Some experimental observations of responses evoked from the cochlea during two-tone stimulation.

When the gerbil cochlea is stimulated with two closely spaced tones, a broad frequency spread of distortion is generated which can be detected in both the meatal sound field and in the round window cochlear microphonic (CM) response. When the frequency ratio f2/f1 is 1.025 or less and f2 = 6 kHz, there is evidence that more than one group of distortion components combines to give the pattern seen in the two cochlear responses. One group is apparently limited to frequencies below 4.5 kHz. This group may be an expression at close stimulus frequency ratios of the 'pulse' described for more widely spaced, lower frequency stimulus conditions, but shown here to be low-pass filtered. Acoustic and CM components within this group show comparable behaviour. A second distortion group consists of a symmetrical distribution of components above and below the stimuli, with no evidence of low-pass filtering. Acoustic and CM manifestations of this symmetrical grouping show little comparable behaviour. It is suggested that interaction could occur between these two groups of distortion to cause some of the complex behaviour reported for 2f1-f2.