Comparison of low- and high-side two-tone suppression in inner hair cell and organ of corti responses

Two-tone interactions were measured from inner hair cells and from the organ of Corti fluid space in the third turn of the guinea pig cochlea. At relatively low stimulus levels, a low-side suppressor caused frequency response functions to become broader. Phase changes exhibited a lag/lead transition around the characteristic frequency in harmony with the change in magnitude. These patterns are similar to those previously documented for a high-side suppressor (Cheatham and Dallos 1989) and suggest that suppression is not simply an attenuation phenomenon since level reductions for single-tone inputs produce response patterns which are mirror images of those obtained for the two-tone conditions. In contrast to the low-level results, data measured at moderately high stimulus levels indicate that the magnitude changes produced by both low- and high-side suppressors are qualitatively similar to changes generated by reducing the input sound level. In other words, ac frequency response functions become narrower, partially reversing the broadening of these functions which occurs as sound level increases. Companion phase measures, however, demonstrate that low- and high-side suppressors, in spite of producing similar changes in filter shape, do not produce similar changes in response phase. In fact, neither of the two-tone conditions produce response patterns similar to the one associated with reducing the input sound level.