Abstract

We explored the human observer's ability to discriminate changes in the spatial frequency or contrast of Gaussian-truncated sine-wave gratings. Two gratings, differing either in their spatial frequency or in contrast, were presented in two temporal intervals. In a 4-altemative forced-choice (4-AFC) paradigm, subjects responded first by determining whether spatial frequency or contrast differed and second by determining in which interval the grating with the higher parametric value was presented. We compare these results to those found when subjects performed a simple discrimination task in which spatial frequency and contrast discrimination were measured in separate 2IFC runs. The Weber fraction (f/f, q/c) was measured as a function of pedestal contrast (0.020.32) and stimulus bandwidth (0.252.0 octaves) at photopic (270 cd/m²) luminance levels. Independent of the type of task, the Weber fraction decreased (1) with increasing stimulus contrast and (2) with decreasing stimulus bandwidth. Discrimination thresholds were a factor of two or more lower when either frequency or contrast was discriminated alone compared to when they were discriminated conjointly. An analysis of the role of suprathreshold stimulus uncertainty in these results will be presented.

© 1990 Optical Society of America