Two-tone interactions in the cochlear microphonic.

Two-tone interactions are explored for the cochlear microphonic (CM) in the guinea pig. Recordings are made from turns one and three using differential electrodes in the perilymphatic space or pipettes placed in scala media through a fenestra over the stria vascularis. We focus on magnitude changes associated with the introduction of appropriate interference tones and on various types of phase shift concomitant with these magnitude variations that have not received documentation in the literature. Based on extensive parametric data, it is suggested that some features of the gross interference phenomenon may be a consequence of the vectorial summation of outputs from contributing hair cell generators. These spatial effects appear to determine phase behavior and the influence of probe frequency on the frequency of maximal interference. In addition, the apparent interval between out defined best frequency (CF) and the frequency of maximal interference is most likely due to an underestimation of CF resulting from phase cancellation between CM-producing hair cell populations. However, after compensating for these spatial effects, several aspects of the CM interference phenomenon seem to be analogous to two-tone suppression in auditory nerve fibers. A direct one-to-one relationship is not implied since the latter reflect the outputs of inner hair cells while CM interference most likely reflects outer hair cell behavior. As a result, the association between suppression and interference must be sought in the process by which outer hair cell influence inner hair cell transduction.