ABSTRACT
It is clear that noise exposure that induces transient changes in thresholds can induce permanent neural pathology. There has been some limited evidence for changes in function on a signal-in-noise test when rats were tested in difficult listening conditions. However, these functional changes were only induced in those cases in which TTS was robust (40–50 dB, 24 h post noise). Moreover, the deficits were only observed at the frequencies at which a permanent noise-induced decrease in ABR Wave I amplitude was measured. New data from nonhuman primates indicate similar patterns of pathology can be induced with significant acute noise exposure while anesthetized. Although this confirms selective synaptopathic injury in a nonhuman primate model, the exposure used to induce this pathology exceeds the daily occupational noise exposure limits for workers in the United States and other countries. The extent to which synaptic pathology, OHC loss, or mixed pathologies, will be induced by occupational exposure is not known, given that occupational exposure will be limited to lower daily doses, but will be repeated 5 days per week over many weeks, months, and years—a condition not tested in animal studies. Although initial data appeared to indicate that increasing recreational noise history was associated with decreasing ABR Wave I amplitude, those results have not been replicated when similar studies were completed in multiple other laboratories. In contrast to these 420negative outcomes, data from music students have revealed the emergence of functional differences during EHF tests and difficult hearing-in-noise tests with differences in SP amplitude but not AP amplitude, data from frequent concert attendees have revealed smaller ABR Wave I/V amplitude ratio but no functional differences, and data from civilians who use firearms recreationally and military personnel with high noise exposure (including firearm use) revealed smaller ABR Wave I amplitudes in those exposed to noise with no functional testing included in that study. Differences in outcomes across participant populations (college student convenience samples, concert goers, music students, military personnel) provide preliminary insight into risk, but the use of different metrics across studies (EHF, ABR, AP, SP, Wave I, Wave V, WIN, QuickSin, NU6, etc.) makes direct comparisons difficult.
