Mark Bullimore


We measured displacement thresholds as a function of contrast for both luminance- and isoluminant color-varying Gabor patches (sinusoidal gratings windowed by a two-dimensional gaussian function). Gabor patches are localized in both space and spatial frequency, with no sharp edges to provide internal spatial referents, and they limit unintended luminance artifacts due to chromatic aberration and retinal variations in spectral sensitivity. All stimuli were invariant in mean luminance (38 cd/m^2) and chromaticity (CIE coordinates x = 0.27; y = 0.30). Grating spatial frequencies of 0.5, 1 and 2 c/deg were used. Two patches were presented sequentially, each for 500 ms. Displacement was produced by altering the grating phase within the stationary gaussian aperture between presentations. Subjects discriminated the direction of displacement (right/left) in a 2AFC staircase, and thresholds were determined by probit analysis of the resulting psychometric functions. For luminance-varying stimuli, displacement thresholds were invariant for contrasts greater than 5 times threshold. At lower contrasts, thresholds increased rapidly with decreasing contrast. At a given spatial frequency and multiple of threshold contrast, displacement thresholds for isoluminant chromatic stimuli were always greater than those for luminance stimuli by a factor of 2-3. For chromatic Gabor patterns, displacement thresholds were inversely proportional to the square root of contrast; the entire displacement threshold/contrast function was well fit by a single line of slope = -0.5. In control experiments subjects were asked to determine the direction of static phase offset (relative to the center of the patch) of a single stimulus presentation. Thresholds, which were virtually identical for both chromatic and luminance patterns,were related to contrast by a single straight line with slope = -0.5. Detection of luminance displacement appears to be mediated by a low-level motion


Year: 1992

Program Number: 2:30 pm

Author Affiliation: n/a

Co-Authors: Karen De Valois

Co-Author Affiliation: n/a

Room: Ireland B