Perceived contrast and stimulus size: experiment and simulation

Perceived contrast functions were determined for three different Gabor patch sizes using magnitude estimation and verified by contrast matching. While thresholds show a significant decrease with decreasing patch size, perceived contrasts are equal and independent of patch size for contrasts above 0.06. Contrast matching was also used to study the apparent contrast of two other spatially limited stimuli; the sum of two orthogonal 4 c/deg sine waves multiplied by a gaussian envelope and the sum of spatially adjacent positive and negative gaussians. Models of contrast perception, based on tuned Gabor spatial filters, were formulated and tested for agreement with our experimental data. A model that pools filter responses across spatial frequencies and orientations was found to be more in agreement with our data than a model that simply uses the response of a single, maximally excited, mechanism to mediate contrast perception. Optimum filter bandwidth was found to be about 1.1 octaves.