Variations in the Effects of Electric Stimulation of the Crossed Olivocochlear Bundle on Cat Single Auditory-Nerve-Fiber Responses to Tone Bursts

Spike discharges were recorded from single auditory-nerve fibers in anesthetized cats. Counts were made of the number of discharges in response to tone bursts at several sound levels presented both alone and preceded by a train of shocks delivered to the decussation of the crossed olivocochlear bundle (COCB). From these counts, a measure 〈Δu〉 of the COCB effect was computed for 141 fibers. 〈Δu〉 is maximum when measured with tone bursts at a fiber's characteristic frequency and decreases for tones of higher or lower frequency. 〈Δu〉 was found to be unrelated to the fibers' spontaneous discharge rates. 〈Δu〉 increases with increasing characteristic frequency up to 6–10 kHz and decreases above 15 kHz. For most fibers, 〈Δu) does not correlate with fiber threshold. Those fibers whose apparent threshold is more than 25 dB above the minimum threshold at their characteristic frequency have low 〈Δu〉's. These results are discussed in relation to previous gross-electrode studies and the anatomical distribution of efferent endings in the cochlea.