Suppression and enhancement of distortion-product otoacoustic emissions byinterference tones above f(2). I. Basic findings in rabbits

The present study measured interference-response areas (IRAs) for distortio n-product otoacoustic emissions (DPOAEs) at 2f(1)-f(2), 3f(1)-2f(2), and 2f (2)-f(1). The IRAs were obtained in either awake or anesthetized rabbits, o r in anesthetized guinea pigs and mice, by sweeping the frequencies and lev els of an interference tone (IT) around a set of f(1) and f(2) primary tone s, at several fixed frequencies and levels, while plotting the effects of t he IT on DPOAE level. An unexpected outcome was the occurrence of regions o f suppression and/or enhancement of DPOAE level when the IT was at a freque ncy slightly less than to more than an octave above f(2). The IRA of the 2f (1)-f(2) DPOAE typically displayed a high-frequency (HF) lobe of suppressio n, while the 2f(2)-f(1) emission often exhibited considerable amounts of en hancement. Moreover, for the 2f(2)-f(1) DPOAE, when enhancement was absent, its IRA usually tuned to a region above f(2) Whether or not suppression/en hancement was observed depended upon primary-tone level and frequency separ ation, as well as on the relative levels of the two primaries. Various phys iological manipulations involving anesthesia, eighth-nerve section, diureti c administration, or pure-tone overstimulation showed that these phenomena were of cochlear origin, and were not dependent upon the acoustic reflex or cochlear-efferent activity. The aftereffects of applying diuretics or over -exposures revealed that suppression/enhancement required the presence of s ensitive, low-level DPOAE-generator sources. Additionally, suppression/enha ncement were general effects in that, in addition to rabbits, they were als o observed in mice and guinea pigs. Further, corresponding plots of DPOAE p hase often revealed areas of differing phase change in the vicinity of the primary tones as compared to regions above f(2). These findings, along with the effects of tonal exposures designed to fatigue regions above f(2), and instances in which; DPOAE level was dependent upon the amount of suppressi on/enhancement, suggested that the interactions of two DPOAE-generator sour ces contributed, in some manner, to these phenomena. (C) 1999 Elsevier Scie nce B.V. All rights reserved.