title: The acoustic two-tone distortions 2f1-f2 and f2-f1 and their possible relation to changes in the operating point of the cochlear amplifier.
abstract: Acoustic two-tone distortions are generated during non-linear mechanical amplification in the cochlea. Generation of the cubic distortion 2f1-f2 depends on asymmetric components of a non-linear transfer function whereas the difference tone f2-f1 relies on symmetric components. Therefore, a change of the operating point and hence the symmetry of the cochlear amplifier could be strongly reflected in the level of the f2-f1 distortion. To test this hypothesis, low-frequency tones (5 Hz) were used to bias the position of the cochlear partition in the gerbil. Phase-correlated changes of f2-f1 occurred at bias tone levels where there were almost no effects on 2f1-f2. Higher levels of the bias tone induced pronounced changes of both distortions. These results are qualitatively in good agreement with the results of a simulation in which the operating point of a Boltzman function was shifted. This function is similar to those used to describe outer hair cell (OHC) transduction. To influence OHC motility, salicylate was injected. It caused a decrease of the 2f1-f2 level and an increase in the level of f2-f1. Such reciprocal changes of both distortions, again, can be interpreted in terms of a shift of the operating point of the cochlear amplifier along a non-linear transfer characteristic. To directly influence the cochlear amplifier, DC current was injected into the scala media. Large negative currents (> -2 microA) caused a pronounced decrease of 2f1-f2 (> 15 dB) and positive currents had more complex effects with increasing and/or decreasing 2f1-f2 distortion level. The effects were time and primary level dependent. Changes of f2-f1 for DC currents > magnitude of mu 2A were in most cases larger compared to 2f1-f2 and reversed for certain primary levels. The current effects probably result from a combination of changing the endocochlear potential and shifting the operating point along a non-linear transfer function.
authors.affiliation: Zoologisches Institut der Universität München, Germany. frank@zi.biologie.uni-muenchen.de
authors.forename: G
authors.initials: G
authors.lastname: Frank
authors.forename: M
authors.initials: M
authors.lastname: Kössl
mesh_terms: D000161:Acoustic Stimulation; D000669:Amplifiers, Electronic; D000818:Animals; D001299:Audiometry; D001309:Auditory Threshold / Q000187:drug effects / Q000502:physiology; D002465:Cell Movement / Q000187:drug effects / Q000502:physiology; D003051:Cochlea / Q000601:surgery*; D003054:Cochlear Implants / Q000592:standards*; D004791:Enzyme Inhibitors / Q000008:administration & dosage / Q000633:toxicity; D005260:Female; D005849:Gerbillinae; D018072:Hair Cells, Auditory, Outer / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D008297:Male; D017084:Otoacoustic Emissions, Spontaneous; D012459:Salicylates / Q000008:administration & dosage / Q000633:toxicity; D020156:Salicylic Acid; D015398:Signal Transduction / Q000502:physiology
chemical_list: D004791:Enzyme Inhibitors; D012459:Salicylates; D020156:Salicylic Acid