Two-tone suppression in inner hair cell responses: correlates of rate suppression in the auditory nerve

Inner hair cell (IHC) recordings were made from second turn of the guinea pig cochlea where characteristic frequencies are approximately 4000 Hz. In order to compare IHC responses with rate suppression measured in the auditory nerve, suppressors were introduced that produced little or no response in the hair cell. The effects of a variable-frequency suppressor on a constant-frequency probe, placed near characteristic frequency, were also investigated since this paradigm is commonly used in single unit experiments. Resulting magnitude changes were measured in the fundamental component of the ac receptor potential and/or in the total dc produced in the region of temporal overlap between the two stimulus inputs. This latter component is especially important when considering how changes in IHC responses relate to decreases in discharge rate in single auditory nerve fibers. Since the ac receptor potential is filtered by the hair cell's basolateral membrane, the dc component probably controls transmitter release at the characteristic frequency of these second-turn IHCs. Based on results from these and previous experiments, a proposal is advanced to explain the evolution of two-tone suppression in the peripheral auditory system. The paper also discusses the use of excitatory versus non-excitatory suppressors and includes a description of two-tone suppression areas at the mechanical, IHC and single unit levels. The explanation of low-side suppression areas is of special interest since hitherto they have been difficult to model (Kim, 1985).