Low-side and multitone suppression in the base of the gerbil cochlea

The cochleas mechanical response to sound stimulation is nonlinear, likely due to saturation of the mechano-electric transduction current that is part of an electromechanical feedback loop. The ability of a second tone or tones to reduce the response to a probe tone is one manifestation of nonlinearity, termed suppression. Using optical coherence tomography to measure motion within the organ of Corti, regional motion variations have been observed. Here, we report on the suppression that occurs within the organ of Corti when a high sound level, low frequency suppressor tone was delivered along with a sweep of discreet single-tones. Responses were measured in the base of the gerbil cochlea at two characteristic frequency locations, with two different directions of observation relative to the sensory tissues anatomical axes. Suppression extended over a wide frequency range in the outer hair cell region, whereas it was typically limited to the characteristic frequency peak in the reticular lamina region and at the basilar membrane. Aspects of the observed suppression were consistent with the effect of a saturating nonlinearity. Recent measurements have noted the three-dimensional nature of organ of Corti motion. The effects of suppression observed here could be due to a combination of reduced motion amplitude and altered vibration axis. Significance StatementThe mammalian auditory organ, the cochlea, relies on a nonlinear active process to achieve sensitivity to low-level sounds and sharp frequency selectivity. Recent work using novel interferometric techniques has revealed complex and nonlinear vibration patterns within the cochleas sensory tissue. In this study, the motion response to a pure tone was reduced by additional "suppressor" tones. The observed motion reduction was consistent with the effect of a saturating nonlinearity, possibly compounded by alterations in the axis of cellular vibration, and thus underscoring the 3-dimensional character of cell-based cochlear mechanical activity.