shifts that were a combination of "pure" TTSs and uncrossed e fferent suppression of cochlear sensitivity. (In normal cats, this binaural tone results in crossed olivocochlear protection that reduces TTS.) Binaur al loud sound did not produce such uncrossed efferent effects in the test c ategory in which the nontest ear had suffered total hearing loss, suggestin g that this uncrossed efferent effect required binaural input to the CNS. I t is noteworthy that, in the absence of this uncrossed efferent suppression , the pure loud sound-alone induced TTSs after binaural exposure were low. Thus in the absence of any efferent effect, the normal-hearing cochlea had a reduced susceptibility to loud tone-induced damage. Finally, the results suggest that, with respect to cochlear actions at high sound levels, uncros sed and crossed efferent pathways may exert different effects at the one ty pe of receptor cell.