Underestimation of auditory fatigue as measured by the compound action potential

The action potential (AP), summating potential (SP), and cochlear microphonic (CM) were measured in rats in response either to clicks or pure tones prior to and following 3 min of exposure to pure tones at a level 5 db less than that which produced maximum CM. The ratio, in decibels, between pre-exposure and post-exposure potentials, for the same exposure and probe stimulus parameters, was taken as an index of decrement. The relative reduction in voltage resulting from the exposure was greater for the SP than for the AP when these potentials were elicited with 20-msec probe tone bursts between 70-80 db SPL having instantaneous rise times. However, for weaker probe levels within 20 db of that yielding AP potentials of 1 mu V, the AP and SP bpth exhibited similar losses. The CM and the click-evoked AP showed essentially no decrement. These results suggest that the SP might be a better indicator of noise-induced auditory decrement (fatigue?) than the CM. Theories of central auditory fatigue may be based on incorrect interpretations of previously published data obtained from cochlear and neural recordings.