Low-frequency masking of brainstem potentials.

The phase and intensity dependence of masking a click by a loud low-frequency tone was examined with brainstem potentials. Wave V latency is practically unaffected; its amplitude, however, is maximally suppressed at a phase of 270 degrees, i.e. rarefaction in the ear canal and basilar membrane displacement towards the scala tympani. Corresponding to subjective threshold, a minor suppression is also observed for the opposite deflection. In the phase of maximal suppression, wave V can be cancelled by a 30 Hz tone of 115 dB SPL up to click intensities of 40 dB HL. With cochlear damage, total suppression can be achieved at even higher click intensities as long as they are close to the subjective threshold. Low-frequency suppression is discussed as a tool applicable for early diagnosis of endolymphatic hydrops.