Stimulus biasing: a comparison between cochlear hair cell and organ of Corti response patterns

Responses from the organ of Corti (OC) fluid space and from individual hair cells are collected for short duration tone pips measured alone and in the presence of a 20 Hz bias tone. Because of the relatively long period of the acoustic bias signal, a probe can be selectively placed at precise locations within a single response period. Since bias effects produced during maximum basilar membrane velocity are negligible, this report documents changes associated with basilar membrane displacements to scala vestibuli and scala tympani. Hair cell response patterns are compared with those measured nearby in the OC to determine the degree to which inner (IHC) and outer hair cells (OHC) contribute to the gross cochlear potentials. It is shown that intracellular OHC and OC dc responses are strongly influenced by the bias while intracellular IHC dc responses are minimally affected. Although an association has been established between the cochlear microphonic and outer hair cell ac receptor potentials, the well correlated changes in the outer hair cell dc receptor potential and the organ of Corti summating potential suggest that gross cochlear potentials reflect dc as well as ac contributions from nearby OHCs. Since IHC responses are not influenced by the bias, they appear to contribute less than OHCs to extracellular potentials. These conclusions, however, are restricted to the moderately high input levels required to produce these effects (Durrant and Dallos, 1974) and to the central region of the cochlea where the recordings are made. Finally, the degree to which bias effects are contaminated by two-tone suppression is discussed.