Two-tone suppression in auditory-nerve fibers: extension of a stimulus-response relationship.

Abstract

Average discharge rate of single auditory-nerve fibers in cats was measured in response to one- and two-tone stimuli. One component (the ’’suppressor tone’’) of each two-tone stimulus was at a frequency (f2) which produced two-tone suppression at some stimulus levels. The other component (excitor tone) produced an increase in rate above the spontaneous rate when presented alone. Fractional response was defined as the driven rate to the two-tone stimulus divided by the driven rate to the excitor alone. Fractional response is thus a quantitative measure of the amount of suppression produced by a suppressor tone. A number of qualitative differences were found in the dependence of fractional response for f2≳CF and f2<CF. For suppressor tone frequencies greater than CF, fractional response depends only on the ratio of suppressor to excitor levels (P2/P1) for a range of excitor levels (P1). For P1 large enough to drive a unit into saturation, fractional response increases with P1. For f2<CF, however, fractional response is a monotonic decreasing function of P2 and P2/P1; it is also a monotonic decreasing function of P1 for P2/P1 fixed. Consistent with these results if the tone level ratio P2/P1 is fixed, rate is a monotonic increasing function of the overall level of a two-tone stimulus for f2≳CF; for f2<CF, rate is typically a nonmonotonic function of overall level. For f2≳CF, slopes of (log) fractional response versus (log) P2/P1 curves are a monotonic decreasing function of suppressor frequency f2. For f2<CF, on the other hand, slope does not depend on f2. If f2 is fixed, and excitor-tone frequency (f1) varied, fractional response decreases with f1 for f2≳CF; for f2<CF, fractional response is independent of f1. Subject Classification: [43]65.42, [43]65.59.