Abstract

A spatial phase discrimination task was used to measure our contrast sensitivity for discriminating positional differences between multifrequency components spanning a 4-octave range of spatial frequencies. Patterns having an optimal phase difference of 90° were used to activate cortical phase-sensitive units in the center of their working range. Previous studies have found that both spatial frequency and spatial extent are important when discriminating phase differences. Gabor filters characterize the rf profiles of cortical phase-sensitive units. This study investigated the gain of multifrequency backgrounds composed of high spatial frequencies having a low fundamental frequency in phase discrimination. The contrast of the background was increased from 1% to 10%. Directionally selective cues were used at all background contrasts between 1% and 5%. Only small differences in the contrast for phase discrimination were found at low background contrasts. An observer’s contrast sensitivity is a nonlinear function of the background contrast when discriminating spatial phase differences. Gabor filters are insufficient for characterizing the processing of spatial phase differences at low background contrasts. Feedback mechanisms that change the gain of these paired filters by providing directionally selective cues must also be proposed.

© 1985 Optical Society of America