Cochlear microphonic enhancement in two tone interactions.

Two tone interaction functions of the cochlear microphonic (CM) were obtained from pigmented guinea pigs. First (basal) cochlear turn recording locations show optimally enhanced levels of CM when the interfering tone (F2) was positioned about 4 kHz above probe tones (F1) of 12 kHz and 20 kHz. Maximum enhancement occurred for equal level tones. No enhancement was seen for a probe tone of 4 kHz. When basal turn cochlear sensitivity was compromised, CM enhancement caused by the interfering tone was altered and only CM reduction was then seen. The CM reduction was the typical characteristic described by many earlier studies. Guinea pigs with various changes in cochlear sensitivity were studied, providing evidence in support of earlier reports that CM interference (both reductions and enhancements) depends on far field vector summation of the outputs of hair cells from a restricted area of the basilar membrane. No CM enhancement was seen in micropipette recordings from within the organ of Corti.