Low-frequency biasing of round window responses in guinea pigs and chinchillas

The acoustic biasing technique using low-frequency sound is of increasing interest to investigators, not only as a means of studying cochlear transduction but also as a promising tool for assessing cochlear pathology such as endolymphatic hydrops. We compared normal modulation patterns of round window responses in guinea pigs and chinchillas, whose low-frequency auditory characteristics are known to be different. A 50-Hz sine wave (90 dB SPL for guinea pigs and 80 dB SPL for chinchillas), which evoked an equivalent magnitude of cochlear microphonics (CM) in both species, was used to modulate the compound action potential (CAP) and the summating potential (SP) elicited by 8-kHz tone bursts. Overall patterns of CAP and SP modulation were almost identical between the two species except for a difference in the phase of 50 Hz CM. The phase of maximum SP enlargement was in accord with that of maximum CAP suppression, which led to inferred basilar membrane (BM) position at maximum scala tympani displacement by between 45 and 90 degrees. More complex or hysteresis effects seemed to be involved in the modulation of CAP and SP magnitude, in addition to the biasing effect owing solely to BM displacement.