The effects of tone exposure on the inner ear functions in the guinea pig:Impact tone vs. steady state tone

The damage-risk criterion (DRC) for hearing supposes that sound exposure wi th equal energy implies equal risk for noise-induced hearing loss (NIHL). W e measured cochlear microphonics (CM), compound action potential (CAP), end ocochlear potential (EP) and K+ ion concentration in the scala media, to se e if the same level of Leq(24h) (impact tone and steady state tone) induced the same physiological changes in the inner ear function or not. Regarding the equal energy principle (EEP), we also examined if the EEP is appopriat e or not at exposure of moderate level tone. We also checked how the time i nterval between impact tones affects or not the inner ear functions at the same Leq(24h) tone exposure. Therefore we used exposure at 1 pulse/second o r 1 pulse/3 seconds and steady state tone exposure at Leq(24h)=90, 85 and 8 0 dB. The results are the following. Both steady state and impact tone expo sure causes change of the electrophysiological data. First, CM maximum outp ut voltage after exposure to impact tone of 115 dB (Leq(24h)=90 dB) was low er than after exposure to a 8 kHz steady state tone of 90 dB. CAP threshold (below 10 mu V) obtained after the 115 and 110 dB exposure of impact tone were 5-10 dB higher than that of steady state tone of 90 dB. The negative E P induced by impact tone exposures showed the same tendency as the CM exper iments. Having more frequent pulses (1 pulse/second vs. to 1 pulse/3 second s) showed more inhibition. The K+ concentration time course remained simila r to the control when the Leq(24h) was low (80 dB). Impact tone exposure in duced stronger effects to the inner ear at exposure of moderate level tone than that of steady state tone of Leq(24h).