Observations on simultaneous perilymphatic motions and cochlear microphonics suppression

Very low frequencies interfere in the intact cochlea with higher frequencies and suppress these depending on the vibration phase of the low-frequency sound. Physiological functions of the body, mediated, for example, by the eardrum or perilymph coupling with the cerebrospinal fluid, cause a low-frequency pressure modulation of the perilymph, which generates a synchronous perilymphatic motion resulting from the unevenly distributed compliances in the cochlea. This slow streaming causes a displacement of the entire basilar membrane, with as a consequence a postponement of the operating point of the mechanoelectrical transducer as a result of the pressure drop in the helicotrema and the narrow apical cochlear turn. In this contribution, interference phenomena are described, which are caused by spontaneous contractions of the tensor tympani muscle and by respiration-synchronous perilymphatic flow. These two test signals have trapezoidal and triangular impulse functions. In both cases, as suppression pattern of the cochlear microphonics level-time function, the second derivative of the pressure-time function was observed. The suppression is found to lie between 1 and 2 dB. It depends on the level of the suppressed sound and shows a compressive nonlinearity.