Neural correlates of auditory fatigue: frequency-dependent changes in activity of single cochlear nerve fibers.

1. These experiments were designed to test whether intense pure tones produced greater depression of cochlear nerve fibers tuned to the exposure frequency or of those tuned to frequencies above the exposure frequency. Spike discharges of single fibers were studied in anesthetized cats before, during, and after exposures lasting 1 min. Exposure frequency was varied relative to each fiber's characteristic frequency (CF), and was either at the CF or 1/2 octave above (+1/2 oct) or 1/2 octave below (-1/2 oct) the CF. Exposure levels were 85 or 90 dB SPL. Effects of the various exposures on driven discharge rates were evaluated using standard test stimuli at each fiber's CF. In addition, nonevoked discharges were measured during the brief quiet intervals between test stimuli ("interstimulus activity") as well as during extended quiet periods ("resting activity"). Major results were as follows: 2. All the exposures resulted in depression of the driven discharge rates; however, these effects were strongly dependent on the exposure frequency. The depression was greatest and endured the longest following -1/2 oct exposures at 90 dB. The CF exposures at 85 and 90 dB were much less depressant, as were exposures at -1/2 oct at 85 dB; these three exposures resulted in very similar recovery functions. The +1/2 oct exposures produced little or no depression, whether at 85 or 90 dB. 3. Interstimulus activity was depressed immediately following all exposures, but recovered to normal quickly than did driven discharge rates. Following exposures at -1/2 oct at 90 dB, recovery was non-monotonic in that an extended period of supernormality preceded the return to normal rates. During this period of elevated activity, the interstimulus activity approached but never exceeded the resting rate of the same fiber. 4. Resting activity recovered even more rapidly than interstimulus activity, being completely normal by 1 min following all exposures. 5. These results constitute the first demonstration that the CF is not necessarily the most depressant exposure frequency for a given cochlear nerve fiber. Further, the results imply that the half-octave (or greater) shifts of the point of maximum hearing loss, so characteristic of auditory fatigue, may be accounted for by frequency-dependent alterations in the responsiveness of cochlear nerve fibers.