Some preliminary observations on the interrelations between two-tone suppression and combination-tone driving in the anteroventral cochlear nucleus of the cat.

Abstract

For units of the anteroventral cochlear nucleus in the cat—as for primary units—response to a best‐frequency tone f1 can be markedly reduced when a higher‐frequency suppressing tone f2 of equal or greater intensity is presented simultaneously at a frequency separation corresponding, in the present experiments, to an optimal distance on the basilar membrane of about 0.5–0.8 mm. More data are needed, but this distance does not seem to vary systematically with best frequency and may be related to critical bandwidth. In further preliminary experiments, we have compared unit response to the combination tone, 2f2−f3, formed when f1 is omitted and f3 is added above f2, with the unit’s response to a three‐tone stimulus. The three‐primary stimulus consists of tones with equispaced frequencies and controllable phase and amplitude relations. Primary tone f1 and combination tone 2f2−f3 coincide at best frequency, while f2 and f3 lie above the tuning curve. The comparison of response to 2f2−f3 ’’alone’’ and of response to the resultant of f1 and 2f2−f3, while f1 phase is varied, allow relative amplitudes to be calculated. They are such that f1 appears suppressed in amplitude by f2 and f3, while 2f2−f3 is correspondingly less than cancellation‐tone estimates. For constant f2 level, 2f2−f3 amplitude reaches maximum when f3 amplitude almost equals f2 and decreases markedly as f3 amplitude exceeds f2. Lastly, when the combination tone resultant composed of 2f1−f2 and 3f2−2f3 lies at best frequency, variation in the phase of f1 produces a marked variation in rate of response to the resultant, appearing to parallel psychophysical results obtained by J. L. Goldstein [’’Aural combination tones,’’ in Frequency Analysis and Periodicity Detection in Hearing, edited by R. Plomp and G. F. Smoorenburg (Sijthoff, Leiden, Netherlands, 1969/1970)].