Contralateral and ipsilateral suppression of the 2f1-f2 distortion product in human subjects.

The amplitude of the 2f1-f2 distortion product was measured under four conditions in human subjects: during simple two-tone stimulation, during stimulation with a third, pure tone in the ipsilateral ear, during contralateral stimulation with broadband noise, and with combined ipsilateral third tone and contralateral noise. The amount of ipsilateral suppression produced varied with suppressor frequency but was maximum close to the f2 primary frequency as previously demonstrated using similar stimulus parameters [A. M. Brown and D. T. Kemp, Hear. Res. 13, 29-37 (1984); F. P. Harris, R. Probst, and L. Xu, ibid. 64, 133-141 (1992)]. Contralateral stimulation also suppressed the level of distortion. During ipsilateral suppression combined with contralateral stimulation, the amount of ipsilateral suppression of the distortion product was reduced. This interaction may indicate a common mechanism for ipsilateral and contralateral suppression. The decrease in ipsilateral suppression under contralateral stimulation was most pronounced at the peak of the ipsilateral suppression tuning curves and this produced some broadening of the function. In the majority of ipsilateral suppression curves, the peak ipsilateral suppression frequency was observed to shift upwards during contralateral stimulation. These results suggest that contralateral stimulation alters the frequency-selective properties of the cochlear response.