Low-frequency responses of inner hair cells: evidence for a mechanical origin of peak splitting

Auditory nerve fibres usually respond to a preferred phase of a low frequency sinusoidal stimulus. However, at high sound pressures fibres may either change the preferred phase of response or respond to more than one phase of the stimulus. This complex firing pattern is known as peak splitting. Hypotheses for the origin of peak splitting have ranged from micromechanical models to models incorporating electrical interactions between inner and outer hair cells. In order to determine the origin of peak splitting, the potential across the IHC synaptic membrane has been measured during stimulation with low frequency tones and it is found that the IHC receptor potentials exhibit peak splitting at sound pressures that coincide with the saturation of the outer hair cell receptor potentials. Current injection experiments show that peak splitting is also recorded in the resistance change of the inner hair cell during acoustic stimulation. It is concluded from this evidence that peak splitting is present in the mechanical input to the IHC.