Spectrum contrast sharpening by lateral suppression: Comb-filtered noise test

Abstract

Spectrum contrast thresholds were measured in normal listeners using comb-filtered signals. The signal spectrum contained 0.075-oct wide ripples. Spectrum contrast varied from 0 (no ripple on the pedestal) to 1 (zero spectrum level between the ripples). Spectrum contrast thresholds were measured at ripple densities from 1 to 10 oct-1 using a two-alternative forced-choice adaptive procedure. The lowest spectrum contrast thresholds of less than 0.1 appeared at a ripple density of 4 oct-1. At lower ripple density down to 1 ocn-1 , thresholds increase up to 0.2. At ripple densities above 4 oct-1, thresholds steeply increased up to 1.0 at densities of 8 to 10 oct-1. In agreement with the excitation-pattern model, threshold increase at high ripple densities may be explained by across-frequency integration in critical bands. However, threshold increase at low ripple densities cannot be explained by a simple critical-band model. The lowest spectrum contrast thresholds at a certain inter-ripple interval indicate the presence of lateral suppression zones in equivalent filter forms. According to this interpretation, the results demonstrate sharpening of spectral contrast due to lateral suppression. The lateral suppression zones may reflect either cochlear lateral suppression, or lateral inhibition in neuronal centers, or both. [Work supported by Russian Science Foundation.]