Effects of crossed-olivocochlear-bundle stimulation on cat auditory nerve fiber responses to tones.

Phase, synchronization index, and average firing rate were calculated from period histograms of tone burst responses obtained from sound level series with and without electrical stimulation of the cross-olivocochlear-bundle (COCB). For most fibers, at low sound levels, COCB stimulation shifted the rate and synchronization index level functions up in sound level but did not shift the phase-level function in the same way. These effects can be accounted for if the stage at which the COCB acts precedes the stage at which analog signals are changed into neural firing patterns with a given rate and synchronization index, but does not precede the stage at which the level dependence of phase is introduced. Some level series show an abrupt phase change and "dips" in rate and synchronization-index level functions at high sound levels. COCB stimulation shifted these abrupt phase changes and dips down in sound level and usually had little effect at sound levels above these abrupt phase changes and dips. The following explanatory hypothesis is developed: excitation of an auditory-nerve fiber is the result of two factors which are out of phase and have different growth functions. The two factors cancel when they are equal in amplitude producing the dips and phase changes. COCB stimulation reduces the more sensitive factor but does not change the other factor so the two factors cancel at a lower sound level with COCB stimulation.